Time-resolved optical Kerr-effect spectroscopy of low-frequency dynamics in Di-L-alanine, poly-L-alanine, and lysozyme in solution

The low-frequency spectra of peptides and proteins in solution have been investigated with optical heterodyne-detected Raman-induced Kerr-effect spectroscopy. Spectra were obtained for di-L-alanine ALA(2) and poly-L-alanine (PLA) in dichloroacetic acid solution. The conformational dependence of those spectra at low frequency has been analyzed. ALA(2) displays a band centered at 50 cm-1, whereas the -helical PLA shows two shoulders at 60 and 140 cm-1. The similarity of the spectral features observed in PLA to those in water can be explained by analogous acoustic translational modes in the hydrogen network of the PLA -helix. The mostly -helical protein lysozyme in aqueous solution has also been investigated and showed significantly more structure with modes at 10, 35, 73, 106, and 164 cm-1.