Identifying Residual Structure in Intrinsically Disordered Systems: A 2D IR Spectroscopic Study of the GVGXPGVG Peptide

The peptide amide-I vibration of a proline turn encodes information on the turn structure. In this study, FTIR, two-dimensional IR spectroscopy and molecular dynamics simulations were employed to characterize the varying turn conformations that exist in the GVGX(L)PGVG family of disordered peptides. This analysis revealed that changing the size of the side chain at the X amino acid site from Gly to Ala to Val substantially alters the conformation of the peptide. To quantify this effect, proline peak shifts and intensity changes were compared to a structure-based spectroscopic model. These simulated spectra were used to assign the population of type-II beta turns, bulged turns, and irregular beta turns for each peptide. Of particular interest was the Val variant commonly found in the protein elastin, which contained a 25% population of irregular beta turns containing two peptide hydrogen bonds to the proline C=O.