Charge stabilization in reaction center protein investigated by optical heterodyne detected transient grating spectroscopy.

Photosynthetic reaction center (RC) pigment protein complex converts the free energy of light into chemical potential of charge pairs with extremely high efficiency. A transient phase in the absorption spectrum in the sub-millisecond time scale is expected to be especially important to examine the conformational gating model of the Q(A)Q(B) to Q(A)Q(B) (here Q(A )and Q(B) are the primary and secondary quinone type electron acceptors, respectively) electron transport. Essential kinetic components at few tens of microseconds scale and at around 200 microseconds have been suggested. We investigated the conformation change of RCs using heterodyne detection of the laser-induced transient grating method. An about 25 microseconds dynamics was observed, which coincides with the one described by the conformational gating model and possibly related to the nonadiabatic intrinsic Q(A)Q(B) to Q(A)Q(B) electron transport. The relative intensity of this component decreased with increasing quinone concentration indicating an initial (P+Q(A))1 or a relaxed (P+Q(A))2 conformational substate. We did not find the decay component at few hundreds of microseconds time scale indicating that there is no large displacement in the RC structure if Q(B) is present. The diffusion coefficient of the RC/LDAO detergent micelles calculated from the kinetic component was D = 3.8 x 10(-11 ) m2/s that agrees fairly well with the number estimated from the Einstein-Stokes relationship, and relates to a hydrodynamic diameter of 11.4 nm of the RC in LDAO micellar solution.