The triplet state in bacterial photosynthesis: Possible mechanisms of the primary photo-act.

In vitro and in vivo triplet state electron paramagnetic resonance (epr) spectra of bacteriochlorophylls (Bchls) show important differences in (a) electron spin polarization (esp), and (b) zero field splitting (ZFS) parameters. The unusual esp and ZFS properties of the observed in vivo triplet state are best interpreted as arising from a short-lived radical pair precursor (not directly observable by epr) formed in or with the special pair of bacteriochlorophyll molecules involved in the primary photo-act.