Abstract
Experimental/theoretical evidence for sustained vibration-assisted electronic (vibronic) coherence in the Photosystem II Reaction Center (PSII RC) indicates that photosynthetic solar-energy conversion might be optimized through the interplay of electronic and vibrational quantum dynamics. This evidence has been obtained by investigating the primary charge separation process in the PSII RC by two-dimensional electronic spectroscopy (2DES) and Redfield modeling of the experimental data. However, while conventional Fourier transform analysis of the 2DES data allows oscillatory signatures of vibronic coherence to be identified in the frequency domain in the form of static 2D frequency maps, the real-time evolution of the coherences is lost. Here we apply for the first time wavelet analysis to the PSII RC 2DES data to obtain time-resolved 2D frequency maps. These maps allow us to demonstrate that (i) coherence between the excitons initiating the two different charge separation pathways is active for more than 500 fs, and (ii) coherence between exciton and charge-transfer states, the reactant and product of the charge separation reaction, respectively; is active for at least 1 ps. These findings imply that the PSII RC employs coherence (i) to sample competing electron transfer pathways, and ii) to perform directed, ultrafast and efficient electron transfer.
Highlights
In oxygenic photosynthesis the site of solar-energy conversion, the photosystem II reaction center (PSII RC), is a membrane-bound pigment-protein complex that converts solar photons into a stable charge-separated state which, in turn, creates an electrochemical potential across the membrane
The continuous wavelet transform (CWT) analysis is applied to the PSII
CWT allows to extract both the frequency and time information contained in 2DES experimental data, a compromise between frequency and time resolution must be found; extremely high frequency resolution will result in poor time resolution and vice versa [a comparison of two limiting cases together with the optimal case used in the analysis presented here can be found in the Supplementary Information (Fig. S3)]
Summary
In oxygenic photosynthesis the site of solar-energy conversion, the photosystem II reaction center (PSII RC), is a membrane-bound pigment-protein complex that converts solar photons into a stable charge-separated state which, in turn, creates an electrochemical potential across the membrane. The horizontal wiggled arrows represent the discussed coherences between electronic states observed as cross-peaks in the 120 and 340 cm−1 time-resolved 2D frequency maps. This figure has been adapted from ref. In our previous work[12], two-dimensional electronic spectroscopy (2DES)[25, 26] and Fourier transform (FT) analysis allowed to visualize the coherence between electronic states in 2D frequency maps [obtained by Fourier transformation of the real rephasing 2D spectra along the population time (T)]. The 120 and 340 cm−1 frequency modes couple to the states involved in charge separation in the PSII RC and allow the complex to rapidly and coherently sample its energy landscape. The most crucial consequence of this coupling is that it allows the complex to sample the ChlD1 and PD1 pathways, and to select the 340 cm−1 mode couples to tmheostht orepetismtaatel schinavrgoelv-esedpianrathtieonPDp1apthatwhawyafyor[(aPsDp2PecDi1f)i+⁎c≈r6e6a0nlimz,a(tPioDn2δo+fPtDh1eδ−d)−⁎is≈o6r7d5nemr,
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