Shifts of bacteriochlorophyll and carotenoid absorption bands linked to cytochrome c-555 photooxidation in Chromatium

Abstract

Shifts in the absorption bands of bacteriochlorophyll and carotenoids in Chromatium vinosum chromatophores were measured after short actinic flashes, under various conditions. The amplitude of the bacteriochlorophyll band shift correlated well with the amount of cytochrome c-555 that was oxidized by P 870 + after a flash. No bacteriochlorophyll band shift appeared to accompany the photooxidation of P 870 itself, nor the oxidation of cytochrome c-552 by P 870 +. The carotenoid band shift also correlated with cytochrome c-555 photooxidation, although a comparatively small carotenoid shift did occur at high redox potentials that permitted only P 870 oxidation. The results explain earlier observations on infrared absorbance changes that had suggested the existence of two different photochemical systems in Chromatium. A single photochemical system accounts for all of the absorbance changes. Previous work has shown that the photooxidations of P 870 and cytochrome c-555 cause similar changes in the electrical charge on the chromatophore membrane. The specific association of the band shifts with cytochrome c-555 photooxidation therefore argues against interpretations of the band shifts based on a light-induced membrane potential.