Bacteriochlorophyll/imidazole and chlorophyll/imidazole complexes are negatively charged in an apolar environment

600 MHz N-15-NMR and 2D homonuclear H-1-H-1 and heteronuclear H-1-N-15-NMR data for bacteriochlorophyll a/imidazole and chlorophyll a/imidazole have b een recorded. Unambiguous assignments of the N-15 signals of the two nitrog ens of imidazole ligated to the Mg of the bacteriochlorophyll a or chloroph yll a were obtained. It follows that the imidazole in complex is deprotonat ed, which implies that the chlorophyll/imidazole carries a full negative ch arge in the apolar solvent environment. This information is potentially use ful in characterizing the nature of the magnesium-histidine interaction and the charge state of chlorophylls coordinated by histidine in photosyntheti c pigment protein complexes. (C) 2000 Elsevier Science B.V. All rights rese rved.