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 congruent to 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 > congruent to 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.