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

Abstract

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: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 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 pi-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 approximately 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to approximately 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 S1 (2(1)Ag-)-->S2 (1(1)Bu+) excited-state transitions which, when subtracted from the energies of the S0 (1(1)Ag-)-->S2 (1(1)Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2(1)Ag-) 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 S1 (1(1)Ag-), S2 (1(1)Bu+), 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 S1 (2(1)Ag-) 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 pi-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon-carbon double bonds in LH2 complexes from purple photosynthetic bacteria.

Figure 1

Structure of the LH2 antenna complex from Rps. acidophila 10050.

Figure 2

Simplified energy flow pathways of LH2 complexes from Car to BChl; a, absorption, ta, transient absorption. Dashed lines represent radiationless processes.

Figure 3

Structures of the all-trans isomers of neurosporene, spheroidene, spheroidenone, and rhodopin glucoside.

Figure 4

Steady-state absorption spectra of LH2 complexes from (A) Rb. sphaeroides G1C, (B) Rb. sphaeroides 2.4.1 (anaerobic), (C) Rb. sphaeroides 2.4.1 (aerobic), and (D) Rps. acidophila 10050 in a buffer at 293 K and in buffer/glycerol (40/60, v/v) at 10 K. The spectra were normalized to their absorption bands at 800 nm.

Figure 5

Emission (Em), fluorescence excitation (Ex), and 1-T (where T is transmittance) spectra of the LH2 complexes in buffer at 293 K, and in buffer/glycerol at 10 K. The emission spectra were detected at 856 nm in (A), (C), and (E), at 877 nm in (B), at 870 nm in (D), at 880 nm in (F), at 872 nm in (G), and at 891 nm in (H).

Figure 6

Transient absorption spectra of LH2 from Rb. sphaeroides G1C at 293 and 10 K.

Figure 7

Transient absorption spectra of LH2 from Rps. acidophila 10050 at 293 and 10 K.

Figure 8

Evolution associated difference spectra (EADS) of LH2 from Rb. sphaeroides G1C.

Figure 9

Evolution associated difference spectra (EADS) of LH2 from Rps. acidophila 10050.

Figure 10

Overlay of the steady-state S₀ (1¹A_g⁻) → S₂ (1¹B_u⁺) absorption (solid line) and the S₁ (2¹A_g⁻) → S₂ (1¹B_u⁺) transient absorption (dashed line) shifted from the NIR region by the indicated amount of energy in cm⁻¹ units. (A) neurosporene, (B) spheroidene, (C) spheroidenone, and (D) rhodopin glucoside. All spectra were taken at 293 K in acetone except spheroidenone, which was measured in hexane.

Figure 11

Energy flow pathways of LH2 complexes applied in the target analysis. hS₁ corresponds to a vibronically excited S₁ (2¹A_g⁻) state of the Car. This intermediate is only evident in the transient absorption spectra and fitted lineshapes from the LH2 complex of Rps. acidophila 10050.

Figure 12

Species associated difference spectra (SADS) of LH2 from Rb. sphaeroides G1C and Rps. acidophila 10050 obtained using global fitting with the target models presented in Figure 11.

Figure 13

(A) Spectral overlap between the absorption of the LH2 Q_X and Q_Y BChl bands from Rb. sphaeroides 2.4.1 and a typical S₁ (2¹A_g⁻) fluorescence trace taken from 3,4,5,6-tetrahydrospheroidene (N = 8) in petroleum ether shifted to correspond to the spectral origins of the S₁ (2¹A_g⁻) → S₀ (1¹A_g⁻) transition of neurosporene (N = 9), spheroidene (N = 10), spheroidenone (N = 10+), and rhodopin glucoside (N = 11). The S₁ (2¹A_g⁻) energies for the Cars were determined from measurements of the S₁ (2¹A_g⁻) → S₂ (1¹B_u⁺) NIR transition energies (Figure 10). (B) Overlap between the absorption spectrum of the LH2 Q_X BChl band and a typical S₂ (1¹B_u⁺) fluorescence trace taken from 3,4,5,6-tetrahydrospheroidene (N = 8) in petroleum ether shifted to correspond to the spectral origins of the S₂ (1¹B_u⁺) → S₀ (1¹A_g⁻) transition of the Cars in LH2 complexes. (C) Overlap between the absorption spectrum of the LH2 Q_X BChl band and the S₂ (1¹B_u⁺) fluorescence taken from neurosporene, spheroidene, spheroidenone, and rhodopin glucoside in n-hexane shifted to correspond to the spectral origins of the S₂ (1¹B_u⁺) → S₀ (1¹A_g⁻) transition of the Cars in LH2 complexes.