KPV: Structure, Naming and Two Forms Under One Name

KPV is one of the smallest peptides in the research market: three residues, lysine, proline and valine. Its size makes it simple to synthesize and simple to verify, and it also means that small differences in form change the molecular weight by a large fraction. This article sets out what the molecule is, where the name comes from, and which figures identify it.

The molecule

KPV is the tripeptide L-lysyl-L-prolyl-L-valine, sequence Lys-Pro-Val. With both ends unmodified its molecular formula is C16H30N4O4, its molecular weight is 342.43 and its CAS number is 67727-97-3. It contains no cysteine, methionine or tryptophan, the residues most prone to oxidation.

Where the sequence comes from

The three residues are the C-terminal end of α-melanocyte-stimulating hormone, positions 11 to 13 of that 13-residue peptide, and the literature refers to the tripeptide as α-MSH(11-13). The material sold for research is chemically synthesized; it is not derived from the hormone.

Two forms under one name

In the parent hormone the N-terminus is acetylated and the C-terminus is amidated. Some suppliers offer the tripeptide in that capped form, Ac-Lys-Pro-Val-NH2, and others as the free peptide; both are commonly labeled KPV. They are different substances. The capped form has the formula C18H33N5O4 and a molecular weight of 383.49, about 41 Da heavier than the free peptide. A reader comparing a published study with a vial needs to know which form each one is.

Confirming what is in a vial

For a molecule this small the identity check is unambiguous. A mass spectrometry value near 342.4 identifies the free peptide; one near 383.5 identifies the capped form. The sequence and the CAS number on the specification should agree with that figure.

A note on counterions

KPV carries two basic groups, the N-terminal amine and the lysine side chain, on a molecule of only 342 Da. Paired with two trifluoroacetate counterions, the salt would be about 40 percent counterion by weight; with two acetates, about 26 percent. For small peptides the gap between the weight of solid and the weight of peptide is correspondingly larger, and net peptide content matters more.

What FLASK lists

The material listed as KPV on this site is the free tripeptide, Lys-Pro-Val, CAS 67727-97-3.

Frequently asked questions

Are both forms sold as KPV the same substance?

No. The free peptide and the acetylated, amidated form differ by about 41 Da and are different substances, although both are commonly labeled KPV.

How can the two forms be told apart?

A mass spectrometry value near 342.4 identifies the free peptide; one near 383.5 identifies the capped form.

Why does net peptide content matter more for KPV?

Two basic groups on a 342 Da molecule mean the salt can be about 40 percent trifluoroacetate or 26 percent acetate by weight.