PRIMARY REACTIONS IN BACTERIAL PHOTOSYNTHESIS—III. REACTIONS OF CAROTENOIDS AND CYTOCHROMES IN ILLUMINATED BACTERIAL CHROMATOPHORES

Summary Washed or air‐dried chromatophores of photosynthetic bacteria show a reversible light‐induced bleaching and red shift of the absorption bands of carotenoids. This change is sensitized by light absorbed by bacteriochlorophyll (BChl). The carotenoid reaction is abolished by exposure to deoxycholate. In dried preparations, the reaction is optimal in the presence of an extremely low water vapor concentration. All of the light‐induced absorbancy changes in chromatophores are suppressed reversibly by exhaustive desiccation. Deoxycholate treatment and drying do not affect the efficiency of energy transfer from carotenoids to BChl in Rhodopseudomonas spheroides chromatophores, nor do these treatments affect the way in which carotenoids modify the absorption spectrum of BChl. The carotenoid reaction in washed or dried chromatophores is interpreted as arising from the diffusion of electrons or holes from the neighborhood of a special BChl component (BChl,) to the carotenoid molecules. Dark‐adapted Chromatium chromatophores show, in addition to the light‐induced alteration of BChl2 the reversible light‐induced oxidation of a cytochrome. Correspondingly, R. spheroides chromatophores show the oxidation of cytochrome c553. The cytochrome reaction occurs reversibly in dried films of Chromatium chromatophores; it is absent in dried chromatophores of R. spheroides. In Chromatium the reactions of cytochrome and BChl, show a close kinetic relationship. It is inferred that BChl, is a photosynthetic reaction center that traps excitation energy and initiates the electron‐transfer processes of photosynthesis.