The primary donor cation P+ in photosynthetic reaction centers of site-directed mutants of Rhodobacter sphaeroides: g-tensor shifts revealed by high-field EPR at 360 GHz/12.8 T

The frozen solution electron paramagnetic resonance spectrum of the primary donor cation P + in reaction centers of site-directed mutants of Rhodobacter ( Rb. ) sphaeroides has been obtained at a microwave frequency ν =360 GHz and a magnetic field B 0 =12.8 T. Due to the high Zeeman resolution of the powder pattern, all three principal components of the rhombic g -tensors at T =160 K could be determined with high accuracy. We compare spectra of the site-directed mutants, in which the axial ligand histidine M202 of the primary donor is replaced by glutamic acid (HE(M202)) or leucine (HL(M202)), with those of the strain R26, whose primary donor is similar to that of the wild type and only lacks the carotenoid. For HE(M202), this is the first determination of its g -tensor with the principal components g xx =2.00335(3), g yy =2.00236(2) and g zz =2.00191(2). While in R26 the primary donor is a bacteriochlorophyll a dimer, the HL(M202) and HE(M202) mutants have previously been shown to be bacteriochlorophyll:bacteriopheophytin heterodimers. Their g -tensor anisotropy Δ g = g xx − g zz shows significant variations in opposite directions when compared with R26, with an increased anisotropy for HE(M202) and a decreased one for HL(M202). Calculations employing Density Functional Theory suggest that the observed shifts originate in different torsional angles of the acetyl group attached to the spin-carrying bacteriochlorophyll half L of the dimer.