Structural studies of viruses by Raman spectroscopy part LXXI - Tyrosine Raman signatures of the filamentous virus Ff are diagnostic of non-hydrogen-bonded phenoxyls: Demonstration by Raman and infrared spectroscopy of p-cresol vapor

p-Cresol is a simple molecular model for the para phenolic side chain of ty rosine. Previously, Siamwiza and co-workers [(1975) Biochemistry 14, 4870-4 876] investigated p-cresol solutions to identify Raman spectroscopic signat ures for different hydrogen-bonding states of the tyrosine phenoxyl group i n proteins. They found that the phenolic moiety exhibits an intense Raman d oublet in the spectral interval 820-860 cm(-1) and that the doublet intensi ty ratio (I-2/I-1, where I-2 and I-1 are Raman peak intensities of the high er- and lower-wavenumber members of the doublet) is diagnostic of specific donor and acceptor roles of the phenoxyl OH group. The range of the doublet intensity ratio in proteins (0.30 < I-2/I-1 < 2.5) was shown to be governe d by Fermi coupling between the phenolic ring-stretching fundamental v(1) a nd the first overtone of the phenolic ring-deformation mode v(16a), such th at when the tyrosine phenoxyl proton is a strong hydrogen-bond donor, I-2/I -1 = 0.30, and when the tyrosine phenoxyl oxygen is a strong hydrogen-bond acceptor, I-2/I-1 = 2.5. Here, we interpret the Raman and infrared spectra of p-cresol vapor and extend the previous correlation to the non-hydrogen-b onded state of the tyrosine phenoxyl group. In the absence of hydrogen bond ing, the Raman intensity of the higher-wavenumber component of the canonica l Fermi doublet is greatly enhanced such that I-2/I-1 = 6.7. Thus, for the non-hydrogen-bonded phenoxyl, the lower-wavenumber member of the Fermi doub let loses most of its Raman intensity. This finding provides a basis for un derstanding the anomalous Raman singlet signature (similar to 854 cm(-1)) o bserved for tyrosine in coat 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 present results for Raman analysis of tyrosine hydrogen-bonding states in other pr oteins are considered.