The relationship between the tyrosine residue 850-830 cm Raman doublet intensity ratio and the aromatic side chain χ torsion angle.

Tyrosine (Tyr) residue in a peptide chain is characterized by the presence of seven Raman markers, referred to as Y (i = 1, …, 7), distributed over the middle wavenumber spectral region. Particularly, the changes observed in the relative intensity of Y and Y markers, appearing as a side by side doublet at ca. 850-830 cm, has received a great attention. Primarily assigned to a Fermi-resonance effect between phenol ring planar and nonplanar modes, former density functional theory calculations led us to affiliate the Y-Y doublet to two distinct fundamental modes. Furthermore, despite the previous assumptions, it was evidenced that the reversal of the doublet intensity ratio cannot be solely explained by hydrogen bonding on the phenol hydroxyl group involved in Tyr. Herein, upon analyzing the observed and theoretical data collected from the cationic species of the tripeptide Gly-Tyr-Gly, the crucial effect of the aromatic side chain orientation, especially that of the χ torsion angle defined around the CC bond, on the Tyr doublet intensity ratio has been evidenced.