A refined model of the chlorosomal antennae of the green bacterium Chlorobium tepidum from proton chemical shift constraints obtained with high-field2-D and 3-D MAS NMR dipolar correlation spectroscopy

Heteronuclear 2-D and 3-D magic-angle spinning NMR dipolar correlation spec troscopy was applied to determine solid-state H-1 shifts for aggregated bac teriochlorophyll c (BChl c) in uniformly C-13- enriched light harvesting ch lorosomes of the green photosynthetic bacterium Chlorobium tepidum. A compl ete assignment of 29 different observable resonances of the 61 protons of t he aggregated BChl c in the intact chlorosomes is obtained, Aggregation shi fts relative to monomeric BChl c in solution are detected for protons attac hed to rings I, II, and III/V and to their side chains. The 2(1)-H-3, 3(2)- H-3, and 3(1)-H resonances are shifted upfield by -2.2, -1, and -3.3 ppm, r espectively, relative to monomeric BChl c in solution. Although the resonan ces are inhomogeneously broadened and reveal considerable global structural heterogeneity, the 5-CH and the 7-Me responses are doubled, which provides evidence for the existence of at least two relatively well-defined structu rally different arrangements. Ab initio quantum chemical modeling studies w ere performed to refine a model for the self-assembled BChl c with two diff erent types of BChl stacks. The BChl in the stacks can adopt either anti- o r syn-configuration of the coordinative bond, where anti and syn designate the relative orientation of the Mg-OH bond relative to the direction of the 17-17(1) bond. The analogy between aggregation shifts for BChl c in the ch lorosome and for self-assembled chlorophyll a/H2O is explored, and a bilaye r model for the tubular supra-structure of sheets of BChl c is proposed, fr om a homology modeling approach.