Selank: seven residues, three prolines

Selank is a heptapeptide with one striking feature: three of its seven residues are proline. That shapes the chain and makes synthesis more demanding than seven residues would suggest.

Thr-Lys-Pro-Arg-Pro-Gly-Pro

The sequence is Thr-Lys-Pro-Arg-Pro-Gly-Pro. Positions three, five and seven are proline; the molecular formula is C₃₃H₅₇N₁₁O₉, the molar mass 751.90 g/mol and the CAS number 129954-34-3. All of it is on the product page.

Proline is the only one of the twenty standard amino acids with a ring that incorporates the backbone. Where proline sits, the chain cannot rotate freely. Three of them in seven residues means the shape is largely fixed, and in solid-phase synthesis couplings at a rigid point run less smoothly than at a flexible one.

Four residues from the original, three added

The first four residues are tuftsin, an endogenous tetrapeptide of the immune system. The three added residues, Pro-Gly-Pro, are an addition that makes the chain more resistant to breakdown: the same design principle as Semax, which also has seven residues and also ends in Pro-Gly-Pro.

The parent sequences of the two, however, have nothing to do with each other. Where the first four residues of Semax come from is set out in Semax explained.

Five certificates, all with a figure

The certificate library holds five certificates of analysis for this substance, all from 10 mg lots, the same fill size that is sold here. Four report a measured purity above 99.8% (one of them 99.848%), and one, lot 2024219, reports 98.82%.

This shop does not state a blanket minimum purity; what counts is the measured value in the certificate for each lot. Where a certificate carries no figure, that is a feature of that document, not a gap in the library; nothing is added after the fact. What to look for is explained in How to read a certificate of analysis.

What makes short chains easy and hard

Six coupling steps are few, and the high values in the certificates follow from that, not from any merit on the supplier's part. The three prolines pull in the other direction, though, because coupling at them runs more slowly than at ordinary residues.

At 751.90 g/mol, mass spectrometry has an easy job: the expected mass is unambiguous and any deviation stands out. Purity should not be confused with peptide content, which is lower because of counterions and residual moisture, and with a molecule this light that difference weighs more heavily than in a long chain.

Before preparing a solution

The product is supplied as a lyophilised powder; what lyophilisation does to the material is explained in Storing lyophilised peptides in the laboratory. It goes into solution with bacteriostatic water, and the glossary entries on reconstitution and concentration explain the terms.

In brief

Seven residues, three of them proline, 751.90 g/mol, CAS 129954-34-3, five certificates of analysis from the matching vial size.

Questions

What is Selank made of?

Seven amino acids: Thr-Lys-Pro-Arg-Pro-Gly-Pro. The first four correspond to the endogenous tetrapeptide tuftsin; the three added residues are a stabilising addition.

Why are three prolines notable?

Proline is the only standard amino acid with a ring in the backbone. At these points the chain cannot rotate freely, and coupling during synthesis runs more slowly.

How many certificates of analysis are there?

Five, all from 10 mg lots. Four report above 99.8%, one reports 98.82%; all can be viewed in the library without an account.