Characterization of the Phenoxyl Radical in Model Complexes for the Cu_B Site of Cytochrome c Oxidase:  Steady-State and Transient Absorption Measurements, UV Resonance Raman Spectroscopy, EPR Spectroscopy, and DFT Calculations for M-BIAIP

Physicochemical properties of the covalently cross-linked tyrosine-histidine-Cu_B (Tyr-His-Cu_B) unit, which is a minimal model complex [M^II-BIAIPBr]Br (M = Cu^II, Zn^II) for the Cu_B site of cytochrome c oxidase, were investigated with steady-state and transient absorption measurements, UV resonance Raman (UVRR) spectroscopy, X-band continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations. The pH dependency of the absorption spectra reveals that the pK_a of the phenolic hydroxyl is ca. 10 for the Cu^II model complex (Cu^II-BIAIP) in the ground state, which is similar to that of p-cresol (tyrosine), contrary to expectations. The bond between Cu^II and nitrogen of cross-linked imidazole cleaves at pH 4.9. We have successfully obtained UVRR spectra of the phenoxyl radical form of BIAIPs and have assigned bands based on the previously reported isotope shifts of Im-Ph (2-(1-imidazoyl)-4-methylphenol) (Aki, M.; Ogura, T.; Naruta, Y.; Le, T. H.; Sato, T.; Kitagawa, T. J. Phys. Chem. A 2002, 106, 3436−3444) in combination with DFT calculations. The upshifts of the phenoxyl vibrational frequencies for 8a (C−C stretching), 7a‘ (C−O stretching), and 19a, and the Raman-intensity enhancements of 19b, 8b, and 14 modes indicate that UVRR spectra are highly sensitive to imidazole-phenol covalent linkage. Both transient absorption measurements and EPR spectra suggest that the Tyr-His-Cu_B unit has only a minor effect on the electronic structure of the phenoxyl radical form, although our experimental results appear to indicate that the cross-linked Tyr radical exhibits no EPR. The role of the Tyr-His-Cu_B unit in the enzyme is discussed in terms of the obtained spectroscopic parameters of the model complex.