PRESSURE-DEPENDENCE OF PRIMARY CHARGE SEPARATION IN A PHOTOSYNTHETIC REACTION-CENTER

Spectral hole burning is used to study the pressure dependence of the Q(y) absorption spectrum and primary charge separation kinetics of the D1-D2-cyt b(559) reaction center complex of photosystem II. The 4.2 K lifetime of P680*, the primary donor state, lengthens from 2.0 ps at 0.1 MPa to 7.0 ps at 267 MPa. Importantly, this effect is irreversible (plastic), in sharp contrast with the elastic effects of pressure on the low-temperature absorption and non-line-narrowed hole spectrum of P680. These observations and data which show that the electron-phonon coupling is weakly dependent on pressure, suggest a model that has the plastic behavior of charge separation kinetics due mainly to the pressure dependence of the energy of the acceptor state and of the variance of the P680*-acceptor energy gap stemming from structural heterogeneity. Nonadiabatic rate expressions, which take into account the distribution of energy gap values, are used to estimate the linear pressure shift of the acceptor state energy for both the superexchange and two-step mechanisms for primary charge separation. For both mechanisms shifts in the vicinity of 1 cm(-1)/MPa are required to explain the data, a value which is not unreasonable based on pressure dependent studies of other systems. The results point to the marriage of hole burning and high pressures as having considerable potential for the study of primary transport dynamics in reaction center and antenna complexes.