A quantum mechanical analysis of the light-harvesting complex 2 from purple photosynthetic bacteria. Insights into the electrostatic effects of transmembrane helices

摘要

We perform a quantum mechanical study of the peptides that are part of theLH2 complex from Rhodopseudomonas acidophila, a non-sulfur purple bacteria thathas the ability of producing chemical energy from photosynthesis. Theelectronic structure calculations indicate that the transmembrane helices ofthese peptides are characterized by dipole moments with a magnitude of ~150 D.When the full nonamer assembly made of eighteen peptides is considered, then amacrodipole of magnitude 704 D is built up from the vector sum of each monomerdipole. The macrodipole is oriented normal to the membrane plane and with thepositive tip toward the cytoplasm thereby indicating that the electronic chargeof the protein scaffold is polarized toward the periplasm. The results obtainedhere suggest that the asymmetric charge distribution of the protein scaffoldcontributes an anisotropic electrostatic environment which differentiates theabsorption properties of the bacteriochlorophyll pigments, B800 and B850,embedded in the LH2 complex.