In a light-driven proton-pump protein, bacteriorhodopsin (BR), protonated Schiff base of the retinal chromophore and Asp85 form ion-pair state, which is stabilized by a bridged water molecule. After light absorption, all-trans to 13-cis photoisomerization takes place, followed by the primary proton transfer from the Schiff base to Asp85 that triggers sequential proton transfer reactions for the pump. Fourier transform infrared (FTIR) spectroscopy first observed O-H stretching vibrations of water during the photocycle of BR, and accurate spectral acquisition has extended the water stretching frequencies into the entire stretching frequency region in D(2)O. This enabled to capture the water molecules hydrating with negative charges, and we have identified the water O-D stretch at 2171 cm(-1) as the bridged water interacting with Asp85. We found that retinal isomerization weakens the hydrogen bond in the K intermediate, but not in the later intermediates such as L, M, and N. On the basis of the observation particularly on the M intermediate, we proposed a model for the mechanism of proton transfer from the Schiff base to Asp85. In the "hydration switch model", hydration of a water molecule is switched in the M intermediate from Asp85 to Asp212. This will have raised the pK(a) of the proton acceptor, and the proton transfer is from the Schiff base to Asp85.