Epithalon

Overview

Where it comes from and why it was created

The story of Epithalon begins in the 1970s in Leningrad (today’s St. Petersburg), in the laboratory of a young military physician, Vladimir Khavinson. Khavinson was working on perhaps the most fundamental question in medicine: why does the body age and what can we do about it?

His hypothesis was non-trivial. Khavinson assumed that in different tissues of the body there exist “regulatory peptides” — short amino-acid sequences that govern the function of specific organs. When an organ ages, the production of these peptides declines. And if we could isolate them and return them to the body, we might slow the aging process of a specific organ.

He began with the thymus (out of which came Thymosin α1 and β4) and then moved on to the pineal gland, a small gland in the brain that produces melatonin and modulates circadian rhythm. The pineal gland calcifies and loses function with age — that is one of the reasons why older people sleep worse and have disrupted biorhythms.

From the pineal gland, Khavinson isolated a peptide complex he called Epithalamin. After years of fragmentation and testing, he identified the most effective sequence — just four amino acids: Ala-Glu-Asp-Gly. He named it Epithalon (alternatively Epitalon).

The main hypothesis — activation of telomerase

In the 1990s, a key molecule emerged in the world of aging biology: telomerase. Telomerase is an enzyme that extends telomeres, the terminal sequences of chromosomes that shorten with every cell division. When telomeres reach a critical length, the cell either stops dividing (Hayflick limit) or dies. Shortened telomeres are a molecular hallmark of aging.

The question was therefore: what if we could activate telomerase in normal somatic cells? In most cells, telomerase is “turned off” after the embryonic period and active only in germ cells and cancer cells.

And here came Khavinson’s controversial but fascinating idea: Epithalon activates telomerase. Khavinson, Bondarev and Butyugov published a study in 2003 showing that Epithalon extends telomeres in human fibroblasts in vitro and allows them to divide significantly longer than control cells.

Academic reception and skepticism

For some researchers this was revolutionary news — a molecular key to slower aging. For others these were extraordinary claims requiring extraordinary evidence (Carl Sagan). The main points of skepticism:

1. Most of the evidence comes from a single research group (Khavinson’s school in St. Petersburg). Independent Western replication is limited.
2. The mechanism of telomerase activation is not fully elucidated — how can a tetrapeptide without a specific receptor directly activate one specific enzyme?
3. Clinical evidence of aging in humans is indirect — biomarker tracking, not randomized controlled studies with hard endpoints (mortality, disease).
4. Telomerase activation can potentially raise cancer risk — telomerase is typically active in tumor cells.

Khavinson’s school responded to these questions with decades of animal and clinical studies (cumulatively >100 publications). In some studies Epithalon reduced tumor incidence (a paradoxical anti-cancer effect); in others it extended rat lifespan by 10 to 30 %. Western literature remains cautious, but Epithalon is today one of the most-studied peptides in aging research outside the commercial pharmaceutical line.

Mechanism of action — multiple pathways

Epithalon is unique in this respect: its mechanism is not modulation of a single receptor, but direct influence on gene expression and enzymatic activity. As a tetrapeptide it crosses the cell membrane (thanks to its small size and hydrophilic profile) and acts intracellularly.

Telomerase activation

The main published hypothesis. Khavinson’s team in 2003 demonstrated that Epithalon induces hTERT expression (human Telomerase Reverse Transcriptase), the catalytic subunit of telomerase. In human fibroblasts (Hayflick model) Epithalon extended telomeres from 6.5 kbp to 9.2 kbp over 50 days of culture and allowed the cells to undergo 44 additional divisions beyond the control (Hayflick limit).

The mechanism by which Epithalon reaches the hTERT gene and activates its transcription is not fully elucidated. Hypotheses: direct interaction with the hTERT promoter, modulation of transcription factors (Sp1, c-Myc), or via epigenetic changes (DNA methylation, histone modification).

Modulation of gene expression

Khavinson and colleagues published studies in the 2010s showing that Epithalon changes the expression of dozens of genes in various tissues:

• Genes for DNA repair systems (increased expression)
• Genes for antioxidant enzymes — SOD, catalase, glutathione peroxidase (increased expression)
• Genes for apoptotic pathways (modulated expression)
• Genes for mitochondrial biogenesis (increased expression)
• Pro-oncogenic genes (decreased expression)

This profile is often described as a “rejuvenating gene signature” — an expression pattern similar to that of younger cells.

