EPR on biological samples beyond the limits of superconducting magnets - The primary donor cation of purple bacterial photosynthesis

The g-matrix of a free radical is an important observable that yields information on its electronic structure. It is usually measured by electron paramagnetic resonance (EPR) under "high field" conditions, where the spectral splitting of the principal g-factor components is larger than the line width due to unresolved hyperfine splitting. For large organic molecules such as the primary electron donor in photosynthetic reaction centers (RC) this usually requires fields above ii T, or, for fields between 3 and 11 T, full deuteration and/or single-crystal work. When trying to obtain improved spectral resolution a major concern is the presence of g-strain which leads to extra line broadening. Here we show that g-strain is negligible for bacterial RCs up to a field of 24 T. We investigated the temperature dependence of the g-anisotropy for RCs from Rhodobacter (Rb.) sphaeroides using different detergents and find that within experimental errors there is no change in the principal g-matrix components up to 200 K. This is the first report of a successful EPR experiment on a biological sample above the limits of superconducting magnets.