A NMR study on the bacteriochlorophyll(BChl) a in a reconstituted subunit of light-harvesting complex

Abstract

Although a number of X-ray crystal structures have been determined for the reaction center and peripheral light-harvesting complex(LH2), molecular organization of the BChl a in core light-harvesting complex (LH1) of purple photosynthetic bacteria has not been determined to atomic resolution. We report here the first observation of NMR signals from intact BChl a molecules in a reconstituted subunit of LH1 complex solubilized in detergent solution, and show several important features of the complexation-induced spectral changes based on the assigned resonances. A modified reconstitution method has been developed for preparing highly concentrated sample appropriate to NMR measurements. Apopolypeptides of LH1 were isolated from Rhodospirillum rubrum. Fully and partially 13C-labeled BChl a molecules were incorporated to form pigment-protein complex. The reconstituted complex exhibits a Qy band at 820 nm, a characteristic of subunit of the LH1 complex. 2D 1H-13C correlation spectra were acquired and show significant signal broadening for all protons and proton-attached carbons of the BChl a, indicating a largely reduced molecular motion upon introduction of the BChl a into the polypeptide-detergent micelles. The most significant feature is that many proton resonances are multi-split with nonsymmetric spectral shapes. The results strongly suggest that BChl a molecules in the B820 subunit exist in a nonequivalent configuration where the corresponding functional groups are surrounded by different local environments. Using selective labeling and comparing with the spectrum of intact BChl a in organic solvent, we were able to specifically assign all the resonances of BChl a in the B820 complex.