Excitonic interaction among three chromophores: An application to the purple membrane of H a l o b a c t e r i u m h a l o b i u m

Abstract

The spectroscopic properties of the purple membrane of Halobacterium halobium in the visible region are examined within the framework of an exciton model for the retinal chromophores in bacteriorhodopsin (bR). Both earlier x‐ray and electron diffraction studies as well as more recent circular dichroism (CD) measurements indicate that considerable chromophore interaction is likely in the purple membrane. This interaction disappears when the protein is solubilized. The exciton model accounts for the red shift of the visible absorption band in the membrane from the absorption peak seen in solubilized bR. It explains the approximate conservation of absorption intensity as well as the changes in the observed CD spectrum when bR is solubilized. The dipole moments of the Cl⁻ salt of the protonated N‐butyl Schiff base of all‐trans retinal in its electronic ground and first dipole allowed excited state have recently been measured. If these are introduced into the exciton model an estimate of the distance of adjacent retinal chromophores in purple membrane is obtained.