Why are there three prolines?

One sits within tuftsin itself; the other two come from the Pro-Gly-Pro extension added for protease resistance. Proline constrains the backbone in a way most peptidases cannot accommodate.

Does Selank oxidise like other peptides?

It has no methionine and no cysteine, so the two most common chemical degradation routes do not apply.

Can Selank be quantified by UV absorbance at 280 nm?

Not reliably. It contains no aromatic residues, so it lacks the absorbance that method depends on.

Selank (Tuftsin Analogue)

Selank Structure, Sequence and Physical Properties

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-232 cited sources

Selank has the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro: seven residues, molecular weight 751.9 Da, formula C33H57N11O9. It contains three prolines, carries a strong positive charge from lysine and arginine, and has neither cysteine nor methionine — the two residues that usually dictate peptide handling.

Key facts

Sequence: H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH
Residues: 7
Molecular weight: 751.9 Da
Molecular formula: C33H57N11O9
Prolines: Three (positions 3, 5 and 7)
Basic residues: Lys2, Arg4
Cysteine: None
Methionine: None

Reading the sequence

In single-letter code the peptide is TKPRPGP: threonine, lysine, proline, arginine, proline, glycine, proline. At 751.9 Da it is small even by research-peptide standards — under a tenth the mass of semaglutide, and lighter than Semax at 813.9 Da despite both being heptapeptides, because Selank has no sulfur-containing residue and a different composition.

Three prolines in seven residues

That is an extremely high proline density. Proline's side chain closes back onto its own backbone nitrogen, removing the amide hydrogen and locking rotation, which imposes strong local conformational rigidity and interrupts regular secondary structure. With three of them, Selank has very little conformational freedom — and correspondingly little for a peptidase to grip.

Research material referenced

Selank 10mg — third-party HPLC tested

View — £24.99

The basic core

Lysine at position 2 and arginine at position 4 give the molecule a strong positive charge at physiological pH. Combined with threonine's hydroxyl and the absence of any large hydrophobic residue, this makes Selank markedly hydrophilic — it dissolves readily in aqueous buffer and has little tendency toward the hydrophobic association that complicates amphipathic peptides.

What is absent matters as much as what is present

Most research peptides carry one of two chemical liabilities: cysteine, which forms and scrambles disulfide bonds, or methionine, whose thioether sulfur oxidises to the sulfoxide and adds 16 Da. Selank has neither. It also has no asparagine or glutamine, removing deamidation as a route. That leaves straightforward hydrolysis as the principal degradation pathway, and the proline content resists even that.

No aromatic residues, and what that costs

There is no tryptophan, tyrosine or phenylalanine in the sequence. That means Selank has very little absorbance near 280 nm, so the standard spectrophotometric method for quantifying peptides does not work well on it. Quantification has to rely on other approaches, which is a practical difference from peptides such as MOTS-c that carry several aromatic residues.

Why it is chemically robust

Small, hydrophilic, heavily proline-constrained, with no oxidation-prone or disulfide-forming residues. Selank is among the more forgiving research peptides to handle — which does not make storage conditions optional, but does mean the specific failure modes that dominate for other compounds largely do not apply.

Quick reference

Property	Value
Sequence	TKPRPGP
Length	7 residues
Molecular weight	751.9 Da
Formula	C33H57N11O9
CAS	129954-34-3
UNII	TS9JR8EP1G
PubChem CID	11765600
Development code	TP-7

Extended research context

The Selank (Tuftsin Analogue) deep dive

Deep dive: preserving a signal rather than supplying one

Selank's most specifically characterised activity is not receptor binding. Work published in the Bulletin of Experimental Biology and Medicine in 2001 and 2002 reported that it inhibits enkephalin-degrading enzymes, measured directly on plasma enkephalinase activity, thereby extending the half-life of endogenous enkephalins rather than acting at opioid receptors itself. That distinction is routinely muddled and it matters: an agonist activates the receptor wherever the drug reaches, at whatever concentration is given, while a degradation inhibitor only lets the ligand the body is already releasing persist longer. The effect is bounded by endogenous release and occurs only where that release happens. The same logic underlies DPP-4 inhibitors in the incretin field, which prolong native GLP-1 rather than supplying an analogue - and instructively, that class produces much smaller effects than the receptor agonists do.

Deep dive: what the sequence does and does not contain

TKPRPGP is unusual among research peptides for what is absent from it. There is no cysteine, so no disulfide bonds form or scramble and no reducing agent is needed. There is no methionine, so the thioether oxidation that adds 16 Da and dominates handling guidance for MOTS-c and Semax does not apply. There is no asparagine or glutamine either, ruling out deamidation. What remains is straightforward hydrolysis, and three prolines in seven residues resist even enzymatic cleavage well, because proline locks the backbone rotation peptidases require. One practical cost of that composition: no aromatic residues means almost no absorbance at 280 nm, so the standard spectrophotometric quantification method does not work on it.

Deep dive: reading a two-literature evidence base

PubMed indexes roughly 135 Selank records against about 690 for its parent peptide tuftsin - an inversion worth noticing, since most designed analogues eventually outgrow the parent they replaced. Most of the tuftsin literature is immunological and predates Selank entirely, so it is not evidence about Selank. Within Selank's own record the split is the same one Semax shows: mechanistic work in internationally indexed journals, assessable directly; clinical work concentrated in Russian-language publications, indexed by translated title and often without accessible English full text. That is evidence which is hard to verify independently, which is not the same as evidence that is absent, and not the same as evidence that is established.

Research applications

▸Enkephalin and enkephalinase pathway research
▸GABAergic receptor expression studies in rodent models
▸BDNF expression research, including intranasal administration routes
▸Comparative work on proline-stabilised peptide design
▸Tuftsin and immunopeptide structure-activity research

Handling checklist

✓Store lyophilised material cold, dry and protected from light
✓No reducing agent needed — the sequence contains no cysteine
✓No methionine oxidation to expect; a +16 Da satellite warrants explanation
✓Do not rely on 280 nm absorbance — there are no aromatic residues
✓Introduce diluent gently against the vial wall; swirl rather than shake
✓Aliquot to avoid repeated freeze-thaw cycles

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Describing Selank as an opioid or opioid agonist

Fix: Reported work describes inhibition of enkephalin-degrading enzymes, not receptor binding. It preserves endogenous enkephalins rather than supplying an agonist.

✗ Treating the benzodiazepine contrast as a safety claim

Fix: The mechanistic difference is real — expression-level rather than direct allosteric modulation — but it supports no comparative claim about safety or dependence.

✗ Citing tuftsin literature as evidence about Selank

Fix: Tuftsin has roughly five times more papers, most of them immunological and predating Selank. They are different compounds.

✗ Reading a BDNF expression change as a demonstrated outcome

Fix: The work measured expression in rat hippocampus. Expression is upstream of function and upstream again of any clinical claim.

✗ Treating Russian registration as equivalent to MHRA approval

Fix: Authorisations are jurisdictional and do not transfer. Selank has never been assessed by the MHRA, EMA or FDA.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Why are there three prolines?: One sits within tuftsin itself; the other two come from the Pro-Gly-Pro extension added for protease resistance. Proline constrains the backbone in a way most peptidases cannot accommodate.
Does Selank oxidise like other peptides?: It has no methionine and no cysteine, so the two most common chemical degradation routes do not apply.
Can Selank be quantified by UV absorbance at 280 nm?: Not reliably. It contains no aromatic residues, so it lacks the absorbance that method depends on.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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