Reconstituted LH2 in multilayer membranes induced by poly-l-lysine: Structure of supramolecular and electronic states

Abstract

To explore the effect of cell membrane stacking on the function of light-harvesting complex 2 (LH2) in purple nonsulfur photosynthetic bacteria, LH2 from Rhodobacter sphaeroides 2.4.1 (R. sph 2.4.1) was reconstituted into lipid bilayer vesicles (LH2@liposome) and further formed multi-layer structure by electrostatic interaction with poly-l-lysine (LH2@liposome/PLL), which was characterized by cryo-electron microscopy (cryo-EM) and TEM. When embedded in liposomes and additionally in multi-layer liposomes, the absorption band, zero-crossing point of CD signals and fluorescence emission of LH2 B850 excitons were observed to uniformly have 1–2 nm red-shifting. Combining with the corresponding fluorescence quench and the generation of shorter-living fluorescence species, a new excitonic species generated through B850 structural splitting was proposed. By FT-Raman and triplet carotenoid dynamics, the structural mechanism was deduced and discussed. Briefly, all environmental changes, including LH2 aggregating and multi-layer membrane stacking, eventually applied forces on B850 exciton molecules mainly through the hydrogen bonding between the C3-acetyl carbonyl groups of B850 BChls and Tyr44 and 45 residues at C-terminus of α-polypeptides. The effect of multi-layer structure on LH2 could be assigned as a kind of photoprotection.