What are peptides

Brief summary for quick readers

Peptides are short chains of amino acids linked by peptide bonds. They form the basis of biological signaling in the human body, where they act as hormones, neurotransmitters, growth factors, immune modulators, and antimicrobial defense molecules. In scientific research, peptides are used as a tool to study physiological processes, develop new diagnostic methods, mechanistically investigate diseases, and test hypotheses for future therapeutics.

This guide is informational material intended exclusively for research and educational purposes. It contains no dosing recommendations, no health claims, and no instructions for use in humans or animals. All peptides offered in the Molequa portfolio are research chemicals, not medicinal products or dietary supplements.

Article contents

1. Introduction: what the article covers and what it does not
2. Chemical definition of a peptide
3. Amino acids as building blocks
4. Peptide structure: from sequence to 3D form
5. Peptides vs proteins: where one ends and the other begins
6. History of peptide research
7. Classification of peptides: endogenous and synthetic
8. Functional categories of peptides in research
9. Mechanisms of action of peptides
10. Peptide manufacturing: SPPS and recombinant production
11. Peptide quality in research: what research-grade quality means
12. Pharmacokinetics of peptides in an experimental context
13. Routes of administration in research models
14. Storage and stability of peptides
15. Reconstitution of lyophilized peptides
16. Regulatory framework for research peptides
17. Safety aspects and limitations
18. Current research directions
19. Frequently asked questions
20. Conclusion and next steps
21. Sources and recommended literature

1. Introduction: what the article covers and what it does not

Peptides are among the most dynamically developing areas of modern biochemistry, endocrinology, and pharmaceutical research. Over the past thirty years, they have evolved from a relatively peripheral category of molecules into one of the most widely used tools in laboratory practice, thanks to their combination of high specificity, modular synthesis, and broad spectrum of physiological functions.

This article covers:

• The chemical definition of peptides and their relationship to amino acids and proteins
• Classification of peptides by origin, structure, and function
• Manufacturing methods and quality criteria in a research context
• Main mechanisms of action in biological systems
• Practical aspects of storage, reconstitution, and stability
• The regulatory framework and definition of “for research use only” status

This article does not contain or address:

• Dosing recommendations for humans or animals
• Instructions for self-administration of peptides
• Clinical indications and treatment of specific diseases
• Claims of efficacy for human use
• Marketing claims about “peptide therapy”

The target audience is researchers, biochemistry and pharmacy students, laboratory workers, and the scientifically oriented public looking for a comprehensive and balanced overview of peptides as a class of molecules.

Before any work with peptides, it is essential to study the primary scientific literature (PubMed, peer-reviewed journals), qualifying materials of the specific product (Certificate of Analysis, technical datasheet), and applicable legislation in your jurisdiction.

2. Chemical definition of a peptide

2.1 Basic definition

A peptide is a molecule made of two or more amino acids linked by a peptide bond. The peptide bond is a special type of covalent amide bond (-CO-NH-) that forms between the carboxyl group (-COOH) of one amino acid and the amine group (-NH₂) of another amino acid. In this reaction one molecule of water is released, which is why the process is called a condensation reaction or dehydration synthesis.

The chemical equation for the formation of the simplest peptide (a dipeptide):

H₂N-CHR₁-COOH + H₂N-CHR₂-COOH → H₂N-CHR₁-CO-NH-CHR₂-COOH + H₂O

where R₁ and R₂ represent the side chains of two different amino acids.

2.2 Classification by chain length

Peptides are conventionally classified by the number of amino acid residues they contain. The boundaries are not strict, but biochemical literature most often uses the following division:

The boundary between a “long peptide” and a “small protein” is largely conventional. Insulin (51 amino acids) is classified in various textbooks sometimes as a polypeptide, sometimes as a peptide hormone, and sometimes as a protein, depending on context.

2.3 Functional definition

From a functional standpoint, a peptide is defined by its biological role. Most endogenous peptides are signaling molecules that:

• Bind to specific receptors (most often GPCRs or tyrosine kinase receptors)
• Trigger a signaling cascade in target cells
• Modulate gene expression, enzymatic activity, or cellular differentiation
• Act at very low concentrations (typically nM to pM)

Three basic functional categories of peptides in the human body are hormones, neurotransmitters, and defense molecules (antimicrobial peptides and immunomodulators).

3. Amino acids as building blocks

3.1 Twenty proteinogenic amino acids

All natural peptides and proteins in the human body are made up of 20 standard (proteinogenic) amino acids encoded by the genetic code. Each amino acid has the same “backbone” consisting of:

• A central carbon atom (α-carbon)
• An amine group (-NH₂)
• A carboxyl group (-COOH)
• A hydrogen atom
• A unique side chain (R group)

The side chain determines the chemical and physical properties of the amino acid: polarity, charge, hydrophobicity, ability to form hydrogen bonds and disulfide bridges.

3.2 Classification of amino acids by side chain

3.3 Cysteine and disulfide bridges

Cysteine deserves separate attention. Its side chain contains a thiol group (-SH), which can oxidatively form a disulfide bridge (-S-S-) with the thiol group of another cysteine residue. Disulfide bridges:

• Stabilize the three-dimensional structure of peptides and proteins
• Are sensitive to reducing environments (cysteine, glutathione, DTT, β-mercaptoethanol)
• Play a critical role in peptides such as oxytocin, vasopressin, somatostatin, AOD-9604, Melanotan II

Loss of disulfide integrity often means loss of biological activity of the peptide. In quality control of research peptides, disulfide integrity is therefore verified by special tests (Ellman’s test, HPLC retention time compared with a reference standard).

3.4 Modified and non-standard amino acids

Research peptides frequently use modified or non-standard amino acids to improve their pharmacological profile. Among the most common are:

• D-amino acids (mirror isomers of L-amino acids). More resistant to peptidases, extending half-life. Examples: D-Phe in Melanotan II, D-Ala in CJC-1295.
• Aib (aminoisobutyric acid). A non-natural α-substituted amino acid. Increases conformational stability and resistance to DPP-IV. Examples: Aib in Semaglutide, Tirzepatide, Retatrutide.
• Norleucine (Nle). Substituted for methionine, which is oxidation-sensitive. Examples: Melanotan II, PT-141.
• Hydroxyproline (Hyp). Hydroxylation of proline. Naturally occurring in collagen.
• Pyroglutamate. Cyclic form of glutamate, often at the N-terminus of peptides.
• N-terminal acetylation. Addition of an acetyl group. Examples: Thymosin α1, Melanotan II.
• C-terminal amidation. Conversion of carboxyl to amide. Often improves stability and activity.

These modifications are classical tools of peptide chemistry, and modern research peptides use them routinely.

