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

TL;DR

This study uses quantum mechanical calculations to analyze the electrostatic effects of transmembrane helices in the LH2 complex from purple bacteria, revealing how protein charge distribution influences pigment absorption.

Contribution

It provides a detailed quantum mechanical insight into the electrostatic properties of LH2 peptides and their impact on pigment behavior, which was not previously characterized at this level.

Findings

Transmembrane helices have dipole moments of ~150 D.

The nonamer assembly creates a macrodipole of 704 D oriented normal to the membrane.

Electrostatic charge distribution affects pigment absorption properties.

Abstract

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