The coupling between electron transfer and protein dynamics has been studied in photosynthetic reaction centers (RC) from Rhodobacter sphaeroides by embedding the protein into room temperature solid trehalose-water matrices. Electron transfer kinetics from the primary quinone acceptor (Q(A)(-)) to the photoxidized donor (P(+)) were measured as a function of the duration of photoexcitation from 20 ns (laser flash) to more than 1 min. Decreasing the water content of the matrix down to approximately 5x10(3) water molecules per RC causes a reversible four-times acceleration of P(+)Q(A)(-) recombination after the laser pulse. By comparing the broadly distributed kinetics observed under these conditions with the ones measured in glycerol-water mixtures at cryogenic temperatures, we conclude that RC relaxation from the dark-adapted to the light-adapted state and thermal fluctuations among conformational substates are hindered in the room temperature matrix over the time scale of tens of milliseconds. When the duration of photoexcitation is increased from a few milliseconds to the second time scale, recombination kinetics of P(+)Q(A)(-) slows down progressively and becomes less distributed, indicating that even in the driest matrices, during continuous illumination, the RC is gaining a limited conformational freedom that results in partial stabilization of P(+)Q(A)(-). This behavior is consistent with a tight structural and dynamical coupling between the protein surface and the trehalose-water matrix.