Excitation energy transfer from the bacteriochlorophyll Soret band to carotenoids in the LH2 light-harvesting complex from Ectothiorhodospira haloalkaliphila is negligible

Abstract

Pathways of intramolecular conversion and intermolecular electronic excitation energy transfer (EET) in the photosynthetic apparatus of purple bacteria remain subject to debate. Here we experimentally tested the possibility of EET from the bacteriochlorophyll (BChl) Soret band to the singlet S level of carotenoids using femtosecond pump-probe measurements and steady-state fluorescence excitation and absorption measurements in the near-ultraviolet and visible spectral ranges. The efficiency of EET from the Soret band of BChl to S of the carotenoids in light-harvesting complex LH2 from the purple bacterium Ectothiorhodospira haloalkaliphila appeared not to exceed a few percent.

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

“…We have previously investigated EET from the BChl Soret band to Cars in the peripheral light-harvesting complexes (LH2) from wild-type (WT) cells of the purple bacterium Ectothiorhodospira (Ect.) haloalkaliphila . We came to the conclusion that there is no such EET, within the accuracy of the measurements (on the order of 5%).…”

“…haloalkaliphila. 9 Remarkably similar E(λ) curves for LH 2s from the three strains of Rhodoblastus (Rbl.) acidophilus were shown in Figure 5 of ref 11, and can be derived for LH2 from Allochromatium vinosum (Figure 3 of ref 12.).…”

“…haloalkaliphila. 9 We came to the conclusion that there is no such EET, within the accuracy of the measurements (on the order of 5%). EET efficiency between pigments can be readily estimated by comparing the excitation spectra of the long-wavelength fluorescence with 1-T spectra, where T is the transmission of the sample (cf., refs 11 and 12).…”

“…Possible ways might be intramolecular conversion to BChl Q x or EET to S 2 of Cars. The latter possibility has been not much investigated, except for very few papers. − This is in part because intramolecular processes are generally considered to be faster than intermolecular ones (Kasha’s rule) …”

Chemistry of porphyrins in fossil plants and animals

“…7). The magnesium(II) complexes derived from phorbin include chlorophyll a (15), chlorophyll b (16), chlorophyll c 1 (19), chlorophyll c 2 (20), chlorophyll c 3 (21), chlorophyll d (17), chlorophyll f (18), 14,39 and the bacteriochlorophylls 22-28, all of which feature a 5membered carbocyclic ring fused to ring C of the porphyrin core structure (Fig. 6 and 7).…”