3.5 Lipidation and albumin binding

A special category of modification is the attachment of a fatty acid to a peptide, typically via a lysine residue with a γ-glutamate linker. This is called lipidation and serves to extend the plasma half-life of the peptide through:

• Reversible binding to serum albumin (the most abundant protein in plasma)
• Slower renal excretion
• Extension from minutes to days

Examples of lipidated peptides in modern research:

• Semaglutide: C18 fatty acid, half-life ~7 days
• Tirzepatide: C20 fatty diacid, half-life ~5 days
• Retatrutide: C20 fatty diacid, half-life ~6 days
• Cagrilintide: C20 fatty diacid, half-life ~6.6 days

Lipidation is one of the most important innovations in peptide chemistry of the past two decades.

4. Peptide structure: from sequence to 3D form

4.1 Primary structure: amino acid sequence

The primary structure of a peptide is its amino acid sequence, that is, the order of individual amino acids from the N-terminus (with a free amine group) to the C-terminus (with a free carboxyl group). The sequence is conventionally written:

• With single-letter abbreviations: for example GEPPPGKPADDAGLV (BPC-157)
• With three-letter abbreviations: for example Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val

The primary structure is fully determined by the gene in natural proteins and dictates all higher levels of structure according to Anfinsen’s dogma (Anfinsen, 1973, Nobel Prize in Chemistry).

4.2 Secondary structure: local arrangement

Secondary structure describes the local arrangement of the peptide chain through hydrogen bonds between the carbonyl group (C=O) and the amide group (N-H) of adjacent peptide bonds. The two most common motifs are:

• α-helix: a right-handed spiral with 3.6 amino acids per turn. Hydrogen bonds between the i-th and (i+4)-th residues. Side chains point outward from the helix. A stable motif for membrane-bound receptors.
• β-sheet: extended chains lying side by side (parallel or antiparallel) connected by hydrogen bonds. They form surfaces and active sites.

Short peptides (under 15 amino acids) typically have flexible conformation in water and adopt more stable structures only after binding to a receptor.

4.3 Tertiary structure: three-dimensional form

Tertiary structure is the overall three-dimensional conformation of the peptide chain, stabilized by:

• Hydrophobic interactions (nonpolar side chains cluster in the center)
• Hydrogen bonds between side chains
• Ionic bonds (salt bridges) between charged groups
• Disulfide bridges (covalent linkage of cysteines)
• Van der Waals forces

For most short research peptides (up to 30 amino acids), tertiary structure is less important than for proteins. Peptides often adopt a defined conformation only after binding to the target receptor (the “induced fit” principle).

4.4 Cyclic peptides

A special category is cyclic peptides, where the N-terminus and C-terminus are covalently joined, or two side chains form a covalent bond (typically a lactam or disulfide). Cyclization provides:

• Conformational rigidity (the peptide has fewer degrees of freedom)
• Higher metabolic stability (proteases attack cyclic structures with more difficulty)
• Often higher binding affinity for the receptor

Examples of cyclic peptides in research:

• Oxytocin, Vasopressin: cyclization via disulfide bridge
• Somatostatin: cyclization via disulfide bridge
• Melanotan II, PT-141: cyclization via lactam bridge (Asp-Lys)
• Setmelanotide: cyclization via lactam bridge

4.5 Conformational flexibility and consequences

Peptides have higher conformational flexibility than proteins, which has two main implications for research:

1. Receptor specificity: a peptide can adapt to the conformation required for receptor binding (allosteric fit), but is therefore also more prone to conformational changes under improper storage.

2. Stability in solution: flexible peptides can aggregate, form amyloid fibrils, or lose activity faster than cyclic or rigidly stabilized analogs.

This is why modern research peptides are designed with chemical stabilizations (Aib substitutions, D-amino acids, cyclization, lipidation).

5. Peptides vs proteins: where one ends and the other begins

These two terms are often confused in popular literature. For the research context, the distinction is significant.

5.1 Comparison table

5.2 Practical implications for research

From the standpoint of laboratory work with research peptides, several important consequences arise:

Peptides are usually administered parenterally (subcutaneously, intramuscularly, intravenously). Gastric peptidases and the acidic environment break down most peptides before they can be absorbed. Exceptions exist (Semaglutide in an oral form with the SNAC absorption enhancer, or small peptides such as BPC-157 with exceptional stability), but they are a minority.

Peptides require milder storage conditions than proteins. A lyophilized peptide at 2 to 8 °C typically has 18 to 24 months of stability. By comparison, many monoclonal antibodies require a strict cold chain and lose activity after brief exposure to room temperature.

Peptides have lower immunogenicity. Short peptides up to 10 amino acids are practically non-immunogenic. Longer peptides (20 to 50 amino acids) can elicit antibody formation, but in pharmaceutical molecules such as Semaglutide or Tirzepatide the incidence of anti-drug antibodies is under 1 %. With protein medicines (recombinant insulin, erythropoietin), immunogenicity is a significantly larger clinical problem.

Synthetic flexibility. Peptides can be manufactured in a controlled manner at gram and kilogram scale via solid-phase peptide synthesis (SPPS). Proteins require recombinant expression in living cells (E. coli, yeast, mammalian cells), which brings complications with post-translational modifications, aggregation, and quality control.

6. History of peptide research

Understanding the historical development of peptide chemistry helps to better understand why some peptides are so prominent in today’s research.

6.1 Key milestones

6.2 Peptide drug market

The global market for peptide therapeutics exceeded USD 50 billion annually in 2024 and is growing at 8 to 12 % per year. As of 2026, more than 80 peptide drugs are globally approved, with another roughly 170 in various phases of clinical trials.

The most significant categories are:

• Insulin and insulin analogs: market over USD 25 billion annually
• GLP-1 and incretin agonists: the fastest-growing category (Semaglutide alone exceeded USD 20 billion annually in 2024)
• GnRH agonists/antagonists: oncology applications (Leuprolide, Degarelix)
• Somatostatin analogs: neuroendocrine tumors (Octreotide)
• Peptide vaccines and immunomodulators: a growing category

This commercial success is also the engine for the development of research peptides as a tool for basic and applied research.

7. Classification of peptides: endogenous and synthetic

7.1 By origin

7.2 By structure

7.3 Endogenous peptides in the human body

The human body produces hundreds of endogenous peptides, which fall into the following main categories:

Pituitary and hypothalamic hormones:

• Oxytocin, Vasopressin, ACTH, GH, TSH, FSH, LH, prolactin
• Releasing hormones: GHRH, GnRH, CRH, TRH, somatostatin

Pancreatic peptides:

• Insulin, Glucagon, Amylin, Somatostatin, Pancreatic polypeptide

Gastrointestinal peptides:

• GLP-1, GIP, Oxyntomodulin, CCK, Gastrin, Secretin, Motilin, VIP, Substance P, Ghrelin

Immune peptides:

• Thymosin α1, Thymosin β4, Defensins, Cathelicidins, LL-37

Neuropeptides:

• Substance P, Endorphins, Enkephalins, Dynorphins, NPY, Galanin, Orexins

Growth factors (technically proteins):

• Insulin-like growth factor (IGF-1)
• Epidermal growth factor (EGF)
• Vascular endothelial growth factor (VEGF)

Mitochondrial peptides (MDPs):

• Humanin, MOTS-c, SHLPs

From this rich endogenous peptide palette comes the majority of modern synthetic analogs used in research and in the clinic.

