Elementary Energy Transfer Pathways in Allochromatium vinosum Photosynthetic Membranes

Abstract

Allochromatium vinosum (formerly Chromatium vinosum) purple bacteria are known to adapt their light-harvesting strategy during growth according to environmental factors such as temperature and average light intensity. Under low light illumination or low ambient temperature conditions, most of the LH2 complexes in the photosynthetic membranes form a B820 exciton with reduced spectral overlap with LH1. To elucidate the reason for this light and temperature adaptation of the LH2 electronic structure, we performed broadband femtosecond transient absorption spectroscopy as a function of excitation wavelength in A. vinosum membranes. A target analysis of the acquired data yielded individual rate constants for all relevant elementary energy transfer (ET) processes. We found that the ET dynamics in high-light-grown membranes was well described by a homogeneous model, with forward and backward rate constants independent of the pump wavelength. Thus, the overall B800→B850→B890→ Reaction Center ET cascade is well described by simple triexponential kinetics. In the low-light-grown membranes, we found that the elementary backward transfer rate constant from B890 to B820 was strongly reduced compared with the corresponding constant from B890 to B850 in high-light-grown samples. The ET dynamics of low-light-grown membranes was strongly dependent on the pump wavelength, clearly showing that the excitation memory is not lost throughout the exciton lifetime. The observed pump energy dependence of the forward and backward ET rate constants suggests exciton diffusion via B850→ B850 transfer steps, making the overall ET dynamics nonexponential. Our results show that disorder plays a crucial role in our understanding of low-light adaptation in A. vinosum.

The light-harvesting complex 2 of Allochromatium vinosum: B800 absorption band splitting and exciton relaxation

“…Note that these time scales are rather similar to the ones previously reported in Ref. [26] although the B800 doublepeak structure has not been very pronounced in the absorption of the studied room temperature case.…”

Energy transfer in purple bacterial photosynthetic units from cells grown in various light intensities

“…In HL adapted ICMs there is the possibility that the RC will be already 'closed' when the energy from an absorbed photon reaches LH1. If this occurs, LH1LH2 energy transfer can take place and excitation can be transferred to another LH1-RC with an 'open' RC (Deinum, et al 1991;Lüer, et al 2015;). However, in low light (LL) RCs are generally 'open' and the scarce photons should be used as efficiently as possible.…”

Excited state absorption spectra of dissolved and aggregated distyrylbenzene: A TD-DFT state and vibronic analysis

“…[1][2][3][4] Meanwhile, PPS is becoming a widespread turn-key like technique, which covers the UV to IR spectral range with widely tunable pump wavelength, timeresolution from milliseconds (ms) down to femtoseconds (fs), and high sensitivity, and matrix-based data analysis methods 5 have, in some cases, allowed the complete analysis of elementary transfer processes by identifying single species PPS spectra and their evolution in time. 3,4,6,7 The sensitivity of the technique has been pushed far enough so that PPS is now routinely used for the detection of elementary loss channel in lasing materials 8 and organic solar cells under operational conditions. 6,9,10 One general difficulty in the analysis of PPS is spectral congestion caused by a multitude of optical probes from various photoexcited states such as singlet, triplet, and charged states.…”