Insights into the Excitonic States of Individual Chlorosomes from Chlorobaculum tepidum

Abstract

Green-sulfur bacteria have evolved a unique light-harvesting apparatus, the chlorosome, by which it is perfectly adapted to thrive photosynthetically under extremely low light conditions. We have used single-particle, optical spectroscopy to study the structure-function relationship of chlorosomes each of which incorporates hundreds of thousands of self-assembled bacteriochlorophyll (BChl) molecules. The electronically excited states of these molecular assemblies are described as Frenkel excitons whose photophysical properties depend crucially on the mutual arrangement of the pigments. The signature of these Frenkel excitons and its relation to the supramolecular organization of the chlorosome becomes accessible by optical spectroscopy. Because subtle spectral features get obscured by ensemble averaging, we have studied individual chlorosomes from wild-type Chlorobaculum tepidum by polarization-resolved fluorescence-excitation spectroscopy. This approach minimizes the inherent sample heterogeneity and allows us to reveal properties of the exciton states without ensemble averaging. The results are compared with predictions from computer simulations of various models of the supramolecular organization of the BChl monomers. We find that the photophysical properties of individual chlorosomes from wild-type Chlorobaculum tepidum are consistent with a (multiwall) helical arrangement of syn-anti stacked BChl molecules in cylinders and/or spirals of different size.

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…[180,181,[183][184][185][186] This has recently been applied to the bchR mutant from Cba. Tepidum, [187] see Figure 9c.…”

Signature of Anomalous Exciton Localization in the Optical Response of Self-Assembled Organic Nanotubes

“…These characteristics are maintained independently of the rolling conditions, as long as the radius is large compared to a unit cell. Interestingly, the recently proposed and frequently studied structure for the chlorosomes in green sulphur bacteria [5,40,41] corresponds to r ¼ 0.44 and ϕ ¼ 36°, on the edge of the above range. Indeed, both optical experiments and simulations have shown clear polarization properties in these systems, indicative of strong exciton delocalization [40].…”

Self‐aggregation of Synthetic Zinc Chlorophyll Derivatives Possessing 31‐Hydroxy or Methoxy Group and 131‐Mono‐ or Dicyanomethylene Moiety in Nonpolar Organic Solvents as Models of Chlorosomal Bacteriochlorophyll‐d Aggregates

“…Zinc 3-hydroxymethyl-13 1 -dicyanomethylene-chlorin 1a prepared by modifying naturally occurring Chl-a did self-aggregate in 1% THF-hexane to form large oligomers similar to the corresponding 13 1 -oxo-analog 3a. The artificial self-aggregate is a new model for natural chlorosomal supramolecules (43)(44)(45)(46)(47)(48)(49). Hydrogen bonding of 3 1 -Z-H (Z = NH, NCH 3 , or O) with 13-C=O as well as coordination of 3 1 -Z with central M (=Mg, Zn, Cd) are usually requisite for such self-aggregation (see Fig.…”