Investigation on chlorosomal antenna geometries: tube, lamella and spiral-type self-aggregates

Abstract

Molecular mechanics calculations and exciton theory have been used to study pigment organization in chlorosomes of green bacteria. Single and double rod, multiple concentric rod, lamella, and Archimedean spiral macrostructures of bacteriochlorophyll c molecules were created and their spectral properties evaluated. The effects of length, width, diameter, and curvature of the macrostructures as well as orientations of monomeric transition dipole moment vectors on the spectral properties of the aggregates were studied. Calculated absorption, linear dichroism, and polarization dependent fluorescence-excitation spectra of the studied long macrostructures were practically identical, but circular dichroism spectra turned out to be very sensitive to geometry and monomeric transition dipole moment orientations of the aggregates. The simulations for long multiple rod and spiral-type macrostructures, observed in recent high-resolution electron microscopy images (Oostergetel et al., FEBS Lett 581:5435-5439, 2007) gave shapes of circular dichroism spectra observed experimentally for chlorosomes. It was shown that the ratio of total circular dichroism intensity to integrated absorption of the Q(y) transition is a good measure of degree of tubular structures in the chlorosomes. Calculations suggest that the broad Q(y) line width of chlorosomes of sulfur bacteria could be due to (1) different orientations of the transition moment vectors in multi-walled rod structures or (2) a variety of Bchl-aggregate structures in the chlorosomes.

Strong coupling between chlorosomes of photosynthetic bacteria and a confined optical cavity mode

“…These are generally oblate spheroid organelles B100-200 nm in length, 30-70 nm in width and 30-40 nm thick 5 . A single chlorosome may contain up to 250,000 bacteriochlorophyll c/d/e (BChl c/d/e) molecules 4,6,7 that are self-assembled into tube-like or lamellae aggregates [8][9][10][11][12][13][14] and are enclosed within in a protein-lipid monolayer 15 . Absorption of a photon results in the creation of a molecular exciton, which is transported to the next subunit of the LHC-a baseplate, which is located on one side of the chlorosome surface.…”

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

“…These structures, enclosed in a lipid monolayer, form an ovoid shaped body with the characteristic size ranging from tens to several hundreds of nanometers. Electron microscopy (EM) [1][2][3] and theoretical studies [4] support the hypothesis that the BChls are organized into tubular elements -rolls, while sheet-like aggregates [2,5] may coexist. The uncertainty in the structural characterization of chlorosomes stems from the large disorder in aggregates composing the chlorosome.…”

The supramolecular organization of self-assembling chlorosomal bacteriochlorophyll c , d , or e mimics

“…Additionally, the absorption spectra are considerably broadened so that a large wavelength range of the solar radiation can be tapped. The spectral and kinetic inhomogeneity encountered in the chlorosomes and the broadened Q y transition, as the recent theoretical study indicates (17), could be due to ''a variety of BChl-aggregate structures'' comprised of upward and downward stacks with 5 1 ⁄2 coordinated magnesium atoms in an asymmetric fashion.…”

“…Another very recent study by solid-state NMR on 13 C-labeled chlorosomes has discussed all previous models and favors a parallel hydrogenbonded dimer model as the most probable one (15,16). Molecular mechanics calculations and exciton theory have been applied in a very thorough theoretical investigation on the different possible geometries such as tube, lamella, or spiral-type for the chlorosomal aggregates (17).…”