Halorhodopsin and bacteriorhodopsin convert light into energy in archaea through light-driven chloride and proton pumps, respectively. Three water molecules are present in their active centers, which presumably stabilize the quadrupole structures and play crucial roles in pumps. The present low-temperature Fourier-transform infrared (FTIR) study revealed that hydration of the negative charges by the internal water molecules is much weaker in halorhodopsin than in bacteriorhodopsin, suggesting that chloride ion is stabilized by weak hydrogen bonds of waters in halorhodopsin. (C) 2004 Elsevier B.V. All rights reserved.