Combinatorial Low‐Volume Synthesis of Well‐Defined Polymers by Enzyme Degassing

The synthesis of well‐defined polymers in a low‐volume, combinatorial fashion has long been a goal in polymer chemistry. Here, we report the preparation of a wide range of highly controlled homo and block co‐polymers by Enz‐RAFT (enzyme‐assisted reversible addition–fragmentation chain transfer) polymerization in microtiter plates in the open atmosphere. The addition of 1 μm glucose oxidase (GOx) to water/solvent mixtures enables polymerization reactions to proceed in extremely low volumes (40 μL) and low radical concentrations. This procedure provides excellent control and high conversions across a range of monomer families and molecular weights, thus avoiding the need to purify for screening applications. This simple technique enables combinatorial polymer synthesis in microtiter plates on the benchtop without the need of highly specialized synthesizers and at much lower volumes than is currently possible by any other technique.     

Spectroscopic investigations of Fe2+ complexation on nontronite clay

Diffuse reflectance (visible) and attenuated total internal reflection Fourier-transform infrared (ATR-FTIR) spectroscopies were used to examine a colloidal nontronite clay in the presence of Fe2+(aq). pH-dependent changes are observed in both types of spectra. In the visible region, a broad feature at approximately 750 nm appears as the pH is raised to circumneutral values. This absorbance band overlaps with a portion of the spectrum of the chemically reduced clay which is dominated by an intervalence charge-transfer transition between Fe2+ and Fe3+ within the mineral structure. The similarities between these spectra suggest that Fe2+(aq) adsorbs to the clay in such a way that it can undergo charge transfer with structural Fe3+ within the clay. ATR-FTIR spectra at pH 5-8 reveal a transformation in the Si-O stretching region between pH 6 and pH 7 with a shift of the component peaks to lower frequency. Taken together, these spectroscopic studies indicate that Fe2+ forms an inner-sphere complex with the clay at higher pH values. The pH threshold for these observed changes in physical and electronic structure is in good agreement with the point of zero charge (pzc) of the hydroxyl groups on the edge surfaces of the clay, suggesting that complexation of Fe2+ to deprotonated edge sites leads to the observed spectroscopic features.     

The determination of glutathione-4-hydroxynonenal (GSHNE), E-4-hydroxynonenal (HNE), and E-1-hydroxynon-2-en-4-one (HNO) in mouse liver tissue by LC-ESI-MS

Glutathione (GSH) conjugation of 4-hydroxy-2(E)-nonenal (HNE) is an efficient means of cellular detoxification. HNE is a byproduct of lipid peroxidation which has shown toxicity but also signaling roles. E-1-hydroxynon-2-en-4-one (HNO) is another byproduct of lipid peroxidation which has the same molecular weight as HNE. This study presents the LC-MS detection of GS-HNE, HNE, and HNO in tissue samples without derivatization and with minimal sample preparation. Tissue samples were taken from wild-type mice and knock-out mice, which have been bred without the RLIP76 transfer protein. Extraction procedures were developed to determine GS-HNE and HNE levels in the mouse liver tissue. A gradient elution LC-MS method was developed for GS-HNE analysis using electrospray ionization and selected ion monitoring (SIM). The HNE/HNO method involves isocratic elution due to instability issues. Higher levels of GSHNE, HNE, and HNO were found in the knock-out animals, due to the absence of the RLIP76 transport mechanism.     

Optical properties of two liquid crystal compounds: a comparative study

The optical properties of two liquid crystalline mesogens S4 (code name) and S5 (code name) have been studied as a function of temperature. Our interest is to compare the macroscopic properties of these two thermotropic mesogens and to study the effect of replacing the OC₆H₁₃ group in S4 by NCS group to produce S5.With this aim in view the thermal variations of birefringence have been determined for both the compounds. The variations of polarizabilities with temperature have also been evaluated from density variations and birefringence studies and the thermal dependence of the order parameters of the two compounds have thereby been determined and compared. The theoretical values of the order parameters have also been calculated using the McMillan and Maier–Saupe potentials and compared with the obtained experimental values.     

Moving protons with pendant amines: proton mobility in a nickel catalyst for oxidation of hydrogen

Proton transport is ubiquitous in chemical and biological processes, including the reduction of dioxygen to water, the reduction of CO(2) to formate, and the production/oxidation of hydrogen. In this work we describe intramolecular proton transfer between Ni and positioned pendant amines for the hydrogen oxidation electrocatalyst [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+) (P(Cy)(2)N(Bn)(2) = 1,5-dibenzyl-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane). Rate constants are determined by variable-temperature one-dimensional NMR techniques and two-dimensional EXSY experiments. Computational studies provide insight into the details of the proton movement and energetics of these complexes. Intramolecular proton exchange processes are observed for two of the three experimentally observable isomers of the doubly protonated Ni(0) complex, [Ni(P(Cy)(2)N(Bn)(2)H)(2)](2+), which have N-H bonds but no Ni-H bonds. For these two isomers, with pendant amines positioned endo to the Ni, the rate constants for proton exchange range from 10(4) to 10(5) s(-1) at 25 °C, depending on isomer and solvent. No exchange is observed for protons on pendant amines positioned exo to the Ni. Analysis of the exchange as a function of temperature provides a barrier for proton exchange of ΔG(?) = 11-12 kcal/mol for both isomers, with little dependence on solvent. Density functional theory calculations and molecular dynamics simulations support the experimental observations, suggesting metal-mediated intramolecular proton transfers between nitrogen atoms, with chair-to-boat isomerizations as the rate-limiting steps. Because of the fast rate of proton movement, this catalyst may be considered a metal center surrounded by a cloud of exchanging protons. The high intramolecular proton mobility provides information directly pertinent to the ability of pendant amines to accelerate proton transfers during catalysis of hydrogen oxidation. These results may also have broader implications for proton movement in homogeneous catalysts and enzymes in general, with specific implications for the proton channel in the Ni-Fe hydrogenase enzyme.     

Synthesis and hydride transfer reactions of cobalt and nickel hydride complexes to BX3 compounds

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].     

Electronic structure and transport properties of early transition metal tripledeckers

The electronic structure and transport properties of the Cp(2)BzM(2) (M = Sc, Ti, and V) tripledeckers are studied by spin polarized density functional theory and nonequilibrium Green's function method considering high-spin and low-spin states. Total energy calculations show that the sandwich structured Cp(2)BzSc(2) exists in a singlet state with no local magnetic moment on the Sc atoms. Cp(2)BzTi(2) in triplet state exists as a distorted tripledecker and is more stable than singlet and quintet states. Cp(2)BzV(2) stabilizes in the quintet state with a spin density of 2.4 on each vanadium atom. Hund's coupling plays a vital role in stabilizing the higher multiplets in case of titanium and vanadium clusters. In bigger clusters like Cp(3)Bz(2)M(4), Sc multidecker has one unpaired spin, Ti multidecker has five unpaired spins, and V multidecker has seven unpaired spins in total. Spin polarized electronic transport is found for all states of vanadium tripledecker and one state of the titanium tripledecker when connected to a gold two probe junction. Moderate to high-spin filter efficiencies are calculated for these states. Cp(2)BzSc(2) shows spin-independent electronic transport for all electronic states when introduced in the gold two probe junction. Current versus voltage curves are reported for selected clusters in the two probe