Ultrahigh field MAS NMR dipolar correlation spectroscopy of the histidine residues in light-harvesting complex II from photosynthetic bacteria reveals partial internal charge transfer in the B850/His complex

Low-temperature N-15 and C-13 CP/MAS (cross-polarization/magic angle spinni ng) NMR has been used to analyze BChl-histidine interactions and the electr onic structure of histidine residues in the light-harvesting complex II (LH 2) of Rhodopseudomonas acidophila. The histidines were selectively labeled at both or one of the two nitrogen sites of the imidazole ring. The resonan ces of histidine nitrogens that are interacting with B850 BChl a have been assigned. Specific N-15 labeling confirmed that it is the tau -nitrogen of histidines which is ligated to Mg2+ of B850 BChl molecules (beta -His30, al pha -His31). The pi -nitrogens of these Mg2+-bound histidines were found to be protonated and may be involved in hydrogen bond interactions. Compariso n of the 2-D MAS NMR homonuclear (C-13-C-13) dipolar correlation spectrum o f [C-13(6), N-15(3)]-histidines in the LH2 complex with model systems in th e solid state reveals two different classes of electronic structures from t he histidines in the LH2. In terms of the C-13 isotropic shifts, one corres ponds to the neutral form of histidine and the other resembles a positively charged histidine species. N-15-C-13 double-CP/MAS NMR data provide eviden ce that the electronic structure of the histidines in the neutral BChl a/Hi s complexes resembles the positive charge character form. While the Mg . . . N-15 isotropic shift confirms a partial positive charge transfer, its ani sotropy is essentially of the lone pair type. This provides evidence that t he hybridization structure corresponding to the neutral form of the imidazo le is capable of "buffering" a significant amount of positive charge.