| Abstract
| - p-Cresol is a simple molecular model for the para phenolic side chain of tyrosine. Previously,Siamwiza and co-workers [(1975) Biochemistry 14, 4870−4876] investigated p-cresol solutions to identifyRaman spectroscopic signatures for different hydrogen-bonding states of the tyrosine phenoxyl group inproteins. They found that the phenolic moiety exhibits an intense Raman doublet in the spectral interval820−860 cm-1 and that the doublet intensity ratio (I2/I1, where I2 and I1 are Raman peak intensities of thehigher- and lower-wavenumber members of the doublet) is diagnostic of specific donor and acceptorroles of the phenoxyl OH group. The range of the doublet intensity ratio in proteins (0.30 < I2/I1< 2.5)was shown to be governed by Fermi coupling between the phenolic ring-stretching fundamental ν1 andthe first overtone of the phenolic ring-deformation mode ν16a, such that when the tyrosine phenoxyl protonis a strong hydrogen-bond donor, I2/I1 = 0.30, and when the tyrosine phenoxyl oxygen is a strong hydrogen-bond acceptor, I2/I1 = 2.5. Here, we interpret the Raman and infrared spectra of p-cresol vapor and extendthe previous correlation to the non-hydrogen-bonded state of the tyrosine phenoxyl group. In the absenceof hydrogen bonding, the Raman intensity of the higher-wavenumber component of the canonical Fermidoublet is greatly enhanced such that I2/I1 = 6.7. Thus, for the non-hydrogen-bonded phenoxyl, the lower-wavenumber member of the Fermi doublet loses most of its Raman intensity. This finding provides abasis for understanding the anomalous Raman singlet signature (∼854 cm-1) observed for tyrosine incoat protein subunits of filamentous viruses Ff and Pf1 [Overman, S. A., et al. (1994) Biochemistry 33,1037−1042; Wen, Z. Q., et al. (1999) Biochemistry 38, 3148−3156]. The implications of the presentresults for Raman analysis of tyrosine hydrogen-bonding states in other proteins are considered.
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