Ultrafast time-resolved carotenoid to-bacteriochlorophyll energy transfer in LH2 complexes from photosynthetic bacteria

Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment−protein complexes isolated from three different strains of photosynthetic bacteria: <i>Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides</i> 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of π-electron conjugated carbon−carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to 53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S<sub>1</sub> (2<sup>1</sup>A<sub>g</sub>−) → S<sub>2</sub> (1<sup>1</sup>B<sub>u</sub><sup>+</sup>) excited-state transitions which, when subtracted from the energies of the S0 (1<sup>1</sup>A<sub>g</sub><sup>−</sup>) → S2 (1<sup>1</sup>B<sub>u</sub><sup>+</sup>) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2<sup>1</sup>A<sub>g</sub>−) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car•+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S<sub>1</sub> (1<sup>1</sup>A<sub>g</sub><sup>−</sup>), S<sub>2</sub> (1<sup>1</sup>B<sub>u</sub><sup>+</sup>), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S<sub>1</sub> (2<sup>1</sup>A<sub>g</sub>−) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of π-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon−carbon double bonds in LH2 complexes from purple photosynthetic bacteria