Quadrupole parameters of nitrogen nuclei in the cation radical P700.+ determined by ESEEM of single crystals of phtosystem I

The cation radical of the primary donor P700 of photosystem I (PS I) is investigated by ESEEM on single crystals of PS I from the cyanobacterium Synechococcus elongatus. The nuclear quadrupole parameters K and η of five different 14N nuclei are determined and 14N hyperfine coupling (hfcʹs) of P700.+ are evaluated. For comparison the cation of methyl-chlorophyllide a is studied in frozen solution. The 14N hfc data agree with published results for the 15N-labeled cation radicals of P700 and chlorophyll a and are discussed in terms of a chlorophyll a dimer model for the cation radical P700.+.