Limitations of Linear Dichroism Spectroscopy for Elucidating Structural Issues of Light-Harvesting Aggregates in Chlorosomes

Abstract

Linear dichroism (LD) spectroscopy is a widely used technique for studying the mutual orientation of the transition-dipole moments of the electronically excited states of molecular aggregates. Often the method is applied to aggregates where detailed information about the geometrical arrangement of the monomers is lacking. However, for complex molecular assemblies where the monomers are assembled hierarchically in tiers of supramolecular structural elements, the method cannot extract well-founded information about the monomer arrangement. Here we discuss this difficulty on the example of chlorosomes, which are the light-harvesting aggregates of photosynthetic green-(non) sulfur bacteria. Chlorosomes consist of hundreds of thousands of bacteriochlorophyll molecules that self-assemble into secondary structural elements of curved lamellar or cylindrical morphology. We exploit data from polarization-resolved fluorescence-excitation spectroscopy performed on single chlorosomes for reconstructing the corresponding LD spectra. This reveals that LD spectroscopy is not suited for benchmarking structural models in particular for complex hierarchically organized molecular supramolecular assemblies.

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

“…Because f ld z1 for all measured LD spectra, we conclude that d ˛½15 ; 110 is consistent with experimental characterization. At the same time, it confirms the very recent conclusion that LD alone is insufficient to determine a particular value (Gu ¨nther et al, 2021). As shown in Figure 3D, the CD spectrum is most sensitive to the chirality and five distinct domains can be identified.…”

Probing size variations of molecular aggregates inside chlorosomes using single-object spectroscopy

“…SOLP spectroscopy gives much more information than LD spectroscopy, which measures the difference in the absorption spectra for two perpendicular polarization directions on an ensemble of oriented objects (molecules, aggregates, and nanostructures). 48 The SOLP studies of chlorosomes from the wild-type and two mutant species of Chlorobaculum tepidum were consistent with a tubular geometry of the BChl aggregates for the mutants and the majority of the wildtype chlorosomes. 30,49 They showed an energy difference between two mutually perpendicular exciton transitions in tubular aggregates, which varied between different chlorosomes.…”

Stress‐Induced Inversion of Linear Dichroism by 4,4′‐Bipyridine Rotation in a Superelastic Organic Single Crystal

“…The orientation and organization of molecules in crystals not only associate with the mechanical responses but also determine the optical properties of materials [34, 35] . In the process of SE or FE deformation, the distinct change in molecular orientations may tune the sum of transition moments for the molecules and lead to profound impacts on the optical properties of crystals.…”