UV resonance Raman study of model complexes of the CuB site of cytochrome c oxidase

A newly designed model complex for the CuB site of cytochrome c oxidase (CcO), that is, Cu coordinated by two free imidazoles and an imidazole covalently linked to p-cresol [CuIIBIAIPBr]Br, (BIAIP =2-[4-[[Bis(1-methyl-1H-imidazol-2-ylmethyl)amino]methyl]-1H-imidazol-1-yl]-4-methylphenol), and related molecules have been investigated with absorption and ultraviolet resonance Raman (UVRR) spectroscopy employing the excitation wavelengths between 220 and 290 nm. Attention was focused on the electron delocalization through the cross-linkage between the phenol and imidazole rings, and the influences by the coordination of CuII to imidazole. In addition to the ν8a and ν8b modes of p-cresol, a number of Raman bands involving vibrations of the imidazole moiety have been intensity-enhanced despite Raman excitation in resonance with the π-π* transition of phenol, indicating appreciable mixing of the π systems of imidazole and phenol rings. Furthermore, two kinds of imidazoles seem to be differential; one is the imidazole linked to p-cresol which yielded Raman bands at 1249, 1191, and 1141 cm−1 for protonated CuII-BIAIP, and the other is one not linked to p-cresol, which yielded an intense band at 1488 cm−1 band. Raman enhancement of the latter mode seems to be caused by preresonance to the lowest π-π* transition of imidazole via the A-term mechanism. The Raman excitation profile (REP) of ν8a mode for the deprotonated phenol of the CuII-complex revealed a weak local maximum corresponding to the La band around 240 nm. Raman enhancement by the La band was relatively weaker for the CuII-complex than for the ZnII-complex and metal-free ligand, suggesting the more extensive mixing of π systems of p-cresol-imidazole through the cross-linkage for the Cu II-complex.