Intensity Borrowing via Excitonic Couplings among Soret and Q_y Transitions of Bacteriochlorophylls in the Pigment Aggregates of Chlorosomes, the Light-Harvesting Antennae of Green Sulfur Bacteria

Abstract

Model calculations of linear dichroism (LD) and circular dichroism (CD) spectra were conducted for the chlorosomes of green sulfur bacteria, Chlorobium phaeobacteroides and Chlorobium tepidum, on the basis of the theory of delocalized exciton. The chlorosomes of Chl. phaeobacteroides and Chl. tepidum contain bacteriochlorophylls (BChls) e and c as the major light-harvesting pigments, respectively. The excitonic couplings among the Soret and Q(y) transitions were considered on the basis of the "rodlike" structural model for the pigment self-aggregates in a chlorosome. Trial simulations were conducted by assuming the B(x) and B(y) transition-dipole vectors to be parallel to the molecular x- and y-axes, respectively. The simulation at this stage could nicely reproduce the larger splitting of the Soret band and more significant enhancement of the Q(y) band upon formation of chlorosome in the BChl e-containing chlorosome than in the BChl c-containing one. Intensity borrowing was indicated to be the key mechanism inducing the enhancement of the Q(y) band in the BChl e-containing chlorosomes. However, the simulated LD and CD spectra in the Soret region showed qualitative disagreement from the observed ones. To resolve the deviations, the directions of the B(x) and B(y) transition-dipole vectors and the orientations of the molecular planes of BChls were adjusted in the next stage. The fine-tuning of these parameters resulted in a striking agreement between the observed and simulated CD and LD spectra over the whole spectral range studied. The best fit was obtained when the B(x) and B(y) transition-dipole vectors were assumed to be rotated 25 degrees clockwise from the molecular x- and y-axes and the molecular planes in the pigment aggregate were tilted 5 degrees from that assumed in the original model without alteration in the direction of the molecular y-axis. The calculated spectral profiles were affected little by the change in the curvatures of the rod surface, showing that the optical spectra of chlorosomes were determined essentially by the local pigment arrangement, but not by the higher-order structure, of the aggregate.

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…For example, in ref. [456], the initially assumed directions of the transition dipole vectors in a rodlike model were modified to obtain a better agreement between exciton calculations and experimental spectra. But, as the atomistic structure is undoubtedly not solved for chlorosomes, there are hardly any electronic-structure calculations available to date.…”

Organization of Bacteriochlorophylls in Individual Chlorosomes from Chlorobaculum tepidum Studied by 2-Dimensional Polarization Fluorescence Microscopy

“…In the chlorosomes, different exciton transitions originate from different transitions of the interacting BChls. 52 The excitonic Q y band (we will call it EQ y hereafter) still has the lowest energy located at ∼750 nm. The excitonic Soret band can still be understood as a combination of EB y and EB x transitions, which are oriented almost perpendicularly to each other.…”

Water soluble chlorophyll binding protein of higher plants: A most suitable model system for basic analyses of pigment–pigment and pigment–protein interactions in chlorophyll protein complexes

“…The orientation of the transition dipole moments with respect to the x-and y-axes of Chls (and bacteriochorophylls) is a matter of long lasting discussions and markedly different values have been reported (Fragata et al, 1995;van Zandvoort et al, 1995;Linke et al, 2008;Shibata et al, 2010). When using the values for the rotational strength of CD spectra determined by Hughes et al (2006) for Chl a in class IIa WSCP we obtained a clockwise Q y deviation of 6-8 • from the y-axis with our best fit on the basis of the structure data of Horigome et al (2007).…”