New proposal for structure of special-pair chlorophyll

Abstract

A new model was proposed for the structure of the special pair of chlorophyll a molecules believed to correspond to the P700 species in green plants and algae. The proposed model, although admittedly speculative, was based upon exciton-theoretical considerations and on in vitro IR and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, 2 chlorophyll a molecules are held together by: 2 strong ring V keto C.dbd.O.sbd.H.sbd.O (or keto C.dbd.O.sbd.H.sbd.N, or keto C.dbd.O.sbd.H.sbd.S) hydrogen bonds and by .pi.-.pi. van der Waals stacking interactions between the 2 chlorophyll a macrocycles. Macrocycle stacking provides the intermolecular .pi.-.pi. overlap necessary to promote the (experimentally observed) delocalization of the unpaired electron of the in vivo radical over 2 chlorophyll .pi. systems. The new model provides an explicit role for the participation of protein in the formation of the chlorophyll special pair. The in vitro system yielded an ESR signal indistinguishable from that of oxidized P700 in Chlorella vulgaris.