Proton ENDOR spectroscopy of the anion radicals of the chlorophyll primary electron acceptors in type I photosynthetic reaction centres

AbstractChlorin anion radicals have been produced by photoaccumulation of photosystem I and the type I reaction centresof the anoxygenic bacteria Chlorobium limicola and Heliobacterium chlorum. Proton electron nuclear double resonance(ENDOR) spectrometry of the photoaccumulated radicals demonstrates that the photoaccumulation technique is re-ducing chlorophyll anions rather than bacteriochlorophyll anions, indicating that photoaccumulation specifically re-duces the primary electron acceptors (A 0 ) in these anoxygenic reaction centres and is not reducing the antennabacteriochlorophylls. Detailed analysis of the in vivo proton ENDOR spectra in comparison with in vitro (bacte-rio)chlorophyll and (bacterio)pheophytin anion radicals points up differences between the environment of the A 0 inphotosystem I and the anoxygenic type I reaction centres. 2003 Elsevier B.V. All rights reserved. 1. IntroductionReaction centres are integral membrane pig-ment–protein complexes that convert the electro-magnetic energy of sunlight into chemicalpotential energy, thereby powering most of thebiological activity on the planet [1]. Reactioncentres are classed into one of two types accordingto the identity of the terminal electron acceptors[2]. In one group these are four-iron/four-sulphurclusters (Fe