Structure and function of photoreaction-centre chlorophyll.

Evidence from electron paramagnetic resonance (e.p.r.) studies suggests that the unpaired spin in oxidized P700+. or P865+. is shared by two special chlorophyll a (Chl a) or bacteriochlorophyll a (Bchl a) molecules respectively. Three classes of models have been proposed for special pair reaction centre chlorophyll: asymmetric, in which one Chl a (or Bchl a) acts as electron donor to a second acting as acceptor; models with translational symmetry only; and models with C2 symmetry. Models with C2 symmetry have been synthesized in vitro with two chlorophyll macrocycles tied together by a covalent link. The singlet and triplet states of the in vitro models have been characterized by e.p.r., nuclear magnetic resonance, and optical studies involving absorption, emission, and lasting behaviour. The fact that lasting occurs only from the folded configuration of the linked dimers suggests the availability of a highly effective nonradiative decay path from the S1 state of the excited open dimer. A radical-pair mechanism that accounts for the unusual spin polarization of the special pair triplet is proposed for the primary photochemistry in the reaction centre.