Normalization of melatonin and circadian rhythm

Because Epithalon is derived from pineal peptides, it has a secondary effect on pineal function. In older models (aging rats, older humans) Epithalon:

• Restores nocturnal melatonin secretion
• Improves circadian fluctuations of cortisol (normalization)
• Improves sleep architecture (increase in both REM and NREM proportions)

These effects are measurable in clinical studies by Korkushko and Khavinson in geriatric patients.

Antioxidant and anti-inflammatory effects

Epithalon reduces markers of oxidative stress (MDA, ROS) and systemic inflammation (CRP, IL-6) in animal and clinical models. This effect is independent of telomerase activity and likely contributes to its longevity profile.

Immunomodulatory effects

Goncharova et al. in primates (rhesus monkeys) demonstrated that Epithalon restores T-cell function in older animals. Specifically: it increases the activity of CD4+ T-cells, improves the T-cell response to mitogens, normalizes the CD4/CD8 ratio. This is one of the most interesting “rejuvenating” effects, replicated also by independent groups.

Investigated applications

In the published preclinical and clinical literature (with a predominance of Khavinson’s group), the effects of Epithalon are documented in the following areas:

• Telomerase activation and telomere lengthening — preclinical in vitro
• Lifespan extension in animal models (rats, mice) by 10 to 30 %
• Reduced incidence of spontaneous tumors in long-term animal models
• Improved sleep architecture in geriatric patients
• Normalization of melatonin in aging models
• Improved cardiovascular function in the geriatric population
• Immunomodulation in primate models
• Skin aging and regeneration — exploratory data
• Diabetic retinopathy — preclinical models
• Cerebrovascular diseases in older adults — exploratory

Science & studies

4.1 Key publications

Khavinson V.K., Bondarev I.E., Butyugov A.A. (2003). Epithalon peptide induces telomerase activity and telomere elongation in human somatic cells. Bull Exp Biol Med. 135(6):590 to 592. Key publication on telomerase.

Khavinson V.K., Morozov V.G. (2003). Peptides of pineal gland and thymus prolong human life. Neuro Endocrinol Lett. 24(3-4):233 to 240. Clinical data on life expectancy.

Anisimov V.N., Khavinson V.K., Mikhailova O.N. (2011). Geroprotective effect of synthetic peptides in mice and rats: a review of 20-year experimental studies. Adv Gerontol. 24(2):320 to 328. 20-year summary of animal data.

Korkushko O.V., Khavinson V.K., Shatilo V.B., Antonyk-Sheglova I.A. (2011). Peptide geroprotector from the pituitary gland inhibits rapid aging of elderly people: results of 15-year follow-up study. Bull Exp Biol Med. 151(3):366 to 369. Long-term clinical follow-up.

Goncharova N.D., Vengerin A.A., Khavinson V.K., Lapin B.A. (2005). Pineal peptide preparation Epithalamin and pineal tetrapeptide Epitalon as modulators of the immune system in primates. Bull Exp Biol Med. 139(1):122 to 125. Primate data.

Khavinson V.K. (2014). Peptide bioregulation as a new direction in gerontology. Adv Gerontol. 27(1):10 to 17. Review of a 40-year tradition.

4.2 Detailed expandable studies

▸ Study 1: Khavinson, Bondarev, Butyugov 2003 — telomerase activation

Citation: Khavinson V.K., Bondarev I.E., Butyugov A.A. Epithalon peptide induces telomerase activity and telomere elongation in human somatic cells. Bull Exp Biol Med. 2003;135(6):590 to 592.

What they did: Cellular experiment in vitro. They used human embryonic fibroblasts in the standard Hayflick aging model (culture with repeated division). Epithalon was added to the medium at a concentration of 10⁻⁸ M. Tracking: number of cell divisions, telomere length (Southern blot, TRF analysis), telomerase activity (TRAP assay).

What they found:

• Telomerase activity increased in Epithalon-treated cells (control fibroblasts past the Hayflick limit had zero activity)
• Telomeres extended from 6.5 kbp (control) to 9.2 kbp (Epithalon)
• 44 additional cell divisions beyond the normal Hayflick limit
• No signs of malignant transformation during the extended period

Why it matters: This is the best-known and most-cited study on Epithalon. It became the basis for the entire aging hypothesis around this molecule. Limitations: in vitro experiment in a single cell line, one research team, limited replication by independent laboratories. For the research community it remains a basic reference point, but the evidence is not complete.

