Review: Internal water molecules as mobile polar groups for light-induced proton translocation in bacteriorhodopsin and rhodopsin as studied by difference FTIR spectroscopy

FTIR spectroscopy is advantageous for detecting changes in polar chemical bonds that participate in bacteriorhodopsin function. Changes in H-bonding of Asp85, Asp96, the Schiff base, and internal water molecules around these residues upon the formation of the L, M, and N photo-intermediates of bacteriorhodopsin were investigated by difference FTIR spectroscopy. The locations and the interactions of these water molecules with the amino acid residues were further revealed by use of mutant pigments. The internal water molecules in the cytoplasmic domain probably work as mobile polar groups in an otherwise apolar environment and act to stabilize the L intermediate, and carrying a proton between the Schiff base and the proton acceptor or donor. Similar internal water molecules were shown to be present in bovine rhodopsin.