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

Article abstract of DOI:10.1016/S0009-2614(00)01117-9

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.