Determination of the absolute orientation of the retinylidene chromophore in purple membrane by a second-harmonic interference technique.

The absolute direction of the retinal chromophore of bacterio-rhodopsin relative to the membrane plane is investigated by using an optical second-harmonic interference technique. Compared with the known adsorbed geometry of free retinylidene Schiff base on a glass substrate, our data indicate the beta-ionone ring of the chromophore of bacteriorhodopsin points away from the cytoplasmic surface of the purple membrane. The implication of this finding is discussed in light of other chemical and structural results on bacteriorhodopsin.