Perturbations to the geometric and electronic structure of the Cu A site: Factors that influence derealization and their contributions to electron transfer

Xiangjin Xie, Serge I. Gorelsky, Ritimukta Sarangi, Dewain K. Garner, Jung Hwang Hee, Keith O. Hodgson, Britt Hedman, Yi Lu, Edward I. Solomon

Research output: Contribution to journalArticle


Using a combination of electronic spectroscopies and DFT calculations, the effect of pH perturbation on the geometric and electronic structure of the CuA site has been defined. Descriptions are developed for high pH (pH = 7) and low pH (pH = 4) forms of CuA azurin and its H120A mutant which address the discrepancies concerning the extent of derealization indicated by multifrequency EPR and ENDOR data (J. Am. Chem. Soc. 2005, 127, 7274; Biophys. J. 2002, 82, 2758). Our resonance Raman and MCD spectra demonstrate that the low pH and H120A mutant forms are essentially identical and are the perturbed forms of the completely delocalized high pH CuA site. However, in going from high pH to low pH, a seven-line hyperfine coupling pattern associated with complete derealization of the electron (S = 1/2) over two Cu coppers (ICu = 3/2) changes into a four-line pattern reflecting apparent localization. DFT calculations show that the unpaired electron is delocalized in the low pH form and reveal that its four-line hyperfine pattern results from the large EPR spectral effects of ∼1% 4s orbital contribution of one Cu to the ground-state spin wave function upon protonative loss of its His ligand. The contribution of the Cu-Cu interaction to electron derealization in this low symmetry protein site is evaluated, and the possible functional significance of the pH-dependent transition in regulating proton-coupled electron transfer in cytochrome c oxidase is discussed.

Original languageEnglish (US)
Pages (from-to)5194-5205
Number of pages12
JournalJournal of the American Chemical Society
Issue number15
StatePublished - Apr 16 2008


ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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