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 prote in from Pseudomonas aeruginosa, have been measured at wavelengths thro ughout the S(Cys) --> Cu(II) charge-transfer absorption band centered at 625 nm in an effort to determine the role of environment and struct ure on the dynamics of excited-state charge transfer. Azurin provides an analogous system to plastocyanin, another Type 1 blue copper protei n, whose excited-state structure and dynamics have been previously det ermined for a number of plant species, Self-consistent analysis of the absorption spectrum and the resulting resonance Raman excitation prof iles using a time-dependent wave packet propagation formalism indicate s 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 p lastocyanin, 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 environm ent in the two proteins reveals specific differences in structure and hydrogen-bonding environment which may explain the differences in obse rved excited-state charge-transfer dynamics of azurin and plastocyanin . The X-ray crystal structures of poplar a plastocyanin and P, aerugin osa azurin suggest that the larger coordination number accounts for th e increased reorganization energy in azurin, and the increased hydroge n bonding at the copper site and/or conformational substates may expla in the greater inhomogeneous component to the absorption line width in azurin.