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. 2008 Mar 27;363(1494):1283-91; discussion 1291.
doi: 10.1098/rstb.2007.2225.

Coupled electron transfers in artificial photosynthesis

Leif Hammarström  1 , Stenbjörn Styring
Affiliations

Coupled electron transfers in artificial photosynthesis

Leif Hammarström et al. Philos Trans R Soc Lond B Biol Sci. .

Abstract

Light-induced charge separation in molecular assemblies has been widely investigated in the context of artificial photosynthesis. Important progress has been made in the fundamental understanding of electron and energy transfer and in stabilizing charge separation by multi-step electron transfer. In the Swedish Consortium for Artificial Photosynthesis, we build on principles from the natural enzyme photosystem II and Fe-hydrogenases. An important theme in this biomimetic effort is that of coupled electron-transfer reactions, which have so far received only little attention. (i) Each absorbed photon leads to charge separation on a single-electron level only, while catalytic water splitting and hydrogen production are multi-electron processes; thus there is the need for controlling accumulative electron transfer on molecular components. (ii) Water splitting and proton reduction at the potential catalysts necessarily require the management of proton release and/or uptake. Far from being just a stoichiometric requirement, this controls the electron transfer processes by proton-coupled electron transfer (PCET). (iii) Redox-active links between the photosensitizers and the catalysts are required to rectify the accumulative electron-transfer reactions, and will often be the starting points of PCET.

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Figures

Figure 1
Figure 1
(a) Schematic of the protein-coupled electron transfer (PCET) reactions on the electron donor side of PSII (based on the structure of Ferreira et al. (2004)). In the PCET reaction (1.1), TyrZ is oxidized by electron transfer to P680+ and proton transfer to His190. In the subsequent PCET reaction, TyrZ is reduced again by electron transferred from the Mn4Ca cluster, while it is not clear from where the proton is transferred. (b) The different oxidation states of the Mn4Ca cluster in the so-called S-state cycle. Each photon excitation leads to a one-step oxidation, and the currently most favoured proton release pattern is indicated.
Figure 2
Figure 2
(a) A Ru–Mn2 complex showing light-induced accumulative electron transfer from the Mn2 to the photo-oxidized Ru unit. (b) A simplified scheme of the ligand exchange and proton release pattern upon oxidation of the Mn2 complex in the presence of water ((i) less than 10% water, (ii) 90% water), as deduced from FTIR and ESI-MS spectroelectrochemistry.
Figure 3
Figure 3
(a) A Mn2-containing triad showing a very long-lived charge separation, with a lifetime of 600 μs at room temperature and approximately 0.5 s in 140 K fluid butyronitrile solution. (b(i)) The diagram shows that the rate of back electron transfer (BET) in our triad lies far down in the Marcus normal region (−ΔG0<λ) in spite of the large driving force, which explains the strong temperature dependence. (ii) Marcus free energy parabolas relevant for the BET. The dashed parabola represents a typical system with moderate reorganization energy for BET that lies in the Marcus inverted region.
Figure 4
Figure 4
The rate constant of tyrosine oxidation as a function of pH for the Ru(bpy)3–tyrosine complex (Sjödin et al. 2000). The solid line is a fit to a Marcus equation (equation (2.1)), with a decrease in ΔG0 of 59 meV per pH unit and λ determined in independent experiments, while the dashed line is a guide for the eye. In a region around pH=10 the kinetics were biphasic.
Figure 5
Figure 5
(a(i,ii)) The rate constant for amino acid oxidation as a function of pH for the Ru(bpy)3–tryptophan and modified Ru(4,4′-CH3COO-bpy)2(bpy)–tyrosine complexes. Note that the rate scale is linear and not logarithmic as in figure 4. The solid lines are fits of the data to equation (2.1), consistent with a pH-dependent concerted reaction, with an additional constant term for the contribution from the stepwise ETPT mechanism. In (ii) the contributions from the two mechanisms are shown as dashed lines. In contrast to the complex of figure 4, the ETPT mechanism dominates up to a pH value of approximately 9, while the concerted mechanism dominates only at higher pH values. The kinetic component of the very rapid oxidation of the tyrosinate anion (τ<20 ns), present at pH values of approximately 10 and higher, is not shown. (b) Transient absorption spectra after photo-oxidation of Ru(bpy)3-tryptophan with Ru(NH3)63+, showing the spectra of the tryptophan radical. The spectra give direct evidence for a switch from a stepwise ETPT mechanism at intermediate pH to a CEP mechanism at high pH. At pH=3, which is below the pKa value of the tryptophan radical (pKa=4.7), the spectrum of the protonated radical is seen. At pH=8, the initial spectrum (t=50 ns; solid line) is from the initially formed protonated radical Trp·H+ that deprotonates with a time constant of approximately 130 ns to give the Trp· spectrum (t=500 ns; solid line with points). At pH=12, already the initial spectrum is that of the deprotonated Trp· radical (t=50 ns).
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