Theory of Anisotropic Circular Dichroism of Excitonically Coupled Systems: Application to the Baseplate of Green Sulfur Bacteria

Abstract

A simple exciton theory for the description of anisotropic circular dichroism (ACD) spectra of multichromophoric systems is presented that is expected to be of general use for the analysis of structure-function relationships of molecular aggregates such as photosynthetic light-harvesting antennae. The theory is applied to the baseplate of green sulfur bacteria. It is demonstrated that only the combined analysis of ACD and circular dichroism (CD) spectra for the present baseplate bacteriochlorophyll (BChl) a dimer allows for an unambiguous determination of the parameters of the exciton Hamiltonian from experimental data. The analysis of experimental absorption and linear dichroism spectra suggests that either the NMR structure has to be refined or in addition to the dimers seen in the NMR structure and in the CD and ACD spectra, BChl a monomers are present in the baseplate carotenosome sample. A refined dimer structure is presented, explaining all four optical spectra.

Polymorphic chiral squaraine crystallites in textured thin films

“…This is expected for the amorphous thin film due to the absence of distinct excitonic molecular coupling . For this microcrystalline thin films of ( R,R )‐PyrSQ‐C1, the CD response is anisotropic . The optical activity tensor α for a monoclinic crystal with space group C 2 has the following non‐diagonal form: …”

Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment

“…2,26 ACD can provide, in principle, additional information inaccessible from CD and LD, that imposes constraints on the possible geometries and electronic couplings between pigment molecules. 20,23,26 The applicability and usefulness of ACD for extracting or validating the excitonic Hamiltonian and cancel each other in the isotropic phase. This was shown both theoretically and experimentally for the chlorosome baseplate protein CsmA in green-sulfur photosynthetic bacteria, which effectively contains dimers of bacteriochlorophyll a.…”

“…In addition to the measured face- ; gray curve -edge-aligned ACD calculated as (3CD iso − ACD face )/2. 22,26 The spectra are plotted in units of ∆A 10 −3 (mOD) and normalized to unity isotropic absorbance at 675 nm. For clarity, the ACD spectra are shifted vertically by 0.5.…”

“…36 Recently, an exciton-theory-derived formalism of ACD has been developed. 26 Here we tested the predictive power of the exciton Hamiltonian proposed earlier 10 by calculating ACD spectra without further refinement of the model parameters.…”

Theoretical Study on the Optical Properties of Multichromophoric Systems Based on an Exciton Approach: Modification Guidelines

“…The excited‐state geometry of the multichromophoric system was optimized using same computational level with the reorganization energy except for the application of the TD‐DFT to the low‐level region of the ONIOM (QM:QM′) method to consider environmental effects in the excited state. In the large molecular aggregates such as crystals, anisotropic effects should be significant for the optical properties . The readers can refer to Refs.…”