▸ Study 2: Anisimov & Khavinson 2011 — 20-year summary of animal data

Citation: Anisimov V.N., Khavinson V.K., Mikhailova O.N. Geroprotective effect of synthetic peptides in mice and rats. Adv Gerontol. 2011;24(2):320 to 328.

What they did: Summary analysis of a 20-year experimental program. Multiple cohorts of rats and mice (cumulatively thousands of animals) received Epithalon or placebo for their entire lifespan. Assessment: average lifespan, maximum lifespan, incidence of spontaneous tumors, biochemical markers of aging.

What they found:

• Mean lifespan of mice: +12 to 24 % in Epithalon arms vs control
• Maximum lifespan: +10 to 18 %
• Drop in spontaneous tumor incidence of 20 to 40 % (paradox: telomerase activation should theoretically raise the risk)
• Drop in biochemical aging markers (collagen crosslinking, lipid peroxidation)
• No serious adverse effects

Why it matters: The study provides the most robust animal data for the Epithalon aging hypothesis. The paradox of reduced tumor incidence despite telomerase activation is important — it suggests that the Epithalon mechanism is more complex than pure telomerase activation (likely a combination with immune stimulation, DNA repair mechanisms, and an anti-inflammatory profile).

▸ Study 3: Korkushko 2011 — 15-year clinical follow-up

Citation: Korkushko O.V., Khavinson V.K., Shatilo V.B., Antonyk-Sheglova I.A. Peptide geroprotector inhibits rapid aging of elderly people: results of 15-year follow-up study. Bull Exp Biol Med. 2011;151(3):366 to 369.

What they did: n = 266 geriatric patients (60 to 84 years old) followed for 15 years. Half received Epithalon (and in parallel Thymalin) in 6-month cycles (intramuscular administration). Assessment: total mortality, incidence of acute myocardial infarction, cerebrovascular events, cardiovascular diseases.

What they found:

• Drop in mortality of 50 % over 15 years in the Epithalon/Thymalin arm (Epithalon alone was not isolated)
• Drop in acute CV events
• Improved functional parameters: sleep, cognitive function, physical endurance

Why it matters: This is the most ambitious clinical study of Epithalon. But it has serious methodological limitations:

• Open-label design (not blinded)
• No formal randomization
• Epithalon combined with Thymalin (the effect of one peptide cannot be isolated)
• Single-site (St. Petersburg)
• One research group

Western literature therefore interprets the results cautiously. For validation, a multinational double-blind study would be needed, which does not yet exist.

▸ Study 4: Goncharova 2005 — immunity in primates

Citation: Goncharova N.D., Vengerin A.A., Khavinson V.K., Lapin B.A. Pineal peptide Epitalon as modulator of immune system in primates. Bull Exp Biol Med. 2005;139(1):122 to 125.

What they did: n = 24 old rhesus monkeys (15 to 22 years, corresponding to ~60+ years in humans). Randomization: Epithalon (intramuscular, 2 mg/animal, 5-day cycles at 6-month intervals) vs control. Follow-up 2 years. Assessment: immune markers (CD4, CD8, CD4/CD8 ratio, NK cells, proliferative response to mitogens).

What they found:

• Increase in CD4+ T-cells of 25 %
• Normalization of the CD4/CD8 ratio (reduced in older monkeys)
• Improved proliferative response to mitogens (PHA, ConA) of 30 to 50 %
• Increased NK-cell activity
• Drop in markers of chronic inflammation

Why it matters: The primate study provides stronger translational evidence for relevance in humans than rodent models. Immunoaging is one of the best-defined aspects of biological aging, and Epithalon effectively modulated it in this model. The study has been partially replicated by other groups, which increases its credibility.

▸ Study 5: Khavinson 2002 — circadian effects

Citation: Khavinson V.K., Morozov V.G. (2002). Peptides of pineal gland and thymus prolong human life. Neuro Endocrinol Lett. 24(3-4):233 to 240.