8. Functional categories of peptides in research

For navigating research peptides, it is useful to know the main functional categories. These categories are not always sharply separated (some peptides fall into several), but they provide a practical navigation scheme.

8.1 Regenerative peptides

The most studied category in the sports and regenerative medicine research literature.

Main molecules:

• BPC-157 (15 aa). A pentadecapeptide isolated from gastric juice. Activates VEGFR2-mediated angiogenesis and the FAK-paxillin pathway. Research focused on tendon healing, gastrointestinal lesions, and vascular damage.
• TB-500 (synthetic fragment of Thymosin β-4, 44 aa). Regulates actin polymerization and mobilizes endothelial progenitor cells. Research on cardiac regeneration, cutaneous wounds, and ophthalmologic indications.
• GHK-Cu (3 aa). The tripeptide Gly-His-Lys with a bound copper ion. Stimulates collagen and elastin. Research on skin regeneration and hair follicles.
• KPV (3 aa). The tripeptide Lys-Pro-Val, the C-terminal fragment of α-MSH. Anti-inflammatory properties.

8.2 Metabolic peptides (the incretin family)

The fastest-growing category of peptide therapy and research.

Main molecules:

• Semaglutide (31 aa). A mono GLP-1 agonist. Semaglutide is the benchmark of modern peptide pharmacology of obesity and diabetes.
• Tirzepatide (39 aa). A dual GIP/GLP-1 agonist. The SURPASS-2 trial demonstrated superiority over Semaglutide.
• Retatrutide (39 aa). A triple GIP/GLP-1/glucagon agonist. Phase 2 trials with weight reduction up to 24.2 %.
• Mazdutide (39 aa). A dual GLP-1/glucagon agonist. Approved in China in 2024 (NMPA) for obesity.
• Cagrilintide (37 aa). A long-acting amylin analog. Phase 3 in combination with Semaglutide as CagriSema.
• Liraglutide (31 aa). Predecessor of Semaglutide, daily dosing.

8.3 Growth hormone secretagogues (GHS)

Peptides that stimulate the endogenous secretion of growth hormone from the pituitary.

Two main mechanism classes:

A. GHRH analogs (stimulate the GHRH receptor):

• Sermorelin (29 aa, fragment GHRH 1-29)
• CJC-1295 (modified GHRH, half-life 6 to 8 days)
• Tesamorelin (approved for HIV-associated lipodystrophy)

B. Ghrelin mimetics / GHRPs (stimulate GHSR-1a):

• Ipamorelin (selective GHRP, does not raise cortisol)
• GHRP-2, GHRP-6 (older generations)
• MK-677 / Ibutamoren (non-peptide oral GHSR-1a agonist)
• Hexarelin

Principle of GHRH + GHRP combination: simultaneous activation of two independent pathways produces a synergistic GH pulse higher than either component alone.

8.4 Skin and cosmetic peptides

A category of peptides used in dermatologic and cosmetic research.

Main molecules:

• GHK-Cu (3 aa with copper). For skin regeneration and hair.
• Melanotan I (Afamelanotide). Approved for erythropoietic protoporphyria (Scenesse).
• Melanotan II. Stronger, non-selective MC agonist. Tan plus libido-inducing effects.
• Argireline (Acetyl Hexapeptide-3). Topical cosmetics, mechanism similar to botulinum toxin.
• Matrixyl, Palmitoyl Pentapeptide-4. Collagen-stimulating peptides.

Special category. Bremelanotide (PT-141) arose from Melanotan II as an MC4R-selective derivative for sexual function. Approved by FDA in 2019 as Vyleesi for HSDD in women.

8.5 Cognitive and neuropeptides

A category with neuromodulatory effects. The strongest research base comes from the Russian and post-Soviet tradition (Institute of Molecular Genetics in Moscow).

Main molecules:

• Selank (7 aa, tuftsin analog). Anxiolytic without sedation. Approved in Russia for GAD.
• Semax (7 aa, ACTH 4-10 analog). Nootropic and neuroprotective. Approved in Russia for cerebrovascular stroke, included on the List of Vital Medicines.
• DSIP (9 aa). Delta sleep-inducing peptide. Modulation of sleep and the HPA axis.
• Cerebrolysin. Complex of peptide fragments from porcine brain. Approved in 50+ countries.
• P21. Fragment of CNTF (ciliary neurotrophic factor). Preclinical data for neurogenesis.
• Dihexa. Angiotensin IV analog. Strong neurotrophic effect in animal models.

8.6 Immune and antimicrobial peptides

• Thymosin α1 (Zadaxin) (28 aa). Approved in 35+ countries for chronic hepatitis B and as a vaccine adjuvant.
• Thymosin β4 / TB-500 (44 aa). Multi-functional regenerative and immune peptide.
• LL-37 (Cathelicidin) (37 aa). Endogenous antimicrobial peptide.
• Defensins (α, β). Antimicrobial peptides of innate immunity.
• Thymulin (FTS). Zinc-dependent nonapeptide from the thymus.

8.7 Anti-aging and mitochondrial peptides

The youngest, but growing category.

• Epithalon (4 aa, AEDG). Activates telomerase in fibroblasts. Khavinson’s Russian tradition.
• MOTS-c (16 aa). Encoded by mitochondrial DNA. AMPK activation, exercise mimetic.
• Humanin (24 aa). Mitochondrial peptide with cytoprotective and antiapoptotic effects.
• SS-31 / Elamipretide (4 aa). Targeted at cardiolipin in the inner mitochondrial membrane.
• GHK-Cu. In some contexts also classified as an anti-aging peptide.

8.8 Reproductive and sexual peptides

• Kisspeptin. Key regulator of GnRH and the HPG axis.
• PT-141 / Bremelanotide (7 aa). MC4R agonist, approved for HSDD.
• Gonadorelin. Synthetic GnRH.
• HCG (technically a glycoprotein). Substitute for LH in hormonal protocols.

9. Mechanisms of action of peptides

Understanding the mechanism of action is key to interpreting research results. Peptides can act through several main mechanisms.

