Natural abundance solid-state carbon NMR studies of photosynthetic reaction centers with photoinduced polarization

Solid-state NMR spectra of natural abundance C-13 in reaction centers from photosynthetic bacteria Rhodobacter sphaeroides R-26 was measured. When the quinone accepters were removed and continuous visible illumination of the sample was provided, exceptionally strong nuclear spin polarization was observed in NMR lines with chemical shifts resembling those of the aromatic carbons in bacteriochlorophyll and bacteriopheophytin. The observation of spin polarized N-15 nuclei in bacteriochlorophyll and bacteriopheophytin was previously demonstrated with nonspecifically N-15-labeled reaction centers. Both the carbon and the nitrogen NMR studies indicate that the polarization is developed on species that carry unpaired electrons in the early electron transfer steps, including the bacteriochlorophyll dimer donor P860 and probably the bacteriopheophytin acceptor I. Both enhanced-absorptive and emissive polarization were seen in the carbon spectrum; most lines were absorptive but the methine carbons of the porphyrin ring (alpha, beta, gamma, delta) exhibited emissive polarization. The change in the sign of the hyperfine coupling at these sites indicates the existence of nodes in the spin density distribution on the tetrapyrrole cofactors flanking each methine carbon bridge.