New Proposal for Structure of Special-Pair Chlorophyll

A new model is 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, is based upon exciton-theoretical considerations and on in vitro infrared and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, two chlorophyll a molecules are held together by (a) two strong ring V keto [Note: See the image of page 1791 for this formatted text] C==O⋯ H-O (or keto C==O⋯ H-N, or keto C==O⋯ H-S) hydrogen bonds and by (b) π -π van der Waals stacking interactions between the two chlorophyll a macrocycles. Macrocycle stacking provides the intermolecular π -π overlap necessary to promote the (experimentally observed) delocalization of the unpaired electron of the in vivo radical over two chlorophyll π 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 yields an electron spin resonance signal indistinguishable from that of oxidized P700 in Chlorella vulgaris.