Ligand-to-Metal Charge-Transfer Dynamics in a Blue Copper Protein Plastocyanin: A Molecular Dynamics Study

Abstract

Equilibrium and nonequilibrium dynamics of a blue copper protein plastocyanin in an oxidized state are studied by molecular dynamics (MD) simulation. Potential energy functions of the lowest seven electronic states, including ligand-to-metal charge-transfer (LMCT) and copper d → d excited states, were taken from our previous work (Ando, K. J. Phys. Chem. B 2004, 108, 3940), which employed ab initio molecular orbital and density functional calculations on the active-site model. The equilibrium MD simulations in the ground state indicate that ligand motions coupled to transition from the ground state to the LMCT state are mostly represented by stretching and bending vibrations of the Cu-S(Cys) distance, Nδ(His)-Cu-Nδ(His) angle, and S(Cys)-Cu-[Nδ(His)]2 trigonal pyramid structure. The nonequilibrium dynamics on the LMCT potential exhibit rapid decays in which surface crossings to the d → d and the first excited states occur in 70-80 fs. The crossing dynamics mostly correlate with cleavage of the Cu-S(Cys) bond and the associated response in the Nδ(His)-Cu-Nδ(His) moiety. The average dynamics of the vertical energy gap coordinates exhibit an overdamped decay with a recurrence oscillation in 500 fs, which shows clear coherence surviving after the ensemble averaging. This oscillation stems mostly from the recoiling motion of the Nδ(His)-Cu-Nδ(His) part. The dynamics of the energy gaps after this coherent oscillation are randomized such that the ensemble average yields flat profiles along time, although each single trajectory exhibits fluctuations with amplitudes large enough to reach surface crossings. These indicate that the relaxation from the LMCT state first occurs via ballistic and coherent potential crossings in 70-80 and 500 fs, followed by thermally activated random transitions.