CLASSICAL MOLECULAR-DYNAMICS SIMULATION OF THE PHOTOINDUCED ELECTRON-TRANSFER DYNAMICS OF PLASTOCYANIN

Classical molecular dynamics simulations are used to investigate the n uclear motions associated with photoinduced electron transfer in plast ocyanin. The blue copper protein is modeled using a molecular mechanic s potential; potential parameters for the copper-protein interactions are determined using an x-ray crystallographic structure and absorptio n and resonance Raman spectra. Molecular dynamics simulations yield a variety of information about the ground (oxidized) and optically excit ed (charge-transfer) states: 1) The probability distribution of the po tential difference between the states, which is used to determine the coordinate and energy displacements, places the states well within the Marcus inverted region. 2) The two-time autocorrelation function of t he difference potential in the ground state and the average of the dif ference potential after instantaneous excitation to the excited state are very similar (confining linear response in this system); their dec ay indicates that vibrational relaxation occurs in about 1 ps in both states. 3) The spectral densities of various internal coordinates begi n to identify the vibrations that affect the optical transition; the s pectral density of the difference potential correlation function shoul d also prove useful in quantum simulations of the back electron transf er. 4) Correlation functions of the protein atomic motions with the di fference potential show that the nuclear motions are correlated over a distance of more than 20 Angstrom, especially along proposed electron transport paths.