EPR at 24 T of the primary donor radical cation from Blastochloris viridis

The <f>g</f>-matrix of photosynthetic pigments has been studied in the last decade due to the advent of high-field EPR techniques. It can be observed when the spectral splitting of the principal <f>g</f>-factor components is larger than the linewidth due to unresolved hyperfine splitting and if there is no <f>g</f>-strain obscuring it. For large organic molecules such as the primary electron donor in photosynthetic reaction centers (RC) this usually requires fields above 11 T, or, for fields between 3 and 11 T, full deuteration and/or single crystal work. Here we present for the first time the fully resolved rhombic EPR spectrum of the primary donor of Blastochloris viridis (formerly called Rhodopseudomonas viridis), a purple photosynthetic bacterium containing bacteriochlorophyll <f>b</f>. As was the case for Rhodobacter sphaeroides, <f>g</f>-strain is negligible for this radical up to a field of 24 T. The temperature dependence of the <f>g</f>-anisotropy is presented and compared with that of the bacteriochlorophyll <f>a</f>-containing Rb. sphaeroides and plant photosystem I. A slight shift in the principal components of the <f>g</f>-matrix is observed at temperatures below 70 K, where it becomes more axial. The experimental work is complemented with theoretical calculations for <f>g</f> using the semi-empirical INDO/S method as implemented in the program ZINDO. The theoretical results generally agree well with the experiment. This indicates that a satisfactory description of the anisotropic <f>g</f>-tensor for radical cations of large planar molecules like the chlorophylls as well as their aggregates, e.g., reaction center primary donor special pairs, is possible with this relatively cheap semi-empirical approach. [Copyright &y& Elsevier]