Spectral heterogeneity and carotenoid-to-bacteriochlorophyll energy transfer in LH2 light-harvesting complexes from Allochromatium vinosum

Abstract

Photosynthetic organisms produce a vast array of spectral forms of antenna pigment-protein complexes to harvest solar energy and also to adapt to growth under the variable environmental conditions of light intensity, temperature, and nutrient availability. This behavior is exemplified by Allochromatium (Alc.) vinosum, a photosynthetic purple sulfur bacterium that produces different types of LH2 light-harvesting complexes in response to variations in growth conditions. In the present work, three different spectral forms of LH2 from Alc. vinosum, B800-820, B800-840, and B800-850, were isolated, purified, and examined using steady-state absorption and fluorescence spectroscopy, and ultrafast time-resolved absorption spectroscopy. The pigment composition of the LH2 complexes was analyzed by high-performance liquid chromatography, and all were found to contain five carotenoids: lycopene, anhydrorhodovibrin, spirilloxanthin, rhodopin, and rhodovibrin. Spectral reconstructions of the absorption and fluorescence excitation spectra based on the pigment composition revealed significantly more spectral heterogeneity in these systems compared to LH2 complexes isolated from other species of purple bacteria. The data also revealed the individual carotenoid-to-bacteriochlorophyll energy transfer efficiencies which were correlated with the kinetic data from the ultrafast transient absorption spectroscopic experiments. This series of LH2 complexes allows a systematic exploration of the factors that determine the spectral properties of the bound pigments and control the rate and efficiency of carotenoid-to-bacteriochlorophyll energy transfer.

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

“…If BChls and Cars are embedded into LHCs, EET between them occurs (see Figure ). The EET from S 2 of Cars to BChl is well documented. ,,− The efficiency of EET from S 2 of Cars to BChl varies widely from ∼20% to almost 100% Figure shows the absorption spectrum (as 1 – T (λ)) and the fluorescence excitation spectrum of LH2 from Alc.…”

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

“…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 and ). The authors of these two papers did not explicitly deal with EET from the BChl Soret to Cars, but a revision of their results also suggests that there may be no such EET (see Figure 5 in ref and Figure 3 in ref ).…”

“…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 and ). The authors of these two papers did not explicitly deal with EET from the BChl Soret to Cars, but a revision of their results also suggests that there may be no such EET (see Figure 5 in ref and Figure 3 in ref ). On the contrary, in a theoretical study it was asserted that “the energy transfer pathway from the Soret states of BChl-a to the 1B u state of lycopene should be extremely efficient and may effectively compete with the internal conversion on the individual BChls-a” .…”

Molecular Physiology of Anaerobic Phototrophic Purple and Green Sulfur Bacteria

“…The complex further contains light-harvesting carotenoids, such as spirilloxanthin. Internal complex of LH1 bacteria Allochromatium vinosum is composed of 2 α and 2 β polypeptide complexes arranged in a circle around the reaction center [86,87]. The model of a possible arrangement of a photosynthetic unit in species of Rhodospirillum, Rhodopseudomonas and Allochromatium genera is presented in Figure 5.…”