9.1 Activation of G-protein-coupled receptors (GPCR)

The most common mechanism of peptide action. GPCRs (G-protein-coupled receptors) are the largest family of membrane receptors in the human genome (approximately 800 members). Characteristic properties:

• Seven transmembrane helices
• Signaling via heterotrimeric G-proteins (Gαs, Gαi, Gαq, Gα12/13)
• Targets for approximately 35 % of all approved drugs

Examples of peptide-activated GPCRs in research:

• GLP-1 receptor (GLP-1R): Gαs → cAMP → PKA. Target for Semaglutide, Liraglutide, Exenatide, Tirzepatide, Retatrutide, Mazdutide.
• GIP receptor (GIPR): Gαs → cAMP. Target for Tirzepatide, Retatrutide.
• Glucagon receptor (GCGR): Gαs → cAMP. Target for Mazdutide, Retatrutide.
• GHSR-1a (Growth Hormone Secretagogue Receptor): Gαq → IP3/DAG → Ca²⁺. Target for Ghrelin, Ipamorelin, GHRP-2/6, MK-677.
• GHRH receptor (GHRH-R): Gαs → cAMP. Target for Sermorelin, CJC-1295, Tesamorelin.
• Melanocortin receptors (MC1R, MC3R, MC4R, MC5R): Gαs → cAMP. Target for Melanotan I/II, PT-141, Setmelanotide.
• Amylin receptors (AMY1/2/3): combination of CTR + RAMP1/2/3. Target for Cagrilintide, Pramlintide.
• Oxytocin receptor (OXTR): Gαq → IP3/DAG. Target for Oxytocin.
• Opioid receptors (μ, δ, κ): Gαi → inhibition of cAMP. Target for endogenous opioids, Enkephalins, Endorphins.

9.2 Activation of receptor tyrosine kinases (RTK)

The second main mechanism. The peptide binds to a transmembrane RTK, which auto-phosphorylates its intracellular domain on tyrosine residues and activates signaling pathways (MAPK, PI3K/AKT, JAK/STAT).

Examples:

• Insulin receptor (IR). Target for Insulin and insulin analogs.
• IGF-1 receptor (IGF-1R). Target for IGF-1.
• EGF receptor (EGFR). Target for EGF.
• VEGF receptor 2 (VEGFR2). Target for VEGF and secondarily by modulation from BPC-157.

9.3 Activation of cytokine receptors (JAK/STAT)

Technically distinct from RTKs, but similar in principle. Cytokine receptors do not have intrinsic kinase activity, but recruit JAK kinases after dimerization.

Examples:

• Growth hormone receptor (GHR): JAK2/STAT5. Target for GH.
• Prolactin receptor: JAK2/STAT5.
• Leptin receptor.

9.4 Intracellular targets

Some peptides cross the cell membrane and act on intracellular targets instead of signaling through membrane receptors.

Examples:

• TB-500 / Thymosin β-4. Binds monomeric G-actin inside the cell and serves as a storage molecule for actin polymerization.
• SS-31 / Elamipretide. Specifically binds cardiolipin in the inner mitochondrial membrane.
• Epithalon. Presumed direct interaction with the hTERT promoter and modulation of gene expression.
• MOTS-c. Translocates from the mitochondrion to the cytoplasm and to the nucleus, where it interacts with NRF2 and antioxidant pathway genes.

9.5 Modulation of gene expression

Some peptides change gene expression through direct or indirect interaction with transcription factors.

Examples:

• Selank and Semax. Modulate expression of genes for the GABA-A receptor (Selank) and BDNF/NGF (Semax).
• MOTS-c. Alters expression of antioxidant pathway and mitochondrial biogenesis genes.
• Epithalon. Induces expression of hTERT.

9.6 Ion channels and membrane effects

Some peptides directly interact with ion channels or destabilize membranes.

Examples:

• Endorphins. Activation of opioid receptors leads to closure of Ca²⁺ channels in neurons.
• Defensins and LL-37. Direct destabilization of bacterial membranes through electrostatic interaction (antimicrobial mechanism).
• Melittin (from bee venom). Formation of transmembrane pores.

9.7 Enzyme modulation

Some peptides act as inhibitors or activators of enzymes instead of receptor signaling.

Examples:

• Glutathione (GSH). Tripeptide antioxidant and substrate for glutathione enzymes.
• Aprotinin. Protease inhibitor.
• ACE inhibitors (Captopril, Lisinopril): although not peptides themselves, they originated from peptide inhibitors.

10. Peptide manufacturing: SPPS and recombinant production

For interpreting the quality of research peptides, it is essential to understand the manufacturing process.

10.1 Solid-Phase Peptide Synthesis (SPPS)

The standard method for most research peptides up to a length of approximately 50 amino acids. It was developed by Robert Bruce Merrifield in 1963 (Nobel Prize in Chemistry 1984).

Principle:

1. The first amino acid is attached to a solid polymer resin via the C-terminal group. The amino acid has a protected amine group (Fmoc or Boc protecting group) and protected side chains.
2. Deprotection. The protecting group is removed from the amine group.
3. Coupling (the bonding reaction). Another amino acid (with protected amine group) and an activating agent (HBTU, HATU, DIC) are added. A new peptide bond forms.
4. Washing. Excess reagents are removed.
5. Repetition of steps 2 to 4, amino acid by amino acid, from C-terminus to N-terminus.
6. Cleavage from the resin. After completion of the sequence, the peptide is released from the resin (typically TFA for Fmoc chemistry).
7. HPLC purification. Incomplete sequences and side products are removed.
8. MS verification. Mass spectrometry confirms identity.
9. Lyophilization. The peptide is frozen and converted by vacuum drying into a stable dry form.

Two main chemistries:

• Fmoc (9-fluorenylmethyloxycarbonyl). Dominates in modern practice. Removal of the protecting group with mildly basic piperidine. Gentler, suitable for most peptides.
• Boc (tert-butyloxycarbonyl). Historically first. Removal of the protecting group with a strong acid (TFA). Cleavage from the resin requires hydrogen fluoride (HF), which is dangerous.

10.2 SPPS efficiency and quality control

In an ideal world, every coupling step in SPPS would proceed at 100 % efficiency. In practice, efficiency is typically 99.0 to 99.8 % per coupling. For a 30-amino-acid peptide this means:

• At 99.5 % efficiency: 0.995³⁰ = 86 % correct target sequence
• At 99.0 % efficiency: 0.990³⁰ = 74 % correct target sequence
• At 98.0 % efficiency: 0.980³⁰ = 54 % correct target sequence

Therefore, HPLC purification is critical. After purification, a top-tier research peptide reaches HPLC purity ≥ 99 %, which is the current “gold standard” for quality research peptides.

For a detailed look at peptide quality, see the separate article: HPLC purity of peptides: what 98 % and above means.

10.3 Recombinant production

For longer peptides and small proteins (typically over 50 amino acids), SPPS is technically demanding and economically unfavorable. In such cases, recombinant production in microorganisms is used.

Principle:

1. The DNA sequence encoding the peptide is inserted into a bacterial plasmid or other expression vector.
2. The vector is transformed into a host: E. coli (most often), yeast (Saccharomyces cerevisiae, Pichia pastoris), mammalian cells (HEK293, CHO), or insect cells (Sf9).
3. The host expresses the peptide in large quantities.
4. The peptide is isolated and purified (typically by affinity chromatography with a His-tag or other fusion marker).
5. If needed, fusion tags are removed and the peptide is lyophilized.