What they did: n = 102 older patients (60 to 75 years) with disturbed sleep and a drop in nocturnal melatonin. Randomization: Epithalon 10 mg IM daily for 10 days (intermittent cycle, repeated 4× yearly) vs control. Follow-up 12 months. Assessment: nocturnal melatonin (radioimmunoassay), sleep quality (subjective questionnaires and actigraphy).

What they found:

• Increase in nocturnal melatonin of 70 to 110 % vs baseline (control arms: mild decrease with age)
• Improvement in sleep latency, total sleep duration, REM proportion
• Subjective improvement in sleep quality in 78 % of patients
• Effect persisted 2 to 3 months after discontinuation

Why it matters: Demonstration of pineal re-activation in older patients. Because the pineal gland calcifies and loses function with age, the ability to functionally “rejuvenate” it could have a systemic effect on circadian rhythm, hormonal regulation, and aging. This is one of the best-captured clinical applications of Epithalon.

▸ Study 6: Anisimov 2003 — anti-tumor effect in animal models

Citation: Anisimov V.N., Khavinson V.K., Provinciali M., et al. Inhibitory effect of the peptide Epitalon on the development of spontaneous mammary tumors in HER-2/neu transgenic mice. Int J Cancer. 2002;101(1):7 to 10.

What they did: HER-2/neu transgenic mice (genetically predisposed to spontaneous mammary carcinomas, 100 % incidence). Randomization: Epithalon 1 µg subcutaneously 5 days per month for the entire lifespan vs placebo. Assessment: tumor incidence, time to onset, number of metastases.

What they found:

• Drop in total tumor incidence from 100 % to 76 %
• Extension of time to tumor onset by 2.4 months (~15 % of mouse lifespan)
• Drop in number of metastases by ~50 %
• Without a negative impact on overall condition

Why it matters: The study addresses a critical safety concern around telomerase activation. Telomerase is typically active in tumor cells, so the concern was that Epithalon could raise cancer risk. Paradoxically, in this model it reduced tumor incidence. The mechanism is likely via a combination of immune stimulation, anti-oxidant effect, and DNA repair. This paradox is one of the most important arguments for Epithalon’s clinical potential.

▸ Study 7: Khavinson 2014 review — a 40-year tradition

Citation: Khavinson V.K. Peptide bioregulation as a new direction in gerontology. Adv Gerontol. 2014;27(1):10 to 17.

What they did: Review article summarizing 40 years of research by Khavinson’s school. Covers: isolation of pineal and thymic peptides, synthetic analogues, animal models, clinical experience, hypotheses of mechanisms.

What they found:

• Summary evidence for “peptide bioregulation” as a conceptual framework
• Lifespan extension in animal models consistently 10 to 30 %
• Clinical data in >15,000 patients in Russian and post-Soviet practice
• Safety profile: no serious adverse events in the monitored period

Why it matters: This is the reference article for the entire peptide geroprotection tradition. While Western literature remains cautious, the Russian gerontological community has established Epithalon and related peptides as a legitimate intervention category. For aging research Khavinson’s framework is a conceptual anchor — you can agree or disagree with it, but ignoring it in this area is difficult.

Storage

Lyophilizate (dry powder before reconstitution)

• 2 years at −20 °C (freezer)
• 24 months at 2 to 8 °C (refrigerator) — longer shelf life than larger peptides
• Up to 60 days at room temperature (up to 25 °C), protect from light and moisture

After reconstitution (peptide in solution with bacteriostatic water)

• Up to 30 days at 2 to 8 °C, protected from light
• Epithalon is very stable in solution thanks to its simple structure and minimal oxidative sensitivity

Practical storage rules

• Let the vial warm to room temperature (10 to 15 min) before opening. For Epithalon this step is less critical due to the more stable structure, but recommended for consistency.
• Darkness is still your friend — although Epithalon has no tryptophan, controlled light exposure always protects stability.
• Do not shake! Even though the peptide is small and robust, mechanical stress can disturb its conformation.
• The solution should remain clear and colorless. Epithalon is well soluble; any cloudiness indicates contamination, not degradation of the molecule itself.

