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 been recorded. Unambiguous assignments of the N-15 signals of the two nitrogens of imidazole ligated to the Mg of the bacteriochlorophyll a or chlorophyll a were obtained. It follows that the imidazole in complex is deprotonated, which implies that the chlorophyll/imidazole carries a full negative charge in the apolar solvent environment. This information is potentially useful in characterizing the nature of the magnesium-histidine interaction and the charge state of chlorophylls coordinated by histidine in photosynthetic pigment protein complexes. (C) 2000 Elsevier Science B.V. All rights reserved.