Understanding CH-Stretching Raman Optical Activity in Ala-Ala Dipeptide

Raman optical activity (ROA) becomes a standard method to monitor peptide conformation. However, the signal in the CH stretching region is particularly difficult to measure and interpret. In order to understand the structural information contained in this part of the spectrum, data obtained on a custom-made ROA spectrometer have been analyzed for the model Ala-Ala molecule, with the help of molecular dynamics (MD) and density functional theory (DFT) computations. The Ala-Ala enantiomers provided the "mirror image" spectra, which proves that the signal can be reliably measured, in spite of a rather low ROA/Raman intensity ratio (~ 210-5). The theoretical modeling indicated that the most intense ROA bands can be attributed to locally asymmetric CH3 and CH vibrations, whereas symmetric methyl CH stretching modes contribute less. A simplified model made it possible to estimate the contribution of local chirality of the two alanine residues to the resultant ROA pattern. In spite of a significant frequency sh...