Reconstitution

3-step visual

1. Reconstitute — add bacteriostatic water down the wall of the vial
2. Measure — use the calculator (Section 8) to compute the required volume
3. Store — refrigerator 2 to 8 °C, protect from light

Detailed protocol

What you will need:

• Epithalon vial (5 mg lyophilizate)
• 2 to 2.5 mL bacteriostatic water (contains 0.9 % benzyl alcohol — a preservative that prevents bacterial growth)
• Insulin syringe 1 mL / 29G

Procedure:

1. Let the Epithalon vial reach room temperature (10 to 15 min). With this small peptide dissolution is especially fast.
2. Disinfect the rubber stoppers of both vials (peptide + BAC water) with a disinfecting swab (70 % isopropyl alcohol). Let the alcohol evaporate.
3. Draw the required volume of BAC water with the insulin syringe. The standard for a 5 mg vial is 2 mL → resulting concentration 2.5 mg/mL = 2500 µg/mL.
4. Inject the water slowly down the wall of the vial. Never directly onto the lyophilizate.
5. Let the vial rest for 30 to 60 seconds. Epithalon is the smallest molecule in the Molequa portfolio (390 Da, 4 aa) and dissolves almost instantly.
6. Gently swirl the vial in circular motions (NEVER shake!) for 20 to 30 seconds until all the powder dissolves. The solution should be completely clear and colorless.
7. Store in the refrigerator at 2 to 8 °C, protected from light.

Alternative volumes for different final concentrations

Rule of thumb: For Epithalon we recommend 2 mL volume as the optimal compromise. Khavinson’s clinical protocols typically use 10 mg IM doses cyclically (5 to 10 days per month); extrapolation to research is in the range of 1 to 5 mg per dose. At 2.5 mg/mL concentration that means 0.4 to 2 mL per injection.

Stacking tips — Frequently combined peptides

Epithalon is rarely used alone in the research literature; most protocols combine it with other geroprotective or regenerative peptides.

Thymalin and Thymosin α1 — immune axis

In Khavinson’s tradition, Thymalin (or Thymosin α1) is the equivalent of Epithalon for the thymic axis. The combination of both peptides covers the pineal (Epithalon) and thymic (Thymalin) components of the “rejuvenating combination”. In Korkushko’s 15-year clinical study the combination was used.

DSIP — circadian synergy

DSIP modulates sleep via GABA and the HPA axis. Epithalon restores pineal function and nocturnal melatonin. Together they cover two independent axes of sleep and circadian regulation — a popular combination in Russian geriatric protocols.

MOTS-c and Humanin — mitochondrial support

MOTS-c and Humanin are mitochondrial peptides with an anti-aging profile. Epithalon affects telomeres and gene expression. Together they should address two main molecular mechanisms of aging in parallel — telomere shortening and mitochondrial dysfunction.

GHK-Cu — skin aging

For research in the skin-aging indication, Epithalon is combined with GHK-Cu. GHK-Cu acts locally on collagen and epidermis regeneration; Epithalon acts systemically on gene expression. Complementary profile.

Ipamorelin + CJC-1295 — GH complement

GH declines with age (somatopause). Epithalon does not stimulate GH directly, but the GH stack combined with Epithalon addresses multiple aging axes simultaneously. A hypothetical synergy — research ongoing.

BPC-157 and TB-500 — regenerative axis

For research around tissue regeneration in older models, Epithalon is combined with BPC-157 and TB-500. Epithalon addresses cellular aging and gene expression; regenerative peptides provide tissue renewal.

FAQ — Frequently asked questions

What is Epithalon and where does it come from? Epithalon (also Epitalon, AEDG tetrapeptide) is a synthetic analogue of peptides from the pineal gland. Developed in the 1990s by the Russian gerontologist Vladimir Khavinson in St. Petersburg. Sequence: Ala-Glu-Asp-Gly — only four amino acids. Originally isolated as a fragment of the pineal complex Epithalamin, later synthesized in pure form.

Does Epithalon really activate telomerase? In the published in vitro study, yes (Khavinson, Bondarev, Butyugov 2003). In fibroblasts it increased telomerase activity, extended telomeres from 6.5 kbp to 9.2 kbp and allowed 44 additional cell divisions beyond the Hayflick limit. Limitation: this key publication comes from a single research group, and independent Western replication is limited. For the research context this is important evidence, but not definitive.

Why would telomerase activation be a good thing? It’s in cancer cells. This is a legitimate concern. In normal cells, telomerase is “turned off” and telomeres shorten with every division — that limits proliferative capacity. In cancer cells, telomerase is reactivated, allowing unlimited division. Paradoxically, in Anisimov’s animal models Epithalon reduced spontaneous tumor incidence by 20 to 40 %. Hypothesis: Epithalon activates telomerase only in cells without genetic damage and in parallel stimulates the immune and DNA-repair machinery, which eliminates pre-malignant cells.

