The role of chirality and plastic crystallinity in the optical and mechanical properties of chlorosomes

Abstract

The most efficient light-harvesting antennae found in nature, chlorosomes, are molecular tubular aggregates (TMAs) assembled by pigments without protein scaffolds. Here, we discuss a classification of chlorosomes as a unique tubular plastic crystal and we attribute the robust energy transfer in chlorosomes to this unique nature. To systematically study the role of supramolecular tube chirality by molecular simulation, a role that has remained unresolved, we share a protocol for generating realistic tubes at atomic resolution. We find that both the optical and the mechanical behavior are strongly dependent on chirality. The optical-chirality relation enables a direct interpretation of experimental spectra in terms of overall tube chirality. The mechanical response shows that the overall chirality regulates the hardness of the tube and provides a new characteristic for relating chlorosomes to distinct chirality. Our protocol also applies to other TMA systems and will inspire other systematic studies beyond lattice models.

Manifestation of Hydrogen Bonding and Exciton Delocalization on the Absorption and Two-Dimensional Electronic Spectra of Chlorosomes

“…The dynamic nature of the disorder gives rise to efficient long-range transfer of the energy absorbed by the antenna as revealed by pump–probe and two-dimensional electronic spectroscopy, and it determines the optical properties of the chlorosomes. For understanding these properties it is, thus, crucial to understand these exciton states and how the localization that inevitably arises from the static heterogeneity is overcome by the dynamic heterogeneity in the plastic crystalline chlorosome self-assembly for rapid redistribution of exciton density and energy transfer …”

“…This may represent a commonality that is preserved across different chlorosomes to sustain their light harvesting function. 17 Due to the close packing of the bacteriochlorophyll molecules, the excited states responsible for light absorption are strongly coupled. 18 At the same time the structural disorder and interaction between different molecules leads to variation in the couplings and excitation energies of the individual bacteriochlorophyll molecules.…”

“…For understanding these properties it is, thus, crucial to understand these exciton states and how the localization that inevitably arises from the static heterogeneity is overcome by the dynamic heterogeneity in the plastic crystalline chlorosome self-assembly for rapid redistribution of exciton density and energy transfer. 17 At an early stage, theoretical descriptions have been developed starting with idealized models considering tubes with a fixed crystalline structure. 18,19 While this allowed for basic understanding of the spectral features, the extreme exciton delocalization of individual exciton states is unphysical, as localization caused by the presence of the disorder is not included.…”

“…This may represent a commonality that is preserved across different chlorosomes to sustain their light harvesting function. 17 …”

“…For understanding these properties it is, thus, crucial to understand these exciton states and how the localization that inevitably arises from the static heterogeneity is overcome by the dynamic heterogeneity in the plastic crystalline chlorosome self-assembly for rapid redistribution of exciton density and energy transfer. 17 …”