The doublet at 850 and 830 cm-1 in the Raman spectra of proteins containing tyrosyl residues has been examined as to its origin and the relation of its components to the environment of the phenyl ring, the state of the phenolic hydroxyl group, and the conformation of the amino acid backbone. Raman spectral studies on numerous model molecules related to tyrosine, including certain deuterium derivatives, show that the doublet is due to Fermi resonance between the ring-breathing vibration and the overtone of an out-of-plane ring-bending vibration of the para-substituted benzenes. Further examination of the effects of pH and solvents on the Fermi doublet and of the crystallographic data demonstrates that the intensity ratio of the two components depends on changes in the relative frequencies of the two vibrations. These in turn are found to be sensitive to the nature of the hydrogen bonding of the phenolic hydroxyl group of its ionization, but much less so to the environment of the phenyl ring and the conformation of the amino acid backbone. By use of the relative intensities of the doublet in model systems where the phenolic hydroxyl group is strongly hydrogen-bonded, weakly hydrogen-bonded, free or ionized, the reported Raman intensities of the doublets observed in the Raman spectra of several proteins have been interpreted. The results are compared with those obtained by other techniques.

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