Abstract
SummaryPhotobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)3]2+ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions are scavenged by laccase to form water instead of H2O2. The net result is the photo accumulation of highly oxidizing [Ru(bpy)3]3+. This study provides ground for the use of laccase in tandem with a light-driven oxidative process and O2 as one-electron transfer relay and as four-electron substrate to be a sustainable final electron acceptor in a photocatalytic process.
Highlights
Electron transfer (ET) reactions are of fundamental importance in biological processes such as photosynthesis and respiration (Winkler, 2005)
At a type 1 (T1) Cu ion site the one-electron oxidation of substrate molecules occurs, and electrons are sequentially transferred inside the protein to a trinuclear copper catalytic center (TNC) where O2 is eventually reduced in a four-electron, four-proton reaction (Jones and Solomon, 2015; Bertrand et al, 2002)
The amplitude of the measured absorption changes at long times, i.e., after decay of the excited state (Figure 1, inset) indicates a yield for the laser-flash-induced formation of the charge-separated state (CSS) of [Ru(bpy)3]3+ and reduced LAC3 (CuI) of only 0.7%. This yield is 17 times lower than what is expected for the products of the quenching process, suggesting that less than 10% of the quenching reactions lead to an effective reduction of laccase despite the high driving force for the ET from the ruthenium to the copper center T1 of –DG = 1.52 eV estimated from the redox potentials (E(RuIII/RuII*) = À0.84 V; E(CuII/CuI) = 0.68 V) (Kalyanasundaram, 1982a; Balland et al, 2008; Bock et al, 1979)
Summary
Electron transfer (ET) reactions are of fundamental importance in biological processes such as photosynthesis and respiration (Winkler, 2005). Chemists and biologists are gathering their efforts to use light energy to drive enzymes to realize both chemical oxidation and reduction reactions of high importance for our societies (Lee et al, 2018; Litman et al, 2018; Guo et al, 2018; Schmermund et al, 2019) These targets are gaining much attention from both a fundamental and applied point of view due to the fact that to date we do not have catalytic systems that can match the activities and specificities of the biological catalysts.
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