Blue light effect on proton pumping by bacteriorhodopsin.

Proton pumping in closed vesicular systems containing bacteriorhodopsin that is initiated by an orange flash, is diminished by a subsequent blue flash. This blue light effect is due to light absorbed by the photocycle intermediate M412 (M), which was formed by the orange flash. A kinetic analysis of the blue-light-induced reduction of proton pumping shows that of the two components of M, only the slowly decaying component is involved in the reduction of proton movement. This may be the first correlation between a proton movement and a specific photochemical intermediate of bacteriorhodopsin. Furthermore, we report that blue light, acting on the slowly decaying intermediate, probably causes a movement of the protons in a direction opposite to that normally seen for light absorbed by bacteriorhodopsin.