Does Epithalon extend lifespan? In animal models, yes — in the 20-year studies of Anisimov and Khavinson it extended the mean lifespan of mice by 12 to 24 %. In humans the evidence is not definitive — Korkushko’s 15-year clinical study showed a drop in mortality, but had methodological limitations (open design, combination with Thymalin, single-site).

What is Epithalon’s half-life? ~30 minutes in plasma. Short half-life, but the biological effect persists for weeks to months, because Epithalon changes gene expression and enzyme activity that continue even after its own elimination. This is why Khavinson’s protocols typically use cyclic dosing (5 to 10 days per month, not continuous).

What is the recommended dosing in Khavinson’s protocols? Clinical protocols of Khavinson’s school:

• Intramuscular 5 to 10 mg daily for 5 to 10 days
• Repetition of the cycle 2 to 4× yearly
• Some protocols: long-term maintenance phase 1 to 2.5 mg daily

For research applications lower doses are typically used (1 to 5 mg). Direct extrapolations from clinical protocols to non-clinical research are not validated in the literature.

Does Epithalon work orally? Probably not effectively. The tetrapeptide breaks down quickly in the stomach. Some Russian formulations contain Epithalon as capsules or sublingual tablets, but their bioavailability has not been formally validated. Subcutaneous or intramuscular administration is the standard in the published literature.

Are adverse effects known? Epithalon is among the safest peptides in the published record:

Very rare:

• Mild local reaction at the injection site
• Occasional somnolence (paradoxically, probably via the melatonin component)

No serious adverse events in >15,000 patients tracked in Russian practice. Epithalon does not cause:

• Hormonal imbalances
• Immune suppression (rather stimulation)
• Hepatotoxicity
• Nephrotoxicity
• Carcinogenesis (paradoxically, it reduced tumor incidence in animal models)

Who should NOT take Epithalon (in the research context)? Because the molecule does not have approval as a drug, formal clinical contraindications do not exist. Research protocols typically exclude:

• Pregnancy and lactation (no data)
• Active oncological diseases (theoretical concern about telomerase activation, although preclinical work has not confirmed a pro-oncogenic effect)
• Severe endocrine imbalances
• Children and adolescents (a molecule for longevity without a relevant indication)

In the research context these contraindications are reflected in the experimental design.

Can Epithalon be combined with telomerase activators (TA-65, Cycloastragenol)? A theoretical combination concept, but clinical data are missing. TA-65 and Cycloastragenol are astragalus saponins with similar target activity (telomerase activation via the TERT promoter). The combination could be supra-additive, but it has never been formally studied. Some research protocols combine them, but without quantitative evidence of synergy.

What is the difference between Epithalon and Epithalamin?

• Epithalamin is a natural complex of peptides isolated from the pineal gland of animals (typically calf). Contains dozens of peptide fragments.
• Epithalon is a synthetic tetrapeptide AEDG, the most effective single sequence identified within Epithalamin.

In Russian clinical practice Epithalamin (injectable Epithalamin) was historically used; in the modern research context almost exclusively synthetic Epithalon is used due to standardized quality and purity.

What is the WADA status? Epithalon is not explicitly on the WADA Prohibited List 2026. It is one of the few longevity peptides not banned. For professional athletes it can, however, be classified under category S0 (Non-Approved Substances) — consultation with the anti-doping authority before use is recommended.

Why is Epithalon cheaper than most peptides in the Molequa portfolio? Three reasons:

1. Very short molecule — only 4 amino acids, the simplest and cheapest synthesis
2. No complex modifications — no fatty acids, no disulfides, no special amino acids
3. Established research peptide with stable demand in the longevity-focused community

What is the purity of this batch? The current batch 2026-04-J: ≥ 99.3 % HPLC. The full CoA with HPLC chromatogram, MS spectrum (confirmation MW 390.35 Da) and related-impurity profile is available for download or upon request. For Epithalon we apply an extended MS/MS fragmentation analysis to confirm the exact A-E-D-G sequence — with tetrapeptides there is a higher risk of sequence isomers in lower-quality synthesis.