Advantages of recombinant production:

• Suitable for longer sequences
• Lower cost per gram at large scale
• Possibility of reproducing post-translational modifications (glycosylation in mammalian cells)

Disadvantages:

• More complex process and higher capital investment
• Higher risk of bacterial endotoxins with E. coli
• Post-translational modifications may differ from native mammalian ones

Insulin is a classic example of the transition from extraction (porcine and bovine pancreas) to recombinant production (E. coli, later yeast), which took place in the 1980s.

10.4 Hybrid approaches

For some complex peptides, hybrid approaches are used:

• Fragment synthesis. The peptide is divided into 2 to 4 fragments, which are prepared by SPPS and then joined chemically (native chemical ligation, NCL).
• Chemoenzymatic synthesis. Part of the peptide is prepared by SPPS, part recombinantly, and the fragments are joined enzymatically.
• Post-synthesis conjugation. Lipidation (Semaglutide, Tirzepatide) or pegylation is performed after the peptide portion is complete.

11. Peptide quality in research: what research-grade quality means

The term “research grade” is used in the peptide market for peptides intended exclusively for research purposes, not for human or veterinary use. The quality of research-grade peptides, however, varies considerably among suppliers, and understanding quality criteria is critical for meaningful research.

For a detailed analysis, see the separate article: Research peptides: what research-grade quality means.

11.1 Six key quality parameters

1. HPLC purity (≥ 98 %, ideally ≥ 99 %)

HPLC (High Performance Liquid Chromatography) is the gold standard for measuring peptide purity. The peptide is passed through a chromatographic column and a UV detector at 220 nm records the elution of individual components. The result is a chromatogram that shows:

• The main peak (target peptide)
• Minor peaks (impurities: truncated sequences, oxidized forms, deamidated forms)

For a quality research peptide, the main peak should represent ≥ 98 % of total area, ideally ≥ 99 %. Some top-tier suppliers (including Molequa) guarantee ≥ 99.0 % as a standard.

2. Peptide identity (Mass Spectrometry, MS)

HPLC purity tells you that it is only one molecule, but does not tell you whether it is the right one. To confirm identity, mass spectrometry (MS) is used, typically ESI-MS or MALDI-TOF MS. MS measures the exact molecular weight of the peptide, which must match the theoretical weight of the target sequence.

For structurally similar peptides (e.g., Tirzepatide, Retatrutide, Mazdutide, Survodutide, Cotadutide, which have a similar backbone), MS/MS fragmentation analysis is often required to verify the exact amino acid sequence.

3. Bacterial endotoxins (LAL test)

Bacterial endotoxins (lipopolysaccharides from the cell wall of gram-negative bacteria) are extremely pyrogenic. Even picomolar concentrations can trigger fever and an inflammatory reaction. For injectable peptides, endotoxin control is therefore critical via the LAL test (Limulus Amebocyte Lysate).

The USP limit for parenteral drugs is < 5 EU/kg body weight per hour. For research peptides, the standard limit is < 0.5 EU/mg peptide.

4. Microbial contamination

A lyophilizate can be contaminated by bacteria or fungi during improper sterile handling. USP <61> defines testing procedures for microbial contamination. A quality research peptide complies with USP <61>.

5. Residual solvents and byproducts

In SPPS, organic solvents (DMF, DCM, NMP, TFA), activating agents (HBTU, HATU), and other chemicals are used. After final purification, these residual solvents must be below ICH Q3C limits. TFA (trifluoroacetic acid) should be below 1 %.

6. Batch stability and Certificate of Analysis (CoA)

For each peptide batch the supplier should provide a Certificate of Analysis (CoA) containing:

• HPLC chromatogram with the main peak and % purity marked
• MS spectrum confirming molecular weight
• LAL test result
• Microbial control
• Residual solvents
• Visual inspection of the lyophilizate
• Date of manufacture and expiration

For a detailed CoA breakdown, see the separate article: CoA for peptides: how to read a certificate of analysis.

11.2 Specific QC parameters for some peptides

Some peptides require additional QC parameters specific to their structure:

• Disulfide integrity (Ellman’s test). Critical for AOD-9604, HGH Fragment 176-191, oxytocin, somatostatin.
• Cyclization integrity. Critical for Melanotan II, PT-141, Setmelanotide (cyclic peptides with a lactam bridge).
• N-acetylation. Critical for Thymosin α1 (without acetylation the peptide loses activity).
• Methionine oxidation. Critical for peptides with methionine (Semax, MOTS-c, DSIP).
• Amyloid aggregation (SEC). Critical for amylin analogs (Cagrilintide, Pramlintide).
• Lipidation completeness. Critical for lipidated peptides (Semaglutide, Tirzepatide, Retatrutide).

A quality research peptide supplier checks these parameters with every relevant batch.

12. Pharmacokinetics of peptides in an experimental context

Pharmacokinetics (PK) describes what the body does to the peptide: absorption, distribution, metabolism, and elimination. Understanding PK is key to interpreting research results.

12.1 Plasma half-life

Plasma half-life (t½) is the time required for the plasma concentration of the peptide to fall to half. For peptides it is extremely variable:

12.2 Strategies for half-life extension

The short half-life of native peptides is the main barrier to their therapeutic use. Modern peptide chemistry has several strategies to extend the half-life:

1. Lipidation (albumin binding)

Attachment of a fatty acid via a γ-glutamate linker. The fatty acid reversibly binds to serum albumin, which acts as a depot and protects the peptide from renal excretion.

• Examples: Semaglutide (C18), Tirzepatide (C20), Retatrutide (C20), Cagrilintide (C20)

2. PEGylation (PEG)

Attachment of a polyethylene glycol chain (PEG). Increases hydrodynamic size and reduces renal filtration.

• Examples: PEG-Interferon, PEG-GCSF, some experimental peptides

3. DAC (Drug Affinity Complex)

A special chemical modification that forms a covalent bond with albumin (in contrast to reversible lipidation).

• Example: CJC-1295 with DAC

4. D-amino acids and Aib substitutions

Replacing L-amino acids with their D mirror isomers or with Aib increases resistance to peptidases.

• Examples: Melanotan II (D-Phe), Aib in Semaglutide/Tirzepatide/Retatrutide

5. Cyclization

Cyclic peptides are more resistant to exopeptidases (enzymes that cleave from the ends).

• Examples: Melanotan II, PT-141, Setmelanotide

6. N-acetylation and C-amidation

Modification of terminal groups stabilizes the peptide against terminal peptidases.

• Examples: Thymosin α1 (N-acetylation), oxytocin (C-amidation)

12.3 Bioavailability and routes of administration

Bioavailability is the fraction of an administered dose that reaches systemic circulation in active form. For peptides it varies greatly by route of administration:

12.4 Distribution and metabolism

Peptides typically distribute in extracellular fluid and blood plasma. They cross the blood-brain barrier to a limited extent (exceptions: DSIP, Selank, Semax with intranasal administration).

Metabolism of peptides proceeds via:

• Peptidases (proteolytic enzymes) in plasma, liver, kidneys
• DPP-IV (dipeptidyl peptidase 4) cleaves GLP-1, GIP, and other incretins at the N-terminus
• Neprilysin in the kidneys
• Carboxypeptidases cleave from the C-terminus

Elimination of metabolites proceeds via urine and bile.

