Hydrogenase enzymes use first-row transition metals to interconvert H2 with protons and electrons, reactions that are important for the storage and recovery of energy from intermittent sources such as solar, hydroelectric, and wind. Here we present Ni(P(Cy)2N(Gly)2)2, a water-soluble molecular electrocatalyst with the amino acid glycine built into the diphosphine ligand framework. Proton transfer between the outer coordination sphere carboxylates and the second coordination sphere pendant amines is rapid, as observed by cyclic voltammetry and FTIR spectroscopy, indicating that the carboxylate groups may participate in proton transfer during catalysis. This complex oxidizes H2 (1-33 s(-1)) at low overpotentials (150-365 mV) over a range of pH values (0.1-9.0) and produces H2 under identical solution conditions (>2400 s(-1) at pH 0.5). Enzymes employ proton channels for the controlled movement of protons over long distances-the results presented here demonstrate the effects of a simple two-component proton channel in a synthetic molecular electrocatalyst.

The tetraruthenium-substituted polyoxometalate Cs(9)[(gamma-PW(10)O(36))(2)Ru(4)O(5)(OH)(H(2)O)(4)] was synthesized and structurally, spectroscopically and electrochemically characterized; it was shown to be a catalyst for visible-light-induced water oxidation.

Four new cyclic 1,5-diaza-3,7-diphosphacyclooctane ligands have been prepared and used to synthesize [Ni(P(Ph)(2)N(R)(2))(2)](2+) complexes in which R is a mono- or dipeptide. These complexes represent a first step in the development of an outer-coordination sphere for this class of complexes that can mimic the outer-coordination sphere of the active sites of hydrogenase enzymes. Importantly, these complexes retain the electrocatalytic activity of the parent [Ni(P(Ph)(2)N(Ph)(2))(2)](2+) complex in an acetonitrile solution with turnover frequencies for hydrogen production ranging from 14 to 25 s(-1) in the presence of p-cyanoanilinium trifluoromethanesulfonate and from 135 to 1000 s(-1) in the presence of protonated dimethylformamide, with moderately low overpotentials, ∼0.3 V. The addition of small amounts of water results in rate increases of 2-7 times. Unlike the parent complex, these complexes demonstrate dynamic structural transformations in solution. These results establish a building block from which larger peptide scaffolding can be added to allow the [Ni(P(R)(2)N(R')(2))(2)](2+) molecular catalytic core to begin to mimic the multifunctional outer-coordination sphere of enzymes.

A new polyoxometalate of earth adundant elements [{Co(4)(μ-OH)(H(2)O)(3)}(Si(2)W(19)O(70))](11-) has been synthesized, characterized and shown to be a water oxidation catalyst. The initial catalytic complex is unstable and slowly undergoes hydrolysis. The hydrolysis products have been isolated and characterized, and their catalytic water oxidation activity is assessed.

We report the incorporation of a simple enzyme-inspired proton channel onto a hydrogen oxidation catalyst. This modification facilitates proton transfer and lowers the overpotential for oxidation of H2 by 300 mV when using water as a base.

The [Ni(P R 2 N R′ 2 ) 2 ] 2+ complexes (where P R 2 N R′ 2 is 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) are fast electrocatalysts for H 2 production and oxidation. Binding of a fifth ligand (CH 3 CN or BF 4 − ) or chair/boat isomerization has the potential to slow catalysis by blocking the addition of H 2 or by incorrectly positioning the pendant amines. We report the structural dynamics of a series of nickel complexes characterized by NMR spectroscopy and theoretical modeling to examine the effects of the fifth ligand for the Ni(II) complexes, including CH 3 CN, BF 4 − , Cl − , and H − , as well as the differences in dynamics between the Ni(II) and Ni(0) oxidation states. A fast exchange process was observed for the [Ni(CH 3 CN)(P R 2 N R′ 2 ) 2 ] 2+ complexes, with rates ranging from 10 4 to 10 7 s −1 depending on the phosphorus and nitrogen substituents on the P R 2 N R′ 2 ligand. This exchange process was identified to occur through a multistep mechanism, which consists of dissociation of the acetonitrile, boat/chair isomerization of each of the four rings (including nitrogen inversion), and reassociation of an acetonitrile on the opposite side of the complex. The rate of the chair/boat inversion was found to be influenced by varying the substituent on the nitrogen atom, but the rate of the overall exchange process is at least an order of magnitude faster than the catalytic rate in acetonitrile, demonstrating that the structural dynamics of the [Ni(CH 3 CN)-(P R 2 N R′ 2 ) 2 ] 2+ complexes do not hinder catalysis. Possible catalytic implications of the coordination of a fifth ligand to the Ni(II) complex are discussed.

A permissive role of nitric oxide (NO) in endothelial cell migration and angiogenesis promoted by vascular endothelial growth factor (VEGF), endothelin, and substance P has previously been established. The present studies were designed to examine the mechanism(s) involved in the NO effect on focal adhesions. Time-lapse videomicroscopy of human umbilical vein endothelial cells (HUVECs) plated on the silicone rubber substrate revealed that unstimulated cells were constantly remodeling the wrinkling pattern, indicative of changing tractional forces. Application of NO donors reversibly decreased the degree of wrinkling, consistent with the release of tractional forces exerted by focal adhesions and stress fibers. Morphometric and immunocytochemical analyses showed that NO inhibited adhesion and spreading of HUVECs and attenuated recruitment of paxillin to focal adhesions. NO also had a profound dose-dependent effect on the formation of stress fibers by HUVECs. De novo formation of focal adhesions in HUVECs was significantly diminished in the presence of NO donors. Migration of HUVECs showed an absolute requirement for the functional NO synthase. NO donors did not interfere with focal adhesion kinase recruitment to focal adhesions but affected the state of its tyrosine phosphorylation, as judged from the results of immunoprecipitation and immunoblotting experiments. Videomicroscopy of HUVECs presented with VEGF in a micropipette showed that the rate of cell migration was slowed down by NO synthase inhibition as well as by inhibition of tyrosine phosphorylation. Collectively, these data indicate that NO reversibly releases tractional forces exerted by spreading endothelial cells via interference with the de novo formation of focal adhesions, tyrosine phosphorylation of components of focal adhesion complexes, and assembly of stress fibers.

Catalytic, peptide-containing metal complexes with a well-defined peptide structure have the potential to enhance molecular catalysts through an enzyme-like outer coordination sphere. Here, we report the synthesis and characterization of an active, peptide-based metal complex built upon the well-characterized hydrogen production catalyst [Ni(P(Ph)2N(Ph))2](2+) (P(Ph)2N(Ph)=1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane). The incorporated peptide maintains its β-hairpin structure when appended to the metal core, and the electrocatalytic activity of the peptide-based metal complex (≈100,000 s(-1)) is enhanced compared to the parent complex ([Ni(P(Ph)2N(APPA))2](2+); ≈50,500 s(-1)). The combination of an active molecular catalyst with a structured peptide provides a scaffold that permits the incorporation of features of an enzyme-like outer-coordination sphere necessary to create molecular electrocatalysts with enhanced functionality.