Protein tuning of excited-state charge-transfer dynamics in azurin

Resonance Raman spectra of azurin from Alcaligenes denitrificans have been measured at wavelengths throughout the S(Cys) --> Cu charge-transfer absorption band to examine the effects of structure and environment on the excited-state charge-transfer dynamics. Measurement of the depolarization ratios suggest that the Raman scattering is enhanced by a single electronic transition. Self-consistent analysis of the absorption spectrum and the resonance Raman fundamental, overtone, and combination bands yields a total inner-sphere reorganization energy of 0.26 +/- 0.02 eV, similar to that for Pseudomonas aeruginosa. The reorganization energy is distributed along similar vibrational modes in the two azurins, although the contribution from each mode is different. To understand these differences, molecular comparisons of the known structures and molecular modeling of the electrostatic field within the protein were performed. The results show that the two proteins have very similar structures, although the copper site itself has a slightly more tetrahedral geometry in A. denitrificans. The resonance Raman spectral differences between these two azurins are interpreted to arise from this copper site structural difference and a greater distribution of charges and/or dipoles near the coordinating histidines in A. denitrificans.