Multichannel carotenoid deactivation in photosynthetic light harvesting as identified by an evolutionary target analysis

Abstract

A new channel of excitation energy deactivation in bacterial light harvesting was recently discovered, which leads to carotenoid triplet population on an ultrafast timescale. Here we show that this mechanism is also active in LH2 of Rhodopseudomonas acidophila through analysis of transient absorption data with an evolutionary target analysis. The algorithm offers flexible testing of kinetic network models with low a priori knowledge requirements. It applies universally to the simultaneous fitting of target state spectra and rate constants to time-wavelength-resolved data. Our best-fit model reproduces correctly the well-known cooling and decay behavior in the S(1) band, but necessitates an additional, clearly distinct singlet state that does not exchange with S(1), promotes ultrafast triplet population and participates in photosynthetic energy transfer.

FIGURE 1

Kinetics at selected wavelengths. (A) The decay at 550 nm probe wavelength is biexponential with a precursor (S^*) that decays within τ = 10.3 ± 0.8 ps into a long-lived state (T) with τ > 300 ps. (B) Kinetics of the S₁ state. The decay at 580-nm probe wavelength is 95% monoexponential with τ = 3.7 ± 0.1 ps. The inset shows the increasingly faster decay times at red-shifted probe wavelengths, reflecting vibrational cooling of the S₁ state. (C) Kinetics of BChl B850. The dashed horizontal line is a guide to the eye, to show the 12% of population growth from slower components.

FIGURE 2

Raw data of transient absorption spectra measurements. Wavelengths have been binned to a 3-nm wavelength resolution with better signal-to-noise. Left, two-dimensional contour plot. Zero absorption change is white, positive yellow/red, and negative green/blue. Right, selected delay cuts. At the earliest time, notice the hotS₁ absorption >600 nm. Intermediate cuts clearly show the two separate peaks at 550 and 580 nm. At very long delays, a positive signal at 550 nm survives, identified as triplet T absorption.

FIGURE 3

Evolutionary target analysis. The experimenter specifies the energy flow network model to be tested (top right), including reasonably wide intervals for the decay rates and for the target state spectra (top left). An initial guess for the shape of the target spectra (bottom left) is optional. The evolutionary algorithm then optimizes the rates and spectra simultaneously, based on an indeterministic mapping of the entire parameter space within the given intervals. For each set of parameters, the time-dependent spectra are calculated and compared to the experimental data. Only the best sets survive and generate the next generation of parameter sets by mutation and recombination.

FIGURE 4

Model of the LH network. All photosynthetic energy transfer is treated inclusively as a state BChl. HotS₁ and S^* are both populated directly from S₂ and there is no transfer between them. S^* undergoes fission into triplet T. Both S₁ and S^* contribute to energy transfer to BChl.

FIGURE 5

Absorption spectra of the best fit model of the excitation energy flow. The vibrationally cold S₁ and T are both blue-shifted of their precursors (hotS₁ and S^*, respectively). The dashed signals from BChl and in the Car S₂ region are equally long-lived as the triplet. (S₂–S₀ bleach is scaled down a factor 3).

FIGURE 6

Transient populations in the optimal LH network model. The inset shows the first picosecond with the decay of S₂ (solid gray) into hotS₁ (dashed black), S^* (solid black), and BChl (dash-dotted black). The main plot shows how these precursor states populate S₁ (dashed gray) and T (solid gray), and let further rise BChl (dash-dotted black).