| Abstract
| - The electronic ground states of the bacteriochlorophyll a type B800 and type B850 in the light-harvesting 2 complex of Rhodopseudomonas acidophila strain 10050 have been characterized by magicangle spinning (MAS) dipolar 13C−13C correlation NMR spectroscopy. Uniformly [13C,15N] enriched light-harvesting 2 (LH2) complexes were prepared biosynthetically, while [13C,15N]-B800 LH2 complexes wereobtained after reconstitution of apoprotein with uniformly [13C,15N]-enriched bacteriochlorophyll cofactors.Extensive sets of isotropic 13C NMR chemical shifts were obtained for each bacteriochlorin ring species inthe LH2 protein. 13C isotropic shifts in the protein have been compared to the corresponding shifts ofmonomeric BChl a dissolved in acetone-d6. Density functional theory calculations were performed to estimatering current effects induced by adjacent cofactors. By correction for the ring current shifts, the 13C shifteffects due to the interactions with the protein matrix were resolved. The chemical shift changes providea clear evidence for a global electronic effect on the B800 and B850 macrocycles, which is attributed tothe dielectrics of the protein environment, in contrast with local effects due to interaction with specific aminoacid residues. Considerable shifts of −6.2 < Δσ < +5.8 ppm are detected for 13C nuclei in both the B800and the B850 bacteriochlorin rings. Because the shift effects for the B800 and B850 are similar, thepolarization of the electronic ground states induced by the protein environment is comparable forboth cofactors and corresponds with a red shift of ∼30 nm relative to the monomeric BChl dissolved inacetone-d6. The electronic coupling between the B850 cofactors due to macrocycle overlap is thepredominant mechanism behind the additional red shift in the B850.
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