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Light-induced charge redistribution in the retinal chromophore is required for initiating the bacteriorhodopsin photocycle.

Bacteriorhodopsin's photocycle is initiated by the retinal chromophore light absorption. It has usually been assumed that light primarily isomerizes a retinal double bond which in turn induces protein conformational alterations and biological activity. We have studied several artificial pigments derived from retinal analogues tailored to substantially reduce the light-induced chromophore polarization. The lack of chromophore polarization was reflected in an undetectable second harmonic generation (SHG) signal. It was revealed that these artificial pigments did not exhibit any detectable light-induced photocycle nor light acceleration of the hydroxylamine-bleaching reaction. We suggest that light-induced retinal polarization triggers protein polarization which controls the course of the isomerization reaction by determining the relative efficiency of forward versus back-branching processes.