Nuclear magnetic resonance secondary shifts of a light-harvesting 2 complex reveal local backbone perturbations induced by its higher-order interactions.

Protein nuclear magnetic resonance (NMR) secondary chemical shifts are widely used to predict the secondary structure, and in solid-state NMR, they are often the only unambiguous structural parameters available. However, the employed prediction methods are empirical in nature, relying on the assumption that secondary shifts are only affected by shielding effects of neighboring atoms. We analyzed the secondary shifts of a photosynthetic membrane protein with a high density of chromophores and very tight packing, the light-harvesting 2 (LH2) complex of Rhodopseudomonas acidophila. A relation was found between secondary shift anomalies and protein-protein or pigment-protein tertiary and quaternary contacts. For several residues, including the bacteriochlorophyll-coordinating histidines (alphaH31 and betaH30) and the phenylalanine alphaF41 that has strongly twisted C(b)-C(a)-C and C(a)-C-N conformations in the LH2 crystal structure, the perturbing effects on the backbone chemical shifts were tested by density functional theory (DFT) calculations. We propose that higher-order interactions in the tightly packed complex can induce localized perturbations of the backbone conformation and electronic structure, related to functional pigment-protein or protein-protein interactions.