Abstract
The light-harvesting in photosynthetic purple bacteria can be tuned in response to the light conditions during cell growth. One of the used strategies is to change the energy of the excitons in the major fight-harvesting complex, commonly known as LH2. In the present study we report the first systematic investigation of the microscopic origin of the exciton tuning using three complexes, namely the common (high-light) and the low-light forms of LH2 from Rps. acidophila plus a third complex analogous to the PucD complex from Rps. palustris. The study is based on the combination of classical molecular dynamics of each complex in a lipid membrane and excitonic calculations based on a multiscale quantum mechanics/molecular mechanics approach including a polarizable embedding. From the comparative analysis, it comes out that the mechanisms that govern the adaptation of the complex to different light conditions use the different H-bonding environment around the bacteriochlorophyll pigments to dynamically control both internal and inter-pigment degrees of freedom. While the former have a large effect on the site energies, the latter significantly change the electronic couplings, but only the combination of the two effects can fully reproduce the tuning of the final excitons and explain the observed spectroscopic differences.
Highlights
Purple bacteria are photosynthetic organisms with a unique light-harvesting (LH) apparatus, which involves two types of pigment–protein complexes, the so called LH2 and LH1.1–4 In the photosynthetic membranes, the LH1 complexes surround the reaction center (RC), while the LH2 complexes are arranged more peripherally around the LH1–RC complex; the ratio of the LH2 complexes to LH1–RC complexes is regulated by the incident light intensity.[5,6] in some species of purple bacteria, an additional adaptation to changing light conditions is possible thanks to the modular composition of the LH complexes
We considered the effect of higher energy Charge transfer (CT) states between adjacent bacteriochlorophylls a (BChl), which can couple with the Qy excitations and nally lead to changes in the exciton energies
We have investigated the origin of the exciton tuning in LH2 complexes when grown in different light conditions
Summary
Purple bacteria are photosynthetic organisms with a unique light-harvesting (LH) apparatus, which involves two types of pigment–protein complexes, the so called LH2 and LH1.1–4 In the photosynthetic membranes, the LH1 complexes surround the reaction center (RC), while the LH2 complexes are arranged more peripherally around the LH1–RC complex; the ratio of the LH2 complexes to LH1–RC complexes is regulated by the incident light intensity.[5,6] in some species of purple bacteria, an additional adaptation to changing light conditions is possible thanks to the modular composition of the LH complexes. By identifying and quantifying the reasons of the successes and the limits of the adopted computational strategy, an explanation of the mechanisms that govern the adaptation to different light conditions is suggested in terms of a delicate interplay between the H-bonding network around the 18-meric ring and “intra” and “inter” pigment mobility. This explanation opens a new scenario for a structurebased mutagenesis strategy which has the goal to control the relative energy of the excitons
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