Excited-State Charge-Transfer Dynamics of Azurin, a Blue Copper Protein, from Resonance Raman Intensities

Resonance Raman spectra of azurin, a 14.6 kDa Type 1 blue copper protein from Pseudomonas aeruginosa, have been measured at wavelengths throughout the S(Cys) → Cu(II) charge-transfer absorption band centered at 625 nm in an effort to determine the role of environment and structure on the dynamics of excited-state charge transfer. Azurin provides an analogous system to plastocyanin, another Type 1 blue copper protein, whose excited-state structure and dynamics have been previously determined for a number of plant species. Self-consistent analysis of the absorption spectrum and the resulting resonance Raman excitation profiles using a time-dependent wave packet propagation formalism indicates that inhomogeneous effects account for the majority of the spectral broadening of the charge-transfer absorption band, in contrast to the primarily homogeneously broadened charge-transfer absorption band in plastocyanin. The total reorganization energy from the resonance Raman enhanced modes was found to be 0.26 ± 0.02 eV, compared to 0.19 ± 0.02 eV for plastocyanin. A detailed comparison of the copper environment in the two proteins reveals specific differences in structure and hydrogen-bonding environment which may explain the differences in observed excited-state charge-transfer dynamics of azurin and plastocyanin. The X-ray crystal structures of poplar a plastocyanin and P. aeruginosa azurin suggest that the larger coordination number accounts for the increased reorganization energy in azurin, and the increased hydrogen bonding at the copper site and/or conformational substates may explain the greater inhomogeneous component to the absorption line width in azurin.