Origin of the doublet at 1360 and 1340 cm−1 in the Raman spectra of tryptophan and related compounds

Raman spectra of indole, skatole, tryptophan and its derivatives including C4-monodeuterated tryptophan were studied with special attention to the origin of the band around 1350 cm−1. The band is observed as a doublet or triplet depending on the compound and solvent, which was ascribed to Fermi resonance between one fundamental and one or two combination bands of the out-of-plane bending vibrations. Removal of the interaction is seen for C4D-tryptophan and some solutions of skatole. The assignments of the component vibrations were made based on the shifts in frequencies upon C4-deuteration. In tryptophan derivatives the doublet (or triplet) peaks exhibit steep changes in relative intensity at the pKa points of the dissociable groups, which is ascribed as arising from slight frequency shifts of a pKa-sensitive component vibration. Relations between the relative intensity and environment of indole were obtained and their possible applicability to the analyses of Raman spectra of proteins is suggested.