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The tyrosine side chain generates a pair of distinctive Raman bands--a Fermi doublet near 850 and 830 cm-1--with relative intensities diagnostic of hydrogen bonding states of the phenolic acceptor and donor atoms Siamwiza et al. (1975) Biochemistry 14, 4870-4876. This structural correlation has been tested extensively and is used widely as an indicator of tyrosine interactions in globular proteins and their assemblies. However, in Ff filamentous viruses (fd, f1, M13) the apparent Fermi doublet intensity ratio (I853/I826 approximately 4.0) is much greater than the maximum predicted or observed in other proteins. To understand this anomaly, we have reevaluated the basis for the Fermi doublet assignment in Ff. We report Raman spectra of site-specific mutants of Ff in which either one (Y21M and Y24M) or both (Y21F/Y24S) tyrosines of the coat protein subunit (pVIII) have been mutated. These Raman data, together with those obtained from Ff virions carrying residue-specific tyrosyl (Y-d4) and phenylalanyl (F-d5) deuterations in pVIII, demonstrate conclusively that the 853 and 826 cm-1 bands of Ff do not constitute a typical tyrosine Fermi doublet: The observed 826 cm-1 Raman band of Ff is due not to tyrosine but to phenylalanine residues of pVIII. The 853 cm-1 Raman band thus constitutes the first known example of a "tyrosine singlet" in the Raman spectrum of a protein. The implications of this finding for Ff virion structure and its relevance to tyrosine markers in other proteins are discussed.
Overman et al. (Tue,) studied this question.