Electron transfer reactions slow down when they become very thermodynamically favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge-separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.

NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni 2+ to Ni 3+ , followed by oxidation to a mixed Ni 3+/4+ state at a potential coincident with the onset of OER activity. Calculations on the 25% Fedoped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe 4+ and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixedmetal oxidation states in heterogeneous catalysts.NiFe oxyhydroxide | oxygen evolution reaction | electrocatalysis | spectroelectrochemistry | density functional theory T he photoelectrochemical conversion of water into O 2 and H 2 is a major focus of energy storage and conversion efforts (1-4), with significant attention directed toward development of efficient catalysts for water oxidation and reduction. Such catalysts should operate at low overpotential, exhibit high selectivity, and be composed of earth-abundant materials. Commercial electrolyzers typically use transition-metal-oxide electrocatalysts for the oxygen evolution reaction (OER) (5, 6), and nickel, nickel-iron, and other mixed-metal oxides are especially effective under alkaline conditions (7,8). Despite the importance and potential future impact of these materials, many features of their catalytic mechanism are poorly understood.Nickel oxyhydroxide has long been associated with OER electrocatalysis (9, 10); however, much of the activity in this material has been shown to arise from the presence of Fe impurities (7, 11). This conclusion complements extensive independent studies demonstrating the effectiveness of NiFebased oxide and oxyhydroxide materials as OER electrocatalysts (12-14), including a survey of nearly 3,500 mixed-metal-oxide compositions, which drew attentio...

BackgroundRetinoblastoma (Rb) is the most common primary intraocular tumor in children. Local treatment of the intraocular disease is usually effective if diagnosed early; however advanced Rb can metastasize through routes that involve invasion of the choroid, sclera and optic nerve or more broadly via the ocular vasculature. Metastatic Rb patients have very high mortality rates. While current therapy for Rb is directed toward blocking tumor cell division and tumor growth, there are no specific treatments targeted to block Rb metastasis. Two such targets are matrix metalloproteinases-2 and -9 (MMP-2, −9), which degrade extracellular matrix as a prerequisite for cellular invasion and have been shown to be involved in other types of cancer metastasis. Cancer Clinical Trials with an anti-MMP-9 therapeutic antibody were recently initiated, prompting us to investigate the role of MMP-2, −9 in Rb metastasis.MethodsWe compare MMP-2, −9 activity in two well-studied Rb cell lines: Y79, which exhibits high metastatic potential and Weri-1, which has low metastatic potential. The effects of inhibitors of MMP-2 (ARP100) and MMP-9 (AG-L-66085) on migration, angiogenesis, and production of immunomodulatory cytokines were determined in both cell lines using qPCR, and ELISA. Cellular migration and potential for invasion were evaluated by the classic wound-healing assay and a Boyden Chamber assay.ResultsOur results showed that both inhibitors had differential effects on the two cell lines, significantly reducing migration in the metastatic Y79 cell line and greatly affecting the viability of Weri-1 cells. The MMP-9 inhibitor (MMP9I) AG-L-66085, diminished the Y79 angiogenic response. In Weri-1 cells, VEGF was significantly reduced and cell viability was decreased by both MMP-2 and MMP-9 inhibitors. Furthermore, inhibition of MMP-2 significantly reduced secretion of TGF-β1 in both Rb models.ConclusionsCollectively, our data indicates MMP-2 and MMP-9 drive metastatic pathways, including migration, viability and secretion of angiogenic factors in Rb cells. These two subtypes of matrix metalloproteinases represent new potential candidates for targeted anti-metastatic therapy for Rb.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-017-3418-y) contains supplementary material, which is available to authorized users.

The first step of the hydrogen evolution reaction, an important reaction for the storage of renewable energy, is the formation of a surface-adsorbed hydrogen atom through proton discharge to the electrode surface, commonly known as the Volmer reaction. Herein a theoretical description of the Volmer reaction is presented. In this formulation, the electronic states are represented in the framework of empirical valence bond theory, and the solvent interactions are treated using a dielectric continuum model in the linear response regime. The nuclear quantum effects of the transferring proton are incorporated by quantization along the proton coordinate. The ground and excited state electron−proton vibronic free energy surfaces are computed as functions of the proton donor−acceptor distance and a collective solvent coordinate. In the fully adiabatic regime, the current densities and Tafel slopes are computed from the ground state vibronic free energy surface. This theory is applied to the proton-coupled electron transfer reaction involving proton discharge from H 3 O + in aqueous solution to a gold electrode. This theoretical model opens the door for future studies, including examination of the effects of vibronic nonadiabaticity, electronic friction, and solvent dynamics.

The selective reduction of O 2 , typically with the goal of forming H 2 O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por OMe ) (por OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e – /2H + or 4e – /4H + reduction of O 2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O 2 reduction to H 2 O 2 or H 2 O in DMF are determined and exhibit a Nernstian dependence on the acid p K a , while the Co III/II redox potential is independent of the acid p K a . The reaction product, H 2 O or H 2 O 2 , is defined by the relationship between the thermodynamic potential for O 2 reduction to H 2 O 2 and the Co III/II redox potential: selective H 2 O 2 formation is observed when the Co III/II potential is below the O 2 /H 2 O 2 potential, while H 2 O formation is observed when the Co III/II potential is above the O 2 /H 2 O 2 potential. Mechanistic studies reveal that the reactions generating H 2 O 2 and H 2 O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus p K a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O 2 to H 2 O.