Primary events in bacterial photosynthesis

Abstract

Our knowledge of the primary charge-separation process in bacterial photosynthesis has been reviewed in the light of recent X-ray structural data on the photosynthetic reaction centre. Attention is drawn to the sequential orthogonality of the various chromophores and the possible involvement of (n, π^*) singlet and/or triplet electronic states as transient participants in the electron-transfer chain is inferred. It is proposed that the initially produced excited singlet state of the special pair lacks the vibrational energy needed to bring about the Franck–Condon nuclear reorganization that is a necessary prerequisite to electronic potential-energy hypersurface crossing in Marcus' theory of electron transfer. Fast isoenergetic internal conversion to a lower-lying electronic state can provide the required F–C reorganization energy and also confer irreversibility on the earliest steps in the charge-separation process. The reason for this is the very high density of vibronic states at the isoenergetic level of the receiving lower electronic state and the consequent large value of the recurrence time for regaining the superposition appropriate to reforming the initial excited electronic state. For the 3 ps step from P to H_L the lower electronic state is probably an (n, π^*) state. For the subsequent 200 ps step from H_L to Q_A a triplet state may be involved. Experimental support for this idea already exists in that removal of the non-haeme Fe atom slows this step by a factor of 50. Additional experiments to test the hypothesis are proposed.