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

600 MHz 15N-NMR and 2D homonuclear 1H–1H and heteronuclear 1H–15N-NMR data for bacteriochlorophyll a/imidazole and chlorophyll a/imidazole have been recorded. Unambiguous assignments of the 15N 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.