Light-induced spin polarization in type I photosynthetic reaction centres

The use of light-induced spin polarization to study the structure and funct ion of type I reaction centres is reviewed. The absorption of light by thes e systems generates a series of sequential radical pairs, which exhibit spi n polarization as a result of the correlation of the unpaired electron spin s. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important struc tural results from such studies on photosystem I (PS I) are summarized. Qui none exchange experiments which demonstrate the influence of protein-cofact or interactions on the polarization patterns are discussed. The results sho w that there are significant differences between the binding sites of the p rimary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I . Studies using spin-polarized EPR transients and spectra to investigate th e electron transfer pathway and kinetics are also reviewed. The results fro m PS I, green-sulphur bacteria and Heliobacteria are compared and the contr oversy surrounding the role of a quinone in the electron transfer in the la tter two systems is discussed. (C) 2001 Elsevier Science B.V. All rights re served.