EVIDENCE FOR CHROMOPHORE-CHROMOPHORE (EXCITON) INTERACTION IN PURPLE MEMBRANE OF HALOBACTERIUM-HALOBIUM

The visible circular dichroic spectrum of the purple membrane of H. halobium includes intense positive and negative bands with cross-over near the wavelength of the absorption maximum. The circular dichroic spectrum has 2 contributions: a positive band due to retinal bound to the protein and positive and negative bands due to the interaction of the chromophores of separate proteins. Bleached purple membrane was regenerated by successive additions of retinal. The shape of the circular dichroic spectrum was dependent on the percent of the retinal binding sites occupied. This dependence strongly suggests that the circular dichroic spectrum of the purple membrane has an important contribution from chromophore-chromophore interactions.