ENDOR study of charge migration in photosynthetic arrays of Rhodobacter sphaeroides.

The chemistry of bacterial photosynthesis begins in the photosynthetic reaction centre (RC), a protein complex containing a series of electron donor and acceptor molecules. Although the pigments of the RC can absorb light to operate the photochemistry, the bulk of the light is captured in special pigmented proteins, the light harvesting complexes (LHCs), that then transfer the energy to the RC. Ordinarily, the LHCs do not participate in chemical reactions during photosynthesis such that LHCs do not become oxidised upon light irradiation. However, upon chemical oxidation in the dark, cation radicals of bacteriochlorophyll (BChl) can be formed in the light harvesting complex 1 (LH1) of Rhodobacter sphaeroides. As observed by continuous-wave electron-paramagnetic resonance (EPR), the charges of the BChl(+) cations migrate rather freely about the LH1 complex as in a molecular wire. Remarkably, these LH1 molecular wires continue to function in the frozen, solid state. To investigate the nature of electron-hole transfer and to corroborate the process as revealed by EPR, electron-nuclear double resonance (ENDOR) was recorded at 80 K. ENDOR observed only monomeric bacteriochlorophyll cations. Their signal intensity decreased with increased oxidation while the EPR signal narrowed and increased in size. At the increased oxidation state, the possibility of spin-spin exchange between two BChl(+)s within LH1 versus electron-hole transfer is addressed. An energy landscape of the BChl(+)s in the LH1 is proposed to explain the EPR and ENDOR results.