Biological diversity of photosynthetic reaction centers and the solid-state photo-CIDNP effect

Abstract

Photosynthetic reaction centers (RCs) from plants, heliobacteria and green sulphur bacteria has been investigated with photochemically induced dynamic nuclear polarization (photo-CIDNP) MAS NMR. In photosystem (PS) I of spinach, all signals appear negative which is proposed by a predominance of the three spin mixing (TSM) over the differential decay (DD) mechanism. There are contrasting magnetic-field dependence of photo-CIDNP of PSI and PSII. For PSII the optimal NMR enhancement factor of ~5000 is observed at 4.7 T, while the strongest light-induced signals of PSI are at 9.4 T. The simulations indicate that difference between bacterial RCs and plant PS I can be due to an increase of the exchange coupling between the donor and acceptor radicals. In the RCs of Chlorobium tepidum the spectra appear negative and can be tentatively assigned to two bacterio chlorophyll a molecules of the donor. The spectral pattern obtained from membrane fragments of Heliobacillus mobilis at 4.7 T, appear to be both positive and negative, which is similar to the pattern observed in the RCs of plant PSII and RCs of Rhodobacter sphaeroides R-26. However, unlike the other RCs, in this system, at 17.6 T, the positive signals undergo a sign change and appear negative.