The Redox Potential Dependence of Coupled Electron Flow in Chromatophores from Rhodopseudomonas Spheroides

Abstract

The nature of the red shift of the carotenoid absorption bands of photosynthetic bacteria has been uncertain since the demonstration that both oxygenation and illumination of suspensions of the intact organisms elicited similar spectral shifts (SMITH & RAMIREZ 1960). The inhibitor, uncoupler and ionophore sensitivity of the shifts (FLEISHHMAN & CLAYTON 1968) strongly suggest that the reactions involved are not simply those of oxidation or reduction of the components of the photosynthetic or respiratory electron transport chains. Carotenoid shifts may also be induced by ATP or pyrophosphate addition to chromatophore suspensions (BALTSCHEFFSKY 1967) or by generating electrical fields across the chromatophore membranes with ion gradients (JACKSON & CROFTS 1969). It seems that the carotenoid red shifts are in some way related to the high energy state of the organism and that this state may be equivalent to or in equilibrium with a trans-membrane electrical potential (see also WITT et al. 1968). This hypothesis is supported by the rapid spectroscopic data reported by JACKSON & CROFTS (1971). In the present communication we have used the kinetic behaviour of the shift as an index of the high-energy state of chromatophores from Rhodopseudomonas spheroides.