Excitation energy transfer in individual light-harvesting chlorosome from green photosynthetic bacterium Chloroflexus aurantiacus at cryogenic temperature

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

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.

Spectral and Structural Variations of Biomimetic Light-Harvesting Nanotubes

Bioinspired, self-assembled nanotubes have been investigated by low-temperature, polarization-resolved single-tube spectroscopy. These assemblies are based on zinc chlorin monomers and are considered as model systems that resemble the secondary structural elements in the natural light-harvesting systems of green (non)sulfur bacteria. Compared to the natural systems, the spectral parameters extracted from the single-nanotube spectra feature distributions with significantly smaller widths, which is ascribed to a tremendous reduction of structural heterogeneity in the artificial systems. Employing quantum chemical molecular modeling the spectra of individual nanotubes can be explained consistently only for a molecular packing model that is fundamentally different from those considered so far for the natural systems. Subsequent theoretical simulations reveal that the remaining spectral variations between single nanotubes can be traced back to small variations of the mutual orientations of the monomer transition dipole moments that are far beyond the resolving power of high-resolution electron microscopy imaging techniques.

Structural Variations in Chlorosomes from Wild-Type and a bchQR Mutant of Chlorobaculum tepidum Revealed by Single-Molecule Spectroscopy

Green sulfur bacteria can grow photosynthetically by absorbing only a few photons per bacteriochlorophyll molecule per day. They contain chlorosomes, perhaps the most efficient light-harvesting antenna system found in photosynthetic organisms. Chlorosomes contain supramolecular structures comprising hundreds of thousands of bacteriochlorophyll molecules, which are properly positioned with respect to one another solely by self-assembly and not by using a protein scaffold as a template for directing the mutual arrangement of the monomers. These two features-high efficiency and self-assembly-have attracted considerable attention for developing light-harvesting systems for artificial photosynthesis. However, reflecting the heterogeneity of the natural system, detailed structural information at atomic resolution of the molecular aggregates is not yet available. Here, we compare the results for chlorosomes from the wild type and two mutants of Chlorobaculum tepidum obtained by polarization-resolved, single-particle fluorescence-excitation spectroscopy and theoretical modeling with results previously obtained from nuclear-magnetic resonance spectroscopy and cryo-electron microscopy. Only the combination of information obtained from all of these techniques allows for an unambiguous description of the molecular packing of bacteriochlorophylls within chlorosomes. In contrast to some suggestions in the literature, we find that, for the chlorosomes from the wild type as well as for those from mutants, the dominant secondary structural element features tubular symmetry following a very similar construction principle. Moreover, the results suggest that the various options for methylation of the bacteriochlorophyll molecules, which are a primary source of the structural (and spectral) heterogeneity of wild-type chlorosome samples, are exploited by nature to achieve improved spectral coverage at the level of individual chlorosomes.

Anisotropic distribution of emitting transition dipoles in chlorosome from Chlorobium tepidum: fluorescence polarization anisotropy study of single chlorosomes

The polarization anisotropy of fluorescence spectra from single chlorosomes isolated from a green sulfur bacterium, Chlorobium (Cb.) tepidum, was observed at 13 K. As the polarizer was rotated, the intensities of the fluorescence bands of both bacteriochlorophyll (BChl)-c self-aggregates and BChl-a in baseplate proteins showed clear oscillations. From the oscillation, the values of the degree of polarization (DP) and the phase shift (PS) between the BChl-c and BChl-a bands were determined for each single chlorosome. The DP versus PS plot for Cb. tepidum chlorosomes showed linear correlations between the PS and the DP values for both BChl-c and BChl-a fluorescence bands. This tendency could be explained from a simulation assuming a random orientation of chlorosomes and a triaxial orientation distribution of emitting transition dipoles within a single chlorosome. The intensity ratios among the X-/Y-/Z-principal transition dipoles were estimated to be 0.3/0.5/1 and 1/0.6/0.1 for the BChl-c and BChl-a fluorescence bands, respectively. Here, the X-, Y-, and Z-axes are perpendicular, parallel to the cytoplasmic membrane, and parallel to the chlorosome long axis, respectively. A theoretical calculation based on the exciton theory was conducted to reproduce the observed triaxial orientation distribution of emitting transition dipoles. The simulation revealed that a deformation introduced to the circular cross section of the rod-shaped BChl-c self-aggregates could qualitatively reproduce results of this study.

Polarized Fluorescence of Aggregated Bacteriochlorophyll c and Baseplate Bacteriochlorophyll a in Single Chlorosomes Isolated from Chloroflexus aurantiacus

The polarization anisotropy of fluorescence from single chlorosomes isolated from a green filamentous bacterium, Chloroflexus aurantiacus, was measured using a confocal laser microscope at 13 K. Each single chlorosome that is floating in a frozen solvent exhibited strong polarization anisotropy of fluorescence. We calculated the degrees of fluorescence polarization for 51 floating single chlorosomes. The value ranged from 0.1 to 0.76 for the BChl-c aggregate in the core chlorosomes and from 0 to 0.4 for the energy acceptor BChl-a in the baseplate protein in the outer membrane. The shifts in polarization angles between the two emission bands were distributed over all the possible values with a sharp peak around 90 degrees , suggesting the perpendicular orientation between the transition dipoles of the fluorescence emission from the BChl-c aggregate and that from BChl-a. A simulation assuming a random orientation of chlorosomes reproduced the experimental results exactly. The analysis further indicated the appreciable contribution of the transition dipole of BChl-c that has an orientation perpendicular to the major polarization axis in each chlorosome. Small values of the degrees of polarization implied the BChl-a transition dipole to be somewhat tilted with respect to the normal of the cytoplasmic membrane to which chlorosomes are attached. These conclusions can be obtained only by observing the fluorescence of single chlorosomes.

Low-temperature fluorescence from single chlorosomes, photosynthetic antenna complexes of green filamentous and sulfur bacteria

Fluorescence spectra of single chlorosomes isolated from a green filamentous bacterium (Chloroflexus (Cfl.) aurantiacus) and a green sulfur bacterium (Chlorobium (Cb.) tepidum) were measured by using a confocal laser microscope at 13 K. Chlorosomes were frozen either in a liquid solution (floating chlorosome) or on a quartz plate after being adsorbed (adsorbed chlorosome). Fluorescence peak wavelengths were shorter for the adsorbed single chlorosomes than for the floating ones. Single floating Cfl. chlorosomes showed a distribution of fluorescence peak positions having a center at 759.0 nm with a full width at half maximum of 6.3 nm. Single floating Cb. chlorosomes showed a 782.7 nm center with a full width at half-maximum of 3.4 nm. The distribution shifted to the blue and became wider with increasing temperature, especially in Cb. chlorosomes, suggesting a large excitonic density of states just above the lowest level. Energy transfer from BChl-c aggregates to BChl-a molecules in the baseplate proteins was observed in the floating chlorosomes but not in the adsorbed ones. A positive correlation was found between the peak wavelength of BChl-c fluorescence and the intensity of BChl-a fluorescence in single Cfl. chlorosomes. The results suggest that the BChl-c aggregates with longer wavelengths of the fluorescence peaks have a more efficient Förster-type energy transfer to the baseplate BChl-a.