Characterization of P700 by FTIR Difference Spectroscopy

Abstract

The primary photochemical act in photosynthesis leads to the generation of the radical cation of a specialized chlorophyll (Chl) or bacteriochlorophyll (BChl) species. For purple bacteria, the primary electron donor is a pair of BChl whose molecular structure has been extensively investigated by various spectroscopic techniques and recently elucidated by X-ray cristallography of the bacterial reaction center. In the absence of high-resolution X-ray data for green plant PS I and PS II, proposals for the structure and bonding interactions of their primary donor rely only on spectroscopy. In particular, the primary donor of PS I, P700, is probably a Chl a dimer, although several spectroscopic studies suggest that the positive charge in the P700+ radical cation is localized on only one of the two Chl molecules that comprise P700 (1–3). Model studies also incorporate a possible keto-enol tautomerization of Chl a upon P700 photooxidation (4,5).