Coupling between Electron Transfer and Protein–Solvent Dynamics: FTIR and Laser-Flash Spectroscopy Studies in Photosynthetic Reaction Center Films at Different Hydration Levels

We report on the relationship between electron transfer, conformational dynamics, and hydration in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides. The kinetics of electron transfer from the photoreduced quinone acceptor (Q(A)(-)) to the photo-oxidized primary donor (P(+)), a charge recombination process sensitive to the conformational dynamics of the RC, has been analyzed at room temperature in dehydrated RC-detergent films as a function of the residual water content under controlled relative humidity (r). The hydration level was evaluated by FTIR spectroscopy from the area of the combination band of water (5155 cm(-1)). Sorption isotherms fit the Hailwood and Horrobin model and indicate a significant contribution to hydration of the detergent belt surrounding the RC. Spectral analysis of the water combination and association (2130 cm(-1)) bands suggests strong rearrangements in the hydrogen-bonding organization upon depletion of the hydration shell of the complex. In parallel with these changes, following dehydration below a critical threshold (r approximately equal 40%), the kinetics of P(+)Q(A)(-) recombination become progressively faster and distributed in rate. When r is decreased from 40% to 10% the average rate constant (k) increases from 15 to 40 s(-1), mimicking the behavior of the hydrated system at cryogenic temperatures. We infer that extensive dehydration inhibits dramatically the relaxation from the dark- to the light-adapted conformation of the RC as well as interconversion among lower tier conformational substates. The RC dynamics probed by P(+)Q(A)(-) recombination appear therefore controlled by the thermal fluctuations of the hydration shell. At r < 10% an additional, much faster ((k) approximately equal 3000 s(-1)) kinetic phase of P(+)Q(A)(-) recombination is observed. We suggest such a fast recombination arises from removal of a pool of RC-bound water molecules which are essential to stabilize the primary charge-separated state at physiological conditions.