Free radicals under control

Significance of hydrogen bonding networks in the proton-coupled electron transfer reactions of photosystem II from a quantum-mechanics perspective

All quantum-mechanical calculations provide insights into the effect of the hydrogen bonding network on the proton-coupled electron transfer at Y_Z and Y_D in photosystem II.

Hydrogen bond assisted photoinduced intramolecular electron transfer and proton coupled electron transfer in an ultrafast time domain using a ruthenium-anthraquinone dyad

PCET kinetics for the formation of charge-separated states was explored by using femtosecond transient absorption spectroscopy. Hydrogen bonding between water and the reduced anthraquinone accounted for thermodynamic and kinetic stabilization.

Enantioselective catalytic β-amination through proton-coupled electron transfer followed by stereocontrolled radical-radical coupling

A new mechanistic approach for the catalytic, enantioselective conjugate addition of nitrogen-based nucleophiles to acceptor-substituted alkenes is reported, which is based on a visible light induced and phosphate base promoted transfer of a single electron from a nitrogen nucleophile to a catalyst-bound acceptor-substituted alkene, followed by a stereocontrolled C-N bond formation through stereocontrolled radical-radical coupling. Specifically, N-aryl carbamates are added to the β-position of α,β-unsaturated 2-acyl imidazoles using a visible light activated photoredox mediator in combination with a chiral-at-rhodium Lewis acid catalyst and a weak phosphate base, affording new C-N bonds in a highly enantioselective fashion with enantioselectivities reaching up to 99% ee and >99 : 1 dr for a menthol-derived carbamate. As an application, the straightforward synthesis of a chiral β-amino acid ester derivative is demonstrated.

A Dinuclear Ruthenium-Based Water Oxidation Catalyst: Use of Non-Innocent Ligand Frameworks for Promoting Multi-Electron Reactions

Insight into how H2 O is oxidized to O2 is envisioned to facilitate the rational design of artificial water oxidation catalysts, which is a vital component in solar-to-fuel conversion schemes. Herein, we report on the mechanistic features associated with a dinuclear Ru-based water oxidation catalyst. The catalytic action of the designed Ru complex was studied by the combined use of high-resolution mass spectrometry, electrochemistry, and quantum chemical calculations. Based on the obtained results, it is suggested that the designed ligand scaffold in Ru complex 1 has a non-innocent behavior, in which metal-ligand cooperation is an important part during the four-electron oxidation of H2 O. This feature is vital for the observed catalytic efficiency and highlights that the preparation of catalysts housing non-innocent molecular frameworks could be a general strategy for accessing efficient catalysts for activation of H2 O.

Reaction dynamics and proton coupled electron transfer: studies of tyrosine-based charge transfer in natural and biomimetic systems

In bioenergetic reactions, electrons are transferred long distances via a hopping mechanism. In photosynthesis and DNA synthesis, the aromatic amino acid residue, tyrosine, functions as an intermediate that is transiently oxidized and reduced during long distance electron transfer. At physiological pH values, oxidation of tyrosine is associated with a deprotonation of the phenolic oxygen, giving rise to a proton coupled electron transfer (PCET) reaction. Tyrosine-based PCET reactions are important in photosystem II, which carries out the light-induced oxidation of water, and in ribonucleotide reductase, which reduces ribonucleotides to form deoxynucleotides. Photosystem II contains two redox-active tyrosines, YD (Y160 in the D2 polypeptide) and YZ (Y161 in the D1 polypeptide). YD forms a light-induced stable radical, while YZ functions as an essential charge relay, oxidizing the catalytic Mn₄CaO₅ cluster on each of four photo-oxidation reactions. In Escherichia coli class 1a RNR, the β2 subunit contains the radical initiator, Y122O•, which is reversibly reduced and oxidized in long range electron transfer with the α2 subunit. In the isolated E. coli β2 subunit, Y122O• is a stable radical, but Y122O• is activated for rapid PCET in an α2β2 substrate/effector complex. Recent results concerning the structure and function of YD, YZ, and Y122 are reviewed here. Comparison is made to recent results derived from bioengineered proteins and biomimetic compounds, in which tyrosine-based charge transfer mechanisms have been investigated. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.