VIP

VIP, vasoactive intestinal peptide, is a twenty-eight-residue peptide of the secretin and glucagon superfamily, carried with a C-terminal amide. It is not a free acid, and the human, rat and porcine sequences are identical.

The amidation is not a detail and is the single most common way this compound is mis-specified. UniProt P01282 records it directly as a modified residue, asparagine amide at precursor position 152, and FDA GSRS records the same modification on aviptadil, the synthetic amidated twenty-eight-mer, as a complete conversion of asparagine to asparaginamide. The amide form has the formula C147H238N44O42S and forty-four nitrogen atoms.

PubChem's records for this compound, CID 16132300 and CID 53314964, carry C147H237N43O43S and a mass near 3326.8. That is the free acid, not the amide: the deposited structure terminates in a carboxylic acid and the nitrogen count is forty-three. A catalogue quoting 3326.8 for VIP is quoting the wrong molecule by about one dalton, so the figure above is calculated from the formula FDA GSRS registers for the amidated peptide, whose own published value is rounded to 3325.

The sequence carries two tyrosines, at positions 10 and 22, and one phenylalanine at position 6, but no tryptophan. That is enough of an aromatic chromophore for absorbance at 280 nm to be a usable concentration method, unlike several of the shorter peptides in this catalogue. There is one methionine, at position 17, and no cysteine.

The molecule contains no proline and no cysteine, so its conformation is set by an amphipathic helix rather than by a ring constraint. UniProt P01282 annotates a helix across residues 2 to 25, evidenced by PDB 8E3Z, the 2.7 Angstrom cryo-electron-microscopy structure of the peptide bound to VPAC1 and a heterotrimeric G protein published by Piper and colleagues in 2022.

Its receptors are VPAC1, the product of VIPR1 (UniProt P32241), and VPAC2, the product of VIPR2 (UniProt P41587), both class B1 G-protein-coupled receptors. It engages PAC1 (UniProt P41586) at lower affinity; the molecular-dynamics work in Piper and colleagues attributes that preference to less stable contacts in the PAC1 receptor core.

No primary source gives a quantitative solubility figure, an adsorption figure, or a forced-degradation study for this peptide. The oxidation and deamidation routes named under storage are read from the composition rather than measured, and are described here as such.