Cation removal or acidification induces a transition of bacteriorhodopsin (BR568) to a blue species (BR605). Fourier transform infrared spectroscopy now reveals that the protein conformational changes accompanying this transition resemble those which occur during the BR568 to M412 transition and associated light-driven proton transport. This and other results suggest that BR605 and M412 both have similar voltage-dependent protein conformations in which a secondary structure is stabilized by a change in the local electric potential sensed by internal charged groups involved in proton transport.
