Charge Transfer Dynamics in Plastocyanin, A Blue Copper Protein, from Resonance Raman Intensities
Ester Fraga, M. Adam Webb, and Glen R. Loppnow
Abstract
The resonance Raman intensities for parsley plastocyanin, a blue copper protein involved in electron transport in plant photosynthesis, have been measured at wavelengths throughout the S(Cys) to Cu charge-transfer 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 absorption band and resulting resonance Raman excitation profiles demonstrates that the charge-transfer absorption band is primarily homogeneously broadened. The homogeneous linewidth is composed of population decay and solvent-induced dephasing. The excited-state lifetime of 20±15 fs calculated here from the observed fluorescence suggests that the charge transfer state decays rapidly via lower-lying ligand-field states. The spectral lineshape dictates that this population decay be modeled as a Gaussian of linewidth 230 wavenumbers. The reorganization energy obtained from the resonance Raman intensities of specific vibrations is 0.19 eV. If the reorganization energy of the protein as measured from the solvent-induced dephasing component of the homogeneous linewidth is included, the observed reorganization energy is 0.25 eV, in quantiitative agreement with a previous upper-limit of 0.3 eV measured for the reorganization energy upon electron transport at the copper site in azurin, a similar blue copper protein. A crude comparison of the reorganization energies upon electron transport and charge transfer suggests that charge transfer may be a somwhat useful model for the geometry changes upon electron transfer. The resonance Raman spectrum indicates that reorganization occurs primarily along normal modes that involve the Cu-S(Cys) stretch, but significant reorganization also occurs along specific normal modes that involve internal cysteine stretches, Cu-N(His) stretches, and protein internal motions. An important result of this work is the two mechanisms by which the protein environment 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 linewidth. These results are compared to those of other methods for determining reorganization energies and are discussed in terms of the role of the environment in controlling electron- and charge-transfer processes.