Spin-torch experiment on reaction centers of Rhodobacter sphaeroides

Abstract

Photosynthesis is the physico-chemical process by which plants, algae and photosynthetic bacteria use light energy to drive the synthesis of organic compounds. Light-induced electron transfer in photosynthetic reaction centers (RCs) is highly efficient, having a quantum yield close to unity. In RCs of Rhodobacter (R.) sphaeroides wild type (WT), the primary electron donor is a bacteriochlorophyll a (BChl) dimer, called the Special Pair P, comprising two dimer halves PL and PM. Two additional BChl cofactors called accessory BChls (BA and BB), two bacteriopheophytins (_A and _B), two quinones and a non-heme iron are organized into two pseudo-symmetric branches named A and B. After the photo excitation the electron is transferred only via the active __A__ branch. On the other hand, in structurally similar RCs, as that of photosystem I, the electron transfer occurs equally over both branches. Neither the reason for the high efficiency nor that of the directionality of the electron transfer has been elucidated so far. To solve these questions, the solid-state photo-chemically induced nuclear polarization (photo-CIDNP) effect with its dramatic enhancement of local NMR signals provides an analytical tool especially suited for studying electron transfer in photosynthetic RCs. In fact, photo-CIDNP MAS NMR has been applied to explore electronic structures of the electron donors and acceptors in RCs