Are syn‐ligated (bacterio)chlorophyll dimers energetic traps in light‐harvesting systems?

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Abstract
A recent study of the stereochemical details of chlorophyll ligation in photosystem I [Balaban et al., Biochim. Biophys. Acta 1556 (2002) 197-207] has revealed that only 14 chlorophylls out of the total 96 are ligated from the same side (syn) as the 17-propionic acid residue which is esterified with phytol. The syn chlorophylls are carefully surrounding the reaction center forming the inner core antenna system and their ligands have been strongly conserved in several species during evolution. We hypothesize here that the two dimers of closely spaced syn chlorophylls which are encountered within roughly 2 nm of P700 are the ultimate energetic traps of this light-harvesting system. Structurally very similar bacteriochlorophyll a dimers are encountered within the Fenna-Matthews-Olson protein complex and within the B850 ring of the LH2 complex of purple bacteria. The non-random disposal of these dimers lends support to our hypothesis that the syn ligation coupled with a strong excitonic interaction leads to the most red-shifted pigments in light-harvesting systems. We would like to encourage both theoretical and experimental studies to either prove or disprove this intriguing structure-function conjecture in view of designing efficient artificial light-harvesting systems.

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