Lack of Excitation Energy Transfer from the Bacteriochlorophyll Soret Band to Carotenoids in Photosynthetic Complexes of Purple Bacteria

Abstract

The excitation energy transfer (EET) from the bacteriochlorophyll (BChl) Soret band to the second excited state(s) (S2) of carotenoids in pigment–protein complexes of purple bacteria was investigated. The efficiency of EET was determined, based on fluorescence excitation and absorption spectra of chromatophores, peripheral light-harvesting complexes (LH2), core complexes (LH1-RC), and pigments in solution. Carotenoid-containing and carotenoid-less samples were compared: LH1-RC and LH2 from Allochromatium minutissimum, Ectothiorhodospira haloalkaliphila, and chromatophores from Rhodobacter sphaeroides and Rhodospirillum rubrum wild type and carotenoid-free strains R-26 and G9. BChl-to-carotenoid EET was absent, or its efficiency was less than the accuracy of the measurements of ∼5%. Quantum chemical calculations support the experimental results: The transition dipole moments of spatially close carotenoid/BChl pairs were found to be nearly orthogonal. The structural arrangements suggest that Soret EET may be lacking for the studied systems, however, EET from carotenoids to Q x appears to be possible.

Electronic Structure Effects Related to the Origin of the Remarkable Near-Infrared Absorption of Blastochloris viridis’ Light Harvesting 1-Reaction Center Complex

“…By analyzing the NTOs for these bands, it is shown that in all of these states, the DHN molecules play an important role in a charge transfer state (see Figure ), the same holds true for the transition from the 540 nm band of B6 . This phenomenon is already reported on plants and bacteria. − However, it is also known that the energy transfer from the BChl Soret band to carotenoids has an extremely low efficiency; in fact, when the DHN molecule is deleted, the 770 nm band is lost (e.g., see the case of system B7 in Figure S12 of the SI).…”

“…This phenomenon is already reported on plants and bacteria. 46−48 However, it is also known that the energy transfer from the BChl Soret band to carotenoids has an extremely low efficiency; 49 in fact, when the DHN molecule is deleted, the 770 nm band is lost (e.g., see the case of system B7 in Figure S12 of the SI).…”

Signature of Quantum Coherence in the Exciton Energy Pathways of the LH2 Photosynthetic Complex

“…In order to compare our results to the experimental results from refs , , we take another step in the calculation and convert the density shown in Figure to current, i.e., to the signal that can be detected in an experiment. The relation between density and currents is easily obtained by simply multiplying the density of a state by the rate of extraction from that state.…”

“…When this is done, it is found that the signal from the lycopene S2 state when excited from the BChls is ∼3% (Figure 2), close to the experimental results. 28,48 The authors of ref 28 argue that the low signal from the lycopenes is due to the blocking of exciton flow from BChls to the lycopenes, which maybe due to the weak coupling between the BChl and lycopene states. 48 However, this argument is inconsistent because if the current is blocked because of weak coupling, then it would have been blocked also when exciting in the opposite pathway, in contradiction to the experimental observations (which show substantial signal in the reverse pathway).…”

“…While this pathway defines the contribution of lycopene to general energy transfer, the opposite direction (BChls toward lycopenes) results in the dissipation of energy at the lycopenes. 28,43,47 A recent experiment examining these pathways in LH2 from the purple bacterium Ectothiorhodospira haloalkaliphila 28,48 showed a surprising asymmetry. These authors measured the emission spectra after excitation, exciting their system from either the Soret level of B850 rings or s2 of carotenoids (which play an equivalent role as the lycopenes in the system studied here).…”