13. Routes of administration in research models

13.1 Subcutaneous (SC)

The most common route for most research peptides in both animal and human clinical models.

Characteristics:

• Injection under the skin, into subcutaneous fat tissue
• Slower absorption than IV but predictable
• Suitable for most peptides with half-lives of hours or days
• An insulin syringe 1 mL with a 29G needle is commonly used for minimal pain

Examples of peptides with SC administration: Semaglutide, Tirzepatide, Retatrutide, Cagrilintide, Mazdutide, BPC-157, TB-500, Ipamorelin, CJC-1295, PT-141, Thymosin α1, AOD-9604, Insulin (multiple).

13.2 Intramuscular (IM)

Injection into a muscle (typically deltoid, gluteus, vastus lateralis). Absorption is somewhat faster than SC, but administration is more painful.

Examples: Some GHRPs, Thymosin β-4 in clinical trials, Tesamorelin.

13.3 Intravenous (IV)

Full bioavailability of 100 % by definition. Suitable for:

• Acute research models
• Clinical trials with precise pharmacokinetic control
• Peptides with an extremely short half-life

Examples of IV administration: TB-500 in clinical trials, REGENERATE-1 program; NAD+ in “NAD therapy”; Cerebrolysin in cerebrovascular applications.

13.4 Intranasal (IN)

Application into the nasal cavity via dropper or spray applicator. Advantages:

• Direct CNS delivery via the nasopharyngeal mucosa and olfactory nerve
• Bypasses the blood-brain barrier
• No injection
• Rapid onset of action (15 to 30 minutes)

Examples of IN administration: Selank, Semax, DSIP, Oxytocin (in some experimental protocols), Cerebrolysin.

13.5 Oral

A very limited category. Gastric peptidases and the acidic environment break down most peptides. Exceptions:

• Rybelsus (Semaglutide + SNAC absorption enhancer). Bioavailability ~1 %, clinically effective.
• BPC-157. Unusually stable, partly active even orally (because it originates from gastric juice).
• MK-677. A non-peptide oral GHSR-1a agonist; it is a small molecule, not a peptide.

13.6 Topical

Application to skin or mucosa. Suitable for skin peptides:

Examples: GHK-Cu (cosmetics), Argireline, Matrixyl, Palmitoyl Pentapeptide-4. Systemic absorption is minimal.

13.7 Sublingual

Under the tongue. Some experimental formulations of GHRPs, but bioavailability is low and inconsistent.

14. Storage and stability of peptides

Correct storage is critical to preserving the biological activity of the peptide. Detailed guide: Storage of peptides: stability, temperature, and light.

14.1 Lyophilizate (dry powder)

A lyophilizate is the dried form of the peptide prepared using the freeze-drying process (sublimation of frozen water under reduced pressure). The lyophilizate is the most stable form of the peptide and typically lasts:

• 2 years at −20 °C (freezer)
• 12 to 18 months at 2 to 8 °C (refrigerator)
• Short-term (days to 30 days) at room temperature (up to 25 °C), protected from light and humidity

For a detailed guide to lyophilized peptides, see the separate article: Lyophilized peptides: significance, advantages, and limits.

14.2 After reconstitution (in solution)

After dissolution in a sterile solvent, the peptide undergoes gradual degradation. The main degradation processes:

• Deamidation (Asn → Asp, Gln → Glu)
• Oxidation (Met, Cys, Trp, His)
• Hydrolysis of the peptide bond (especially Asp-Pro, Asp-Gly)
• Racemization (L → D)
• Aggregation (amyloid for sensitive peptides)

Shelf life after reconstitution depends on the solvent:

• Bacteriostatic water (with 0.9 % benzyl alcohol): 28+ days at 2 to 8 °C
• Sterile water (WFI) without preservative: 1 to 7 days at 2 to 8 °C
• Saline 0.9 % NaCl: 7 to 14 days at 2 to 8 °C

14.3 Factors affecting stability

15. Reconstitution of lyophilized peptides

Reconstitution is the process of bringing a lyophilized peptide back into solution by adding a sterile solvent. For a detailed procedure, see the separate article: Reconstitution of peptides in a laboratory context.

15.1 Solvent choice

15.2 Standard procedure

1. Equilibration. Warm the lyophilizate and the solvent to room temperature (15 to 20 minutes).
2. Disinfection. Disinfect the rubber stoppers of both vials with alcohol pads.
3. Solvent withdrawal. Use a sterile insulin syringe 1 mL / 29G.
4. Slow application. Inject slowly along the wall of the peptide vial. Never directly onto the lyophilizate (disperses the peptide).
5. Gentle dissolution. Circular swirling for 30 to 60 seconds. Never shake.
6. Visual inspection. The solution must be completely clear. Any cloudiness indicates degradation or contamination.
7. Storage. Refrigerator 2 to 8 °C, protection from light.

15.3 Concentration calculation

Concentration formula:

Concentration (mg/mL) = Mass of peptide (mg) / Volume of solvent (mL)

Example: 5 mg of peptide dissolved in 2 mL of bacteriostatic water gives a concentration of 2.5 mg/mL = 2,500 µg/mL.

On an insulin U-100 syringe (1 mL = 100 units), units represent volume in hundredths of a milliliter:

• 1 unit = 0.01 mL
• 10 units = 0.1 mL
• 100 units = 1 mL

For a dose of 250 µg at a concentration of 2,500 µg/mL:

• Volume = 250 µg / 2,500 µg/mL = 0.1 mL = 10 units

16. Regulatory framework for research peptides

16.1 Definition of “for research use only” status

In the context of research peptides, “for research use only” (also “research chemicals,” “research material”) means:

• Not registered as a medicinal product by any regulator (FDA, EMA, ŠÚKL, etc.)
• Not intended for human or veterinary use
• Sold exclusively for laboratory, academic, and scientific applications
• No clinical claims may be made

For a detailed analysis, see the separate article: Research notice: why it matters for peptides.

16.2 Approved peptide drugs vs research peptides

Some peptides exist in parallel under both statuses:

Other peptides are not approved anywhere and are exclusively research:

• BPC-157, TB-500, Retatrutide (Phase 3), Cagrilintide (Phase 3), Ipamorelin, CJC-1295, AOD-9604 (Phase 2b failed), HGH Fragment 176-191, Melanotan II, Epithalon, MOTS-c, Humanin, DSIP, GHK-Cu (cosmetic use yes, but not as a medicine), MK-677 (Phase 3 failed).

