Function of the Diiron Cluster of Escherichia coli Class Ia Ribonucleotide Reductase in Proton-Coupled Electron Transfer

The class Ia ribonucleotide reductase (RNR) from Escherichia coli employs a free-radical mechanism, which involves bidirectional translocation of a radical equivalent or “hole” over a distance of ~35 Å from the stable diferric/tyrosyl-radical (Y[subscript 122]•) cofactor in the β subunit to cysteine 439 (C[subscript 439]) in the active site of the α subunit. This long-range, intersubunit electron transfer occurs by a multistep “hopping” mechanism via formation of transient amino acid radicals along a specific pathway and is thought to be conformationally gated and coupled to local proton transfers. Whereas constituent amino acids of the hopping pathway have been identified, details of the proton-transfer steps and conformational gating within the β sununit have remained obscure; specific proton couples have been proposed, but no direct evidence has been provided. In the key first step, the reduction of Y[subscript 122]• by the first residue in the hopping pathway, a water ligand to Fe[subscript 1] of the diferric cluster was suggested to donate a proton to yield the neutral Y[subscript 122]. Here we show that forward radical translocation is associated with perturbation of the Mössbauer spectrum of the diferric cluster, especially the quadrupole doublet associated with Fe[subscript 1]. Density functional theory (DFT) calculations verify the consistency of the experimentally observed perturbation with that expected for deprotonation of the Fe[subscript 1]-coordinated water ligand. The results thus provide the first evidence that the diiron cluster of this prototypical class Ia RNR functions not only in its well-known role as generator of the enzyme’s essential Y[subscript 122]•, but also directly in catalysis.National Institutes of Health (U.S.) (GM-29595

Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition

Alkanethiolate protected gold nanoparticles are one of the most widely used systems in modern science and technology, where the emergent electronic properties of the gold core are valued for use in applications such as plasmonic solar cells, photocatalysis, and photothermal heating. Though choice in alkane chain length is not often discussed as a way in which to control the electronic properties of these nanoparticles, we show that the chain length of the alkyl tail exerts clear control over the electronic properties of the gold core, as determined by conduction electron spin resonance spectroscopy. The control exerted by chain length is reported on by changes to the g-factor of the metallic electrons, which we can relate to the average surface potential on the gold core. We propose that the surface potential is modulated by direct charge donation from the ligand to the metal, resulting from the formation of a chemical bond. Furthermore, the degree of charge transfer is controlled by differences between the dielectric constant of the medium and the ligand shell. Together, these observations are used to construct a simple electrostatic model that provides a framework for understanding how surface chemistry can be used to modulate the electronic properties of gold nanoparticles

Structural, Electronic, and Magnetic Characterization of a Dinuclear Zinc Complex Containing TCNQ^– and a μ‑[TCNQ–TCNQ]^2– Ligand

A dinuclear zinc complex containing both a σ-dimerized 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligand ([TCNQ–TCNQ]^2–) and TCNQ^– was synthesized for the first time. This is the first instance of a single molecular complex with a bridging [TCNQ–TCNQ]^2– ligand. Each zinc center is coordinated with two 2,2′-bipyrimidines and one TCNQ^–, and the remaining coordination site is occupied by a [TCNQ–TCNQ]^2– ligand, which bridges the two zinc centers. The complex facilitates π-stacking of TCNQ^– ligands when crystallized, which gives rise to a near-IR charge-transfer transition and strong antiferromagnetic coupling

An EPR/ENDOR, Mössbauer, and quantum chemical investigation of diiron complexes mimicking the active oxidized state of [FeFe]hydrogenase

We present a study of two compounds closely resembling the active site of [FeFe]hydrogenase, which catalyzes the reversible heterolytic splitting of molecular hydrogen. Utilizing Mössbauer and advanced electron paramagnetic resonance techniques, we were able to resolve the electronic structure of these model compounds in great detail. The experimental results are also compared with quantum-chemical calculations in order to gain more insight into their electronic properties. The obtained data allow us to better understand the function of the native hydrogen catalyst