Using 4-Cyanophenylalanine to Probe the Degree of Water Exposure in a Peptide Hydrogel

Abstract

A peptide comprised of residues 37-49 of human semenogelin I (SgI 37-49), sequence KGSFSIQYTYHVD, spontaneously aggregates in water in a pH dependent manner to generate a hydrogel. Structural analysis by Fourier transform infrared (FTIR) spectroscopy indicates that the aggregated peptide fibrils consist of extended β;-sheet. Three SgI 37-49 mutants were synthesized in which the aromatic residues, phenylalanine and tyrosine, were each substituted for 4-cyanophenylalanine, FCN, to generate F4FCN, Y8FCN, and Y10FCN. The frequency of the nitrile group in the hydrogel state was then assessed with FTIR. The cyano group, with a stretch vibration of 2237 cm−1 in water and 2228 cm−1 in a non-polar solvent, such as tetrahydrofuran, has been used previously to probe the degree of water exposure, inferred through extent of hydrogen bonding, in aggregating systems such as the islet amyloid peptide, IAPP. Our studies are consistent with the core residue in position 8 of SgI 37-49 being embedded in the hydrogel whereas the flanking positions four and 10 appear more solvent exposed, in agreement with their positions in the sequence. These results suggest that the N- and C-terminal ends of the self-assembled peptides within the aggregated fibrils might be at least partly frayed.