Triplet excited state spectra and dynamics of carotenoids from the thermophilic purple photosynthetic bacterium Thermochromatium tepidum

Abstract

Light-harvesting complex 2 from the anoxygenic phototrophic purple bacterium Thermochromatium tepidum was purified and studied by steady-state absorption, fluorescence and flash photolysis spectroscopy. Steady-state absorption and fluorescence measurements show that carotenoids play a negligible role as supportive energy donors and transfer excitation to bacteriochlorophyll-a with low energy transfer efficiency of ~30%. HPLC analysis determined that the dominant carotenoids in the complex are rhodopin and spirilloxanthin. Carotenoid excited triplet state formation upon direct (carotenoid) or indirect (bacteriochlorophyll-a Q(x) band) excitation shows that carotenoid triplets are mostly localized on spirilloxanthin. In addition, no triplet excitation transfer between carotenoids was observed. Such specific carotenoid composition and spectroscopic results strongly suggest that this organism optimized carotenoid composition in the light-harvesting complex 2 in order to maximize photoprotective capabilities of carotenoids but subsequently drastically suppressed their supporting role in light-harvesting process.

Ultrafast time-resolved spectroscopy of the light-harvesting complex 2 (LH2) from the photosynthetic bacterium Thermochromatium tepidum

“…The room temperature (RT) absorption spectrum contains features associated with bacteriochlorophyll a (BChl) (Soret, Q x , B800 and B850) and a broad carotenoid (Car) band that is not well resolved. Interestingly, on lowering the temperature, the split of the B800 band to two distinct bands is observed (Niedzwiedzki et al 2011;van Dijk et al 1998). Initially, the presence of two bands around 800 nm in the LH2 was suggested to be associated with a mixture of two different spectral forms of the same LH2 that cannot be biochemically separated (van Dijk et al 1998).…”

“…Work done by Yang et al (2010) shows that the dominant Cars are rhodopin (*66%, N = 11), spirilloxanthin (*20%, N = 13), and anhydrorhodovibrin (*9%, N = 12). In work done by Niedzwiedzki et al (2011) the values are as follows: rhodopin (35%, N = 11), spirilloxanthin (40%, N = 13), rhodovibrin (N = 12), and anhydrorhodovibrin (N = 12) (25%). Despite the observed differences in relative ratios, qualitatively the results are similar and may simply reflect a flexibility of the Car biosynthesis path in this bacterium under different growth conditions.…”

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

“…The column was maintained at 30°C throughout the duration of the chromatographic run using a Phenomenex TS-130 heater. The mobile phase consisted of an isocratic delivery of 9:1 (v/v) hexane:tetrahydrofuran as previously described (Niedzwiedzki et al 2011b(Niedzwiedzki et al , 2012. The flow rate was 1.5 mL/min.…”

“…The flow rate was 1.5 mL/min. The carotenoids were identified based on their PDA absorption spectra and retention times by comparison with data obtained from previous investigations (Niedzwiedzki et al 2011b).…”

“…Thus, they are very likely due to carotenoid triplet-triplet absorption. Carotenoid triplet states are readily formed in antenna complexes by triplet energy transfer from BChl a (Monger et al 1976;Farhoosh et al 1994;Feng et al 2004;Kakitani et al 2007;Niedzwiedzki et al 2011b). On the basis of triplet-triplet absorption spectra previously recorded in LH1 and LH2 complexes (Feng et al 2004;Kakitani et al 2007), the signals at 543 and 568 nm can be assigned to rhodopin (N = 11), and rhodovibrin plus anhydrorhodovibrin (N = 12), respectively.…”