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 α-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 α-helix. The mostly α-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.