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 S₂ 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 S₂ of the carotenoids in light-harvesting complex LH2 from the purple bacterium Ectothiorhodospira haloalkaliphila appeared not to exceed a few percent.

Appendix

We estimate the degree of sample bleaching when short laser pulses λ = 350 nm pass through the rotating cell with L = 1.5 mm layer of LH2 preparation.

Excitation laser pulse energy was measured to be W = 0.24 µJ, it thus including n₀ = W/(hc/λ) = 0.41 × 10¹² quanta. Since absorption at 340 nm is D₃₄₀ = 0.350, the number of quanta absorbed in the cell is n = n₀·(1–10^−D340) = 0.227 × 10¹². These quanta were absorbed within the cylindrical volume containing the LH2 sample; since the cylinder diameter d = 0.3 mm, the illuminated cylinder volume V = 1.06 × 10⁻⁷ liter. Although the exact composition of the LH2 complex of Ect. haloalkaliphila is not known, it is assumed to be similar to LH2 composition of Rps. acidophila, which includes p = 18 molecules of BChl B850, q = 9 molecules of BChl B800, and r = 9 molecules of Cars.

Only BChl B850 molecules absorb light at 846 nm, so with the known molar extinction coefficient ε = 1.33 × 10⁵ and measured absorbance D₈₄₆ = 0.457 we calculate the molar concentration of BChl B850 in the sample, C_B850 = D₈₄₆/(ε₈₄₆·L) = 22.9 × 10⁻⁶ M, and the number of BChl B850 and B800 molecules in the illuminated volume: N_B850 = C_B850·N_A·V = 1.46 × 10¹², N_A being Avogadro’s number, N_B800 = N_B850·q/p = 0.731 × 10¹², total BChl number N_BChl = N_B850 + N_B800 = 2.19 × 10¹². Similarly, Car molecules number N_C = N_B850·r/p = 0.731 × 10¹².

Both BChl and Car molecules contribute to the absorbance at 340 nm, the contribution of Car molecules being about β = 0.05 (see Fig. 4). This means that n_C = n·β = 1.14 × 10¹⁰ quanta were absorbed by Car molecules, while the remaining n_B = n·(1−β) = 2.16 × 10¹¹ quanta were absorbed by BChl molecules.

Absorption of excitation quanta by some Car molecules appears as a proportional reduction in the Car absorption band centered at 500 nm, where the LH2 preparation absorbance was measured to be D₅₀₀ = 0.287. Obviously, the excitation of f_C = F_C/n_C = 0.0155 fraction of Car molecules would result in ΔD₅₀₀ = f_C·D₅₀₀ = 0.00446 bleaching at 500 nm. Indeed, the calculated bleaching ΔD₅₀₀ = 0.00446 is close to the experimentally measured value ΔA₅₀₀ = 0.00416, differing by only 6–7%.