The role of water in retinal complexation to bacterio-opsin.

A system is described that allows for the delineation of the factors that effect the complexation of retinal to the apoprotein of bacteriorhodopsin. This complexation is investigated in various states of hydration, in H2O and D2O, at a variety of pH levels, with mutant membranes and labeled retinals. The complexation reaction was also investigated using absorption spectroscopy and vibrational spectra using difference Fourier transform infrared spectroscopy. The results demonstrate the crucial role of water in controlling the protein conformations that lead to protein/ligand binding reactions and begin to shed new light on the protein control of a reaction that normally cannot take place in an aqueous medium.