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Molecular Insight in the Optical Response of Tubular Chlorosomal Assemblies

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posted on 07.06.2019, 00:00 by Xinmeng Li, Francesco Buda, Huub J. M. de Groot, G. J. Agur. Sevink
A systematic procedure is developed for determining unique stackings of BChl molecules in different types of chlorosomes directly from characterization data, using information about: (i) the dimeric unit cell from nuclear magnetic resonance (NMR), (ii) local stacking distances from cryo-electron microscopy (cryo-EM), and (iii) large-scale chirality from absorption, linear-dichroism, and circular-dichroism spectra. To enable a comparison with optical data, we have employed a Frenkel Hamiltonian formalism to calculate optical properties of preassembled tubes of realistic 120 nm length. Our analysis for the first time explains the variability of optical signals measured for chlorosomes and points at chiral angles δ = 105* for BChl c and δ = 19.8° for BChl d that satisfy all information contained in parts i–iii within experimental and computational accuracy, where the asterisk denotes that the large-scale tube curvature is reversed. We also find that the dynamic disorder is not sensitive to a particular chirality. To investigate the role of disorder on excitonic features, we have represented correlated deviations from a crystalline order due to thermal energy, as extracted from all-atom molecular dynamics (AAMD) calculations, directly in the coupling terms of the Frenkel Hamiltonian, assuming that variations of site energies can be neglected. The use of AAMD snapshot for sampling relevant molecular conformations allows us to study molecular origins of important emergent phenomena in chlorosomes. We find that our model reproduces two mechanismsexciton localization and level crossingthat have been proposed to play a key role in the transfer of excitonic energy. This highlights the importance of the usually disregarded rotation-related electronic coupling variations in the exciton properties.