Characterization of the Phenoxyl Radical in Model Complexes for the CuB Site of Cytochrome c Oxidase: Steady-State and Transient Absorption Measurements, UV Resonance Raman Spectrosc

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• 作者：Yasutomo Nagano ; Jin-Gang Liu ; Yoshinori Naruta ; Tadaaki Ikoma ; Shozo Tero-Kubota ; Teizo Kitagawa
• 刊名：Journal of the American Chemical Society
• 出版年：2006
• 出版时间：November 15, 2006
• 年：2006
• 卷：128
• 期：45
• 页码：14560 - 14570
• 全文大小：173K
• 年卷期：v.128,no.45(November 15, 2006)
• ISSN：1520-5126

文摘

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 coxidase, 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 phenolichydroxyl is ca. 10 for the Cu^II model complex (Cu^II-BIAIP) in the ground state, which is similar to that ofp-cresol (tyrosine), contrary to expectations. The bond between Cu^II and nitrogen of cross-linked imidazolecleaves at pH 4.9. We have successfully obtained UVRR spectra of the phenoxyl radical form of BIAIPsand 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 frequenciesfor 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 aminor effect on the electronic structure of the phenoxyl radical form, although our experimental resultsappear to indicate that the cross-linked Tyr radical exhibits no EPR. The role of the Tyr-His-Cu_B unit in theenzyme is discussed in terms of the obtained spectroscopic parameters of the model complex.