CHARGE-TRANSFER DYNAMICS IN PLASTOCYANIN, A BLUE COPPER PROTEIN, FROMRESONANCE RAMAN INTENSITIES

The resonance Raman intensities for parsley plastocyanin, a blue coppe r protein involved in electron transport in plant photosynthesis, have been measured at wavelengths throughout the S(Cys) --> Cu charge-tran sfer absorption band centered at 597 nm in an effort to determine the structural and dynamic role of inner- and outer-sphere reorganization in the kinetics of charge transfer. Self-consistent analysis of the ab sorption band and the resulting resonance Raman excitation profiles de monstrates that the charge-transfer absorption band is primarily homog eneously broadened. The homogeneous line width is composed of populati on decay and solvent-induced dephasing. The excited-state lifetime of 20 +/- 15 fs calculated here from the observed fluorescence suggests t hat the charge-transfer state decays rapidly via lower-lying ligand-fi eld states. The spectral line shape dictates that this population deca y be modeled as a Gaussian of line width 230 cm(-1). The reorganizatio n energy obtained from the resonance Raman intensities of specific vib rations is 0.19 eV. if the reorganization energy of the protein as mea sured from the solvent-induced dephasing component of the homogeneous line width is included, the observed reorganization energy is 0.25 eV, in quantitative agreement with a previous upper limit of 0.3 eV measu red for the reorganization energy upon electron transport at the coppe r site in azurin, a similar blue copper protein. A crude comparison of the reorganization energies upon electron transport and charge transf er suggests that charge transfer may be a somewhat useful model for th e geometry changes upon electron transfer. The resonance Raman spectru m indicates that reorganization occurs primarily along normal modes th at involve the Cu-S(Cys) stretch, but significant reorganization also occurs along specific normal modes that involve internal cysteine stre tches, Cu-N(His) stretches, and protein internal motions. An important result of this work is the two mechanisms by which the protein enviro nment contributes to the reorganization energy: through coupling into specific resonance-enhanced normal modes and through a solvent-induced dephasing contribution as evidenced by the homogeneous line width. Th ese results are compared to those of other methods for determining reo rganization energies and are discussed in terms of the role of the env ironment in controlling electron- and charge-transfer processes.