Selank

What Selank is, structurally

Selank is a heptapeptide with a free N-terminal amine and a free C-terminal acid.

The formula reconciles with the sequence exactly. Eleven nitrogens comes from seven backbone amides, one for the lysine side chain and three for the arginine guanidinium, with nothing unaccounted for.

The dominant feature is proline content. Three of seven residues are proline, at positions three, five and seven, so nearly half the chain has a side chain looping back onto its own backbone nitrogen. Each removes an amide hydrogen and restricts rotation, making this a constrained molecule rather than a flexible chain, and the C-terminal Pro-Gly-Pro is particularly stiff.

Proline has a second consequence that matters more for analysis than structure. The bond preceding a proline interconverts between cis and trans far more readily than an ordinary peptide bond, and slowly, over seconds to minutes. A molecule with three prolines therefore exists as a mixture of slowly interconverting conformers rather than one population, with an easily misread effect on chromatography covered below.

Charge is strongly cationic: lysine and arginine side chains plus the free N-terminal amine against a single C-terminal acid, with no acidic side chain anywhere. The peptide is highly polar and water soluble, though no quantitative figure is quoted here.

There is no cysteine, no methionine and no aromatic residue in the sequence, so the formula contains no sulfur and the molecule has no 280 nm chromophore.

Reconstitution and handling

Sterile water is the default solvent, and with no metal centre present PBS or an assay buffer are equally suitable. Add diluent down the vial wall, swirl gently, and let it clarify. Vortexing serves no purpose here.

No reducing agent is required, since the formula contains no sulfur for one to act on.

Adsorption is worth allowing for at low concentration, and low-protein-binding tubes reduce measured loss. Three positive charges and no hydrophobic surface make negatively charged surfaces such as untreated glass the likelier route rather than polypropylene.

This is laboratory preparation chemistry, not dosing, administration, or protocol guidance of any kind.

Storage and stability

Store the lyophilized powder at -20 degrees C, sealed, desiccated and protected from light, and let a cold vial warm before opening. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thawing repeatedly.

This is a chemically robust sequence, for compositional reasons. No cysteine to oxidise, no methionine to form a sulfoxide, no tryptophan to photo-oxidise, no asparagine or glutamine to deamidate, no aspartate to isomerise. Every route that dominates peptide degradation requires a residue this molecule lacks.

The prolines add further resistance. Proline blocks most exopeptidase activity at the bond preceding it, and three across seven residues leave little unprotected backbone.

What remains is ordinary amide hydrolysis under strong acid or alkali, slow near neutral pH. Cold, dry, near-neutral storage suffices, and there is no single vulnerable residue to protect.

These are storage conditions for the material, not dosing or administration guidance.

How Selank is tested

Reversed-phase HPLC establishes chromatographic purity as area percent. With no aromatic residue there is no 280 nm absorbance, so detection sits near 214 nm. Small, highly charged and strongly polar, it retains weakly on C18 and elutes early, so ion-pairing conditions matter more than for a neutral or acidic peptide.

The proline content creates a chromatographic artefact easily mistaken for an impurity. Slow cis-trans interconversion at the bonds preceding the three prolines means the sample is a mixture of conformers on the timescale of the separation, and they can resolve partially, giving peak broadening, shoulders or apparent doubling that reflects conformational equilibrium rather than chemical heterogeneity. Two things distinguish it from a real impurity: it is temperature-dependent, coalescing into a sharper single peak as column temperature rises, and the resolved features share an identical mass. A purity figure that ignores it understates the material.

Area percent is a chromatographic measure rather than peptide content by weight. Solid-phase synthesis delivers a salt, commonly trifluoroacetate or acetate, and on a 752 g/mol peptide with three basic sites the counter-ion load is proportionally high.

Mass spectrometry confirms identity and settles the conformer question: multiple chromatographic peaks sharing one mass are conformers, while a real impurity differs in mass.

These describe general methodology, not a claim about any particular batch.

Handling FAQ

The chromatogram shows a broadened or split peak. Is the material impure? Not necessarily. Three prolines mean the peptide exists as slowly interconverting cis and trans conformers, which can partially resolve during a separation. Raising the column temperature usually coalesces them, and mass spectrometry settles it: identical masses indicate conformers, not impurities.

Does this need protection from air or light? Not for chemical reasons. The sequence contains no sulfur and no aromatic residue, so the oxidation and photo-oxidation routes that matter elsewhere are absent. Ordinary sealed, cold, dry storage is sufficient.

Full specifications for Selank.