All characteristic Raman markers of tyrosine and tyrosinate originate from phenol ring fundamental vibrations

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Abstract

Raman data collected from free amino acid and peptide chains permit assignment of the seven markers located at approximately 1616, 1606, 1210, 1178, 850, 830, and 643 cm −1 to tyrosine. The effects induced by labile hydrogen deuteration, temperature, concentration, and protonation–deprotonation on these Raman markers were analyzed. Following closely a recent multiconformational analysis on phenylalanine, we could confirm the predominance of gauche ‐ / gauche ‐ rotamers with respect to the side chain χ 1 / χ 2 torsion angles in both tyrosine and tyrosinate. Calculated Raman spectra, based on the consideration of both implicit and explicit hydration models, have revealed the effect of hydrogen bonding of water molecules to the phenol ring hydroxyl group of tyrosine. A special attention has been paid to the 850/830 cm −1 tyrosine doublet, for which no intensity ratio inversion could be remarked when the phenol hydroxyl group acts as a hydrogen bond donor or acceptor. On the contrary, this intensity ratio appeared to be strongly dependent on the hydrophobic/hydrophilic balance of the interactions, in which the phenol ring is involved. Based on the present theoretical calculations, the components of the tyrosine doublet might originate from two independent fundamental modes of the phenol ring: one with an in‐plane character and the other with an out‐of‐plane character. As a consequence, the widely spread Fermi resonance‐based description of the tyrosine doublet appears to be unjustified. The latter interpretation simply results from the insufficiencies of the simple model compound used at that time for analyzing the vibrational features of a more complex molecular system such as tyrosine. Copyright © 2015 John Wiley & Sons, Ltd.

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