16.3 Global regulatory bodies

16.4 WADA Prohibited List

WADA (World Anti-Doping Agency) maintains the list of prohibited substances in professional sport. For professional athletes it is important to know the status of peptides:

Prohibited (S2 — Peptide Hormones, Growth Factors, Related Substances):

• All GLP-1 agonists (Semaglutide, Tirzepatide, Retatrutide, Mazdutide, Liraglutide, Exenatide)
• Cagrilintide (S2)
• GH and GHRH analogs (Sermorelin, CJC-1295, Tesamorelin)
• GHRPs (Ipamorelin, GHRP-2/6, Hexarelin)
• MK-677 (since 2013)
• AOD-9604, HGH Fragment 176-191
• Erythropoietin and analogs
• Insulin (outside approved medical use)

Prohibited (S0 — Non-Approved Substances):

• BPC-157 (since 2022)
• Other non-approved peptides

Not explicitly prohibited as of 2026:

• DSIP, Selank, Semax, Epithalon (may be covered by S0 under broader interpretation)
• NAD+ and precursors (NMN, NR)
• MOTS-c, Humanin

For professional athletes, consultation with an anti-doping authority before using any peptide is recommended.

17. Safety aspects and limitations

17.1 Immunogenicity

Short peptides (up to 10 amino acids) are practically non-immunogenic. Longer peptides (20 to 50 amino acids) can elicit formation of anti-drug antibodies (ADA):

• Semaglutide: ADA in ~0.5 % of patients, without effect on efficacy
• Tirzepatide: ADA in 5 to 9 %, without effect on efficacy
• Liraglutide: ADA in ~5 %, without effect on efficacy

ADA with peptides are rarely clinically significant. With protein medicines (recombinant insulin, EPO, monoclonal antibodies) ADA is a more frequent problem.

17.2 Main categories of side effects

Side effects of peptides are strongly dependent on the target receptor:

17.3 Contraindications and limitations

Universal contraindications for most peptides in a research context:

• Pregnancy and lactation (most peptides have not been tested)
• Active oncological conditions (especially for GHS, due to IGF-1 elevation)
• Severe heart/liver/kidney disease (requires individual assessment)
• Known allergy to the specific peptide or excipient

Specific contraindications:

• GLP-1 agonists: MEN-2 syndrome, medullary thyroid carcinoma, gastroparesis
• Melanotan I/II: Multiple dysplastic nevi, history of melanoma
• Thymosin α1: Active autoimmune diseases, transplantation

17.4 Limitations of research data

For most research peptides (especially those without regulatory approval) the following applies:

• Limited long-term safety data in humans
• No robust Phase 3 human clinical trials
• Most data from animal models
• Variability of results across laboratories
• Often a single research group dominates publications (for example Khavinson for Epithalon, Sikiric for BPC-157)

A researcher should weigh the strength of evidence and understand the limitations of the primary literature.

18. Current research directions

18.1 Multi-receptor agonists

The most dynamic area of the 2020s. After the success of Tirzepatide (dual GIP/GLP-1, 2022), the entire space opened for polyagonists:

• Retatrutide (triple GIP/GLP-1/glucagon). Phase 3 ongoing.
• Mazdutide (dual GLP-1/glucagon). Approved in China 2024.
• Survodutide (Boehringer Ingelheim, dual GLP-1/glucagon). Phase 3.
• CagriSema (fixed combination Semaglutide + Cagrilintide). Phase 3 REDEFINE 1: −25.3 %.
• Triretides of future generations (FGF21-GLP-1-glucagon).

18.2 Mitochondrial peptides

After the discovery of MOTS-c in 2015 and Humanin in 2001, a new category of mitochondrial-derived peptides (MDPs) opened:

• MOTS-c, Humanin, SHLP1-6
• SS-31 / Elamipretide (Phase 3 for primary mitochondrial myopathy)
• Research in aging, neurodegeneration, metabolic dysfunction

18.3 Peptide vaccines

Personalized peptide vaccines for oncology are in Phase 1/2 trials. The best-known example is mRNA-4157 (Moderna), although it is not strictly a peptide vaccine (it encodes peptides).

18.4 Antimicrobial peptides (AMPs)

In an era of rising antibiotic resistance, AMPs are attractive. Defensins, LL-37, and synthetic analogs are being tested in Phase 1/2 for resistant infections.

18.5 Peptides in neurodegeneration

Semaglutide in the EVOKE Phase 3 trial for Alzheimer’s disease (results 2025/2026) may open an entirely new indication for GLP-1 agonists.

18.6 Geroprotection

The Khavinson school, Sinclair’s work on NAD+/sirtuins, MOTS-c, Humanin. Anti-aging peptides are becoming a legitimate research field.

19. Frequently asked questions

Question 1: What is the difference between a peptide and a protein?

Answer: The main difference is chain size. Peptides typically have 2 to 50 amino acids, proteins more than 50. The boundary is conventional. Insulin (51 amino acids) is called in various contexts either a peptide hormone or a small protein. Functionally, peptides are usually signaling molecules (hormones, neurotransmitters); proteins are usually structural, enzymatic, or transport molecules.

Question 2: Are research peptides approved as medicines?

Answer: Some yes, most no. Approved peptide medicines include Insulin, Semaglutide (Ozempic, Wegovy), Tirzepatide (Mounjaro, Zepbound), Liraglutide, Bremelanotide (Vyleesi), Thymosin α1 (Zadaxin in 35+ countries), and dozens of others. Many research peptides (BPC-157, TB-500, MOTS-c, Epithalon, Retatrutide, Cagrilintide, and others) have no regulatory approval and are sold exclusively for research purposes. For details see Research notice for peptides.

Question 3: What is the difference between a native and a synthetic peptide?

Answer: A native peptide is one naturally produced by the body (for example endogenous insulin in the pancreas). A synthetic peptide is manufactured chemically (via SPPS) or recombinantly (in microorganisms). A chemically correctly synthesized peptide is identical to the native one (same amino acid sequence, same structure, same biological activity). Some peptides are modified analogs of native ones (for example Semaglutide is a modified GLP-1 with lipidation for a long half-life). Modified analogs have intentionally different properties from the natives.

Question 4: Why are peptides administered mostly by injection?

Answer: Gastric peptidases and the acidic environment break down most peptides before they can be absorbed. Oral bioavailability is typically below 5 % (often below 1 %). Subcutaneous or intramuscular injection bypasses the GI tract and provides 70 to 90 % bioavailability. There are exceptions:

• Rybelsus (oral Semaglutide with SNAC absorption enhancer, bioavailability ~1 %)
• MK-677 (Ibutamoren, a non-peptide molecule, fully orally bioavailable)
• BPC-157 (partly active even orally due to origin from gastric juice)
• Intranasal administration (Selank, Semax, DSIP) is also a non-invasive alternative

Question 5: What is the significance of HPLC purity ≥ 99 %?

Answer: HPLC purity indicates what % of the isolated peptide is the target molecule vs side products (truncated sequences, oxidized forms, deamidated forms). At 95 % purity the vial has 5× more impurities than at 99 %. For quality research, 99 % is the gold standard. Some impurities may be toxic, immunogenic, or may skew research results. Detailed breakdown: HPLC purity of peptides: what 98 % and above means.

Question 6: What does “for research use only” status mean?

Answer: “For research use only” status (exclusively for research purposes) means that the peptide:

• Is not registered as a medicine by any regulator
• Is not intended for human or veterinary use
• Is sold exclusively to qualified researchers, academic institutions, and laboratories for scientific applications
• No clinical or health claims may be made

In many jurisdictions the sale of research peptides is legal, but with strict restriction of use. Detailed breakdown: Research notice: why it matters for peptides.

Question 7: How is peptide quality verified before research use?

Answer: Standard quality parameters:

1. HPLC purity ≥ 99 % (gold standard)
2. MS identification (confirmation of molecular weight)
3. LAL endotoxin test (< 0.5 EU/mg)
4. Microbial control (USP <61>)
5. CoA documentation for the specific batch
6. Specific parameters per peptide (disulfide integrity, cyclization, lipidation, N-acetylation)

A quality supplier provides a Certificate of Analysis (CoA) with every batch. Detailed guide: CoA for peptides: how to read a certificate of analysis.

Question 8: What is the optimal storage of lyophilized peptides?

Answer:

• Long-term: −20 °C (freezer), 2 to 3 years stability
• Routine: 2 to 8 °C (refrigerator), 12 to 18 months stability
• Short-term: up to 30 days at room temperature (up to 25 °C)
• Always protect from light and humidity

After reconstitution (in bacteriostatic water) the peptide is stable for 28+ days at 2 to 8 °C. Detailed guide: Storage of peptides: stability, temperature, and light.

20. Conclusion and next steps

Peptides represent one of the most rapidly developing categories in modern biomedical and biochemical literature. From the discovery of insulin a century ago, they have become a fundamental tool in endocrinology, neurology, regenerative medicine, oncology, immunology, and cosmetics.

Key points from this guide:

• Peptides are short chains of amino acids (typically 2 to 50), functionally between amino acids and proteins
• Producible via solid-phase synthesis (SPPS) or recombinantly
• Main function: signaling molecules with high specificity toward receptors
• Main functional categories: regenerative, metabolic, GH-stimulating, cosmetic, cognitive, immune, anti-aging
• Globally approved ~80 peptides as medicines, most others are research-only
• Peptide quality is measured via HPLC purity, MS identification, LAL test, and CoA documentation
• Exclusively research use for most non-registered peptides

20.1 Further reading in the Molequa guide series

To deepen specific topics we recommend separate guides:

Pillar and support articles:

• Research peptides: what research-grade quality means
• CoA for peptides: how to read a certificate of analysis
• HPLC purity of peptides: what 98 % and above means
• Lyophilized peptides: significance, advantages, and limits
• Reconstitution of peptides in a laboratory context
• Storage of peptides: stability, temperature, and light
• Research notice: why it matters for peptides

Molecule profiles:

• BPC-157 in research: overview of available data
• TB-500 and Thymosin β-4 in the research context
• Semaglutide in research and clinical practice
• Tirzepatide: a dual incretin agonist
• Ipamorelin + CJC-1295: growth hormone secretagogues

20.2 CTA blocks

For researchers: See the research peptides category → Contact scientific support →

21. Sources and recommended literature

21.1 Reference publications

1. Wang L., Wang N., Zhang W., et al. (2022). Therapeutic peptides: current applications and future directions. Signal Transduct Target Ther. 7(1):48.
2. Muttenthaler M., King G.F., Adams D.J., Alewood P.F. (2021). Trends in peptide drug discovery. Nat Rev Drug Discov. 20(4):309 to 325.
3. Lau J.L., Dunn M.K. (2018). Therapeutic peptides: historical perspectives, current development trends, and future directions. Bioorg Med Chem. 26(10):2700 to 2707.
4. Merrifield R.B. (1963). Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 85(14):2149 to 2154.
5. Anfinsen C.B. (1973). Principles that govern the folding of protein chains. Science. 181(4096):223 to 230.
6. Goldstein A.L., Hannappel E., Sosne G., Kleinman H.K. (2012). Thymosin α1 and β4: regenerative peptides. Expert Opin Biol Ther. 12(1):37 to 51.
7. Sikiric P., Seiwerth S., Rucman R., et al. (2011). Stable gastric pentadecapeptide BPC 157: novel therapy in gastrointestinal tract. Curr Pharm Des. 17(16):1612 to 1632.
8. Knudsen L.B., Lau J. (2019). The discovery and development of liraglutide and semaglutide. Front Endocrinol. 10:155.
9. Frías J.P., Davies M.J., Rosenstock J., et al. (2021). Tirzepatide versus semaglutide once weekly (SURPASS-2). N Engl J Med. 385(6):503 to 515.
10. Khavinson V.K. (2014). Peptide bioregulation as a new direction in gerontology. Adv Gerontol. 27(1):10 to 17.
11. Lee C., Zeng J., Drew B.G., et al. (2015). MOTS-c promotes metabolic homeostasis. Cell Metab. 21(3):443 to 454.
12. Imai S., Guarente L. (2014). NAD+ and sirtuins in aging and disease. Trends Cell Biol. 24(8):464 to 471.

21.2 Regulatory and qualitative documents

• United States Pharmacopeia (USP): General Chapters <71> Sterility, <85> Endotoxins, <61> Microbial contamination, <797> Sterile Compounding
• European Pharmacopoeia (Ph. Eur.): Monograph 0169 (Water for injections), Monograph 0008 (Parenteral preparations)
• ICH Q3C: Residual solvents
• FDA Drug Approval Database: www.fda.gov/drugs/drug-approvals-and-databases
• EMA EPAR Database: www.ema.europa.eu
• IUPAC-IUB Joint Commission on Biochemical Nomenclature: Nomenclature and symbology for amino acids and peptides
• WADA Prohibited List 2026: www.wada-ama.org

21.3 Online databases

• PubMed: pubmed.ncbi.nlm.nih.gov (primary scientific literature)
• ClinicalTrials.gov: clinicaltrials.gov (clinical trials)
• UniProt: www.uniprot.org (peptide and protein sequences)
• Protein Data Bank (PDB): www.rcsb.org (3D structures)

Legal notice

This guide is informational material intended exclusively for research and educational purposes. It does not constitute medical recommendations, does not constitute clinical advice, and does not serve as instructions for human or veterinary use. All peptides mentioned in this article that are not approved by the relevant regulators (FDA, EMA, ŠÚKL, etc.) as medicinal products are exclusively research chemicals intended for laboratory and scientific applications. Before any work with peptides, study the primary scientific literature, the technical datasheet of the specific product, and the Certificate of Analysis. Comply with applicable legislation in your jurisdiction. Molequa accepts no responsibility for improper use of information outside the research context.

Author: Molequa Research Team Last updated: May 2026 Target article length: ~8,500 words Schema markup: Article + FAQPage + BreadcrumbList Competitive keywords: peptides, research peptides, what are peptides, peptide structure, peptide quality, lyophilized peptide, HPLC purity, peptide CoA