ENDOR and ESEEM of the 15N labelled radical cations of chlorophyll a and the primary donor P700 in photosystem I Original Research Article

The hyperfine couplings of the nitrogen nuclei in the radical cations of both 15N-labelled chlorophyll a and the primary donor P700 in Photosystem I of Synechococcus elongatus and spinach (Spinacea oleracea) in frozen solutions were investigated by ENDOR and, for confirmation, by two-dimensional ESEEM techniques. In addition, 1H ENDOR experiments were performed on these compounds. The experimental 15N hyperfine couplings of the chlorophyll a radical cation are compared with theoretical ones obtained by RHF-INDO/SP calculations and with the respective hyperfine couplings in the closely related 15N-bacteriochlorophyll a radical cation. Based on the observed 15N and 1H hyperfine couplings two possible models are discussed for P700+: (a) the special pair model with a strongly asymmetric spin density distribution over the dimer halves; (b) the model of a strongly perturbed chlorophyll a monomer.