Whole Cell Nuclear Magnetic Resonance Characterization of Two Photochemically Active States of the Photosynthetic Reaction Center in Heliobacteria

Applying photo-CIDNP (photochemically induced dynamic nuclear polarization) MAS (magic angle spinning) nuclear magnetic resonance to whole cells of Heliobacillus (Hb.) mobilis, we demonstrate that heliobacterial reaction centers are operational in two different states as indicated by the occurrence of a light induced spin correlated radical pair. A culture maintained anaerobically is called Braunstoff (German for brown substance). After exposure to oxygen, Braunstoff is converted to Grunstoff (green substance) as indicated by a color change due to the conversion of BChl g to Chl a(F). It is shown that electron transfer occurs symmetrically via both branches of cofactors in both forms. The donor and acceptor cofactors remain identical and unchanged upon conversion, while the intermediate accessory cofactors are transformed from BChl g to Chl a(F). The donor triplet state in Braunstoff is localized on the special pair donor and lives for 100 mu s, demonstrating the absence of nearby carotenoids. In Grunstoff, the donor triplet becomes mobile and appears to be formed on an accessory cofactor.