The Kinetics of Light Induced Carotenoid Changes in Rhodopseudomonas spheroides and their Relation to Electrical Field Generation across the Chromatophore Membrane

Abstract

The shift of the carotenoid absorption spectrum in chromatophores prepared from Rhodo pseudomonas spheroides induced by different types of actinic illumination has been resolved into several distinct, kinetic phases. A saturating 20 ns pulse from a Q‐switched ruby laser elicited a spectral change more rapid than the response time of the mesuring apparatus (1 μs). A rapid change 10–20% greater than this was produced during a saturating 200 μs xenon flash. Additional slower phases (half‐resetimes approximately 300 μs and 5 ms) could be distingushed after the xenon flash. Continuous illumination produced a rapid change of similar extent to that produced by xenon flash but with a risetime limited by the light intensity, and an equally extensive slow change of half‐risetime approximately 83 ms. During prolonged illumination the carotenoid shift decayed by about 50% with an approximate half‐decay time of 30 s. The effects of certain electron‐transport inhibitors, uncoupling agents, ion‐transporting antibotics and permeant anions on these phases and upon the decay after illumination have been investigated. None of these agents markedly affected the rapid phase of the shift induced by flash illumination. The relation of this phase to membrane potential was indicated by the fact that its decay was stimulated during electrophoretic ion flux. These observations are consistent with the suggestion tht the rapid phase of the carotenoid shift is in response to separation of charge across the chromatophore membrane, between reaction cente bacteriochlorophyll and the primary acceptor. The effects of these specific chemical modifies on the slower pahses of the carotenoid shift were complex. They are discussed in terms of a model in which thes phases reflect the generation of a membrane potential as a result of electrogenic oxidation‐reduction processees. Antimycin A partly inhibited electron flow through such an electrogenic site(s). Valinomycin delayed the formation of the membrane potential by catalysing an initial, electrophoretic K+ efflux. Uncoupling agents causeed a partial but permanent collapse of the electrical field. Permeant anions produced a slight lowering of the steady‐state electrical field by raising the membrane conductivity. The contribution of the membrane potential to the high‐energy state leading to ATP synthesis is discussed. Copyright © 1971, Wiley Blackwell. All rights reserved