UV resonance Raman characterization of model compounds of Tyr244 of bovine cytochrome c oxidase in its neutral, deprotonated anionic, and deprotonated neutral radical forms:: Effects of covalent binding between tyrosine and histidine

A model compound of Tyr(244)-His(240) of bovine cytochrome c oxidase was synthesized and examined with UV resonance Raman (UVRR) as well as UV absorption spectroscopy and pH titration, Owing to the covalent link-age between imidazole and phenol, the pK(a) of phenolic OH and imidazolic KdeltaH groups were lowered by 1.1 and 2.3, respectively. UVRR measurements of ortho-imidazole-bound para-cresol (IMlozengeCrOH), its deprotonated anion (ImlozengeCrO(-)), and deprotonated neutral radical (ImlozengeCrO(.)), and their imidazole perdeuterated, cresol perdeuterated, and O-18 derivatives allowed assignments of Raman hands to the imidazole and phenol modes. Unexpectedly, some of imidazole vibrations were resonance enhanced upon pipi* transition of phenol, although they were not observed for the corresponding equimolar mixture of imidazole and p-cresol, indicating delocalization of pi electrons between the imidazole and phenol rings via the covalent linkage. Such features were appreciably changed by incorporation of a bulky group at imidazole C-2 position, causing staggered conformation. The C-O stretching RR band was observed at 1530 cm(-1) only for a radical state. The imidazole substituent at the C-2 position seems to increase appreciably a double bond character of the CO bond in the radical state than its absence. The Y8a band is not shifted upon deprotonation, although it was downshifted by 12 cm(-1) for the unmodified p-cresol. The UVRR difference spectra of the anion and radical with regard to the neutral states are discussed in relation with the corresponding difference spectra of the enzyme.