Effects of hydrogen bonding on the tyrosine Raman bands in the 1300–1150 cm⁻¹ region

Raman spectra of p ‐cresol, a model compound for tyrosine, were measured in solutions of various solvents, paying special attention to the effects of hydrogen bonding on the Raman bands in the 1300–1150 cm −1 region. The frequency of the ν 7a (CO stretch) band was found to be sensitive to the state of hydrogen bonding at the phenolic hydroxyl group. It occurs at 1275–1265 cm −1 in proton‐donating states, 1240–1230 cm −1 in proton‐accepting states and around 1255 cm −1 in weakly or non‐hydrogen‐bonding states. This relationship between the ν 7a′ frequency and hydrogen bonding was verified in the Raman spectra of L‐tyrosine and its derivatives in the crystalline state. Analysis of the crystal Raman spectra further suggested that the ν 7a (CC stretch) frequency also serves as a marker, though less sensitive, of hydrogen bonding and the ν 9a (CH bend) frequency reflects the displacement of the OH hydrogen atom from the plane of benzene ring, which may be induced by hydrogen bonding. These Raman bands are strong with UV excitation and are expected to be useful in characterizing tyrosine side‐chains in peptides and proteins by UV resonance Raman spectroscopy.