The Relationship between the Spatial Arrangement of Pigments and Exciton Transition Moments in Photosynthetic Light-Harvesting Complexes.

Roman Y Pishchalnikov,Denis D Chesalin,Andrei P Razjivin

doi:10.3390/ijms221810031

Abstract

Considering bacteriochlorophyll molecules embedded in the protein matrix of the light-harvesting complexes of purple bacteria (known as LH2 and LH1-RC) as examples of systems of interacting pigment molecules, we investigated the relationship between the spatial arrangement of the pigments and their exciton transition moments. Based on the recently reported crystal structures of LH2 and LH1-RC and the outcomes of previous theoretical studies, as well as adopting the Frenkel exciton Hamiltonian for two-level molecules, we performed visualizations of the LH2 and LH1 exciton transition moments. To make the electron transition moments in the exciton representation invariant with respect to the position of the system in space, a system of pigments must be translated to the center of mass before starting the calculations. As a result, the visualization of the transition moments for LH2 provided the following pattern: two strong transitions were outside of LH2 and the other two were perpendicular and at the center of LH2. The antenna of LH1-RC was characterized as having the same location of the strongest moments in the center of the complex, exactly as in the B850 ring, which actually coincides with the RC. Considering LH2 and LH1 as supermolecules, each of which has excitation energies and corresponding transition moments, we propose that the outer transitions of LH2 can be important for inter-complex energy exchange, while the inner transitions keep the energy in the complex; moreover, in the case of LH1, the inner transitions increased the rate of antenna-to-RC energy transfer.

Highlights

Taking into account the current number of studies [1] devoted to the comprehensive analysis of energy migration in photosynthetic pigment–protein light-harvesting complexes (LHCs), it is difficult to imagine that there are still some aspects of either physical or chemical phenomena occurring in LHCs that have not been touched upon by the keen investigators of photosynthesis
Along with a protein shell serving as a rigid skeleton, each LHC contains chlorophylls or bacteriochlorophylls, pheophytins, and carotenoids—the chromophores responsible for visible light absorption, energy transport, and charge separation [3]
Like the drosophila fly, which was the key object of research on the nature of genetic mutations, LHCs of purple bacteria have become a classic example of the successful application of quantum theories from solid physics to simulate the linear and nonlinear optical response of pigment–protein complexes [4]

Summary

Introduction

Taking into account the current number of studies [1] devoted to the comprehensive analysis of energy migration in photosynthetic pigment–protein light-harvesting complexes (LHCs), it is difficult to imagine that there are still some aspects of either physical or chemical phenomena occurring in LHCs that have not been touched upon by the keen investigators of photosynthesis. The chromophore locations are rigidly fixed in LHCs, creating unique reciprocal orientations of pigments for each complex [4] These orientations are not random; they determine a matrix of interaction energies between molecules that provides the characteristic paths of energy migration in the complex [5]. The photosynthetic apparatus of purple bacteria contains two types of LHCs—the core and peripheral complexes (named LH1 and LH2, respectively). Both LH1 and LH2 consist of small one-helix α- and β-subunits that form heterodimers as the main building blocks of the LHCs and serve as binding sites for BChl molecules [7,8,9]. Being absorbed by BChl molecules of LH2 and LH1, light quanta are transformed into excited states of the antenna and sequentially relax to the excited states of the RC, where the chemical reactions of charge separation occur [10,11,12,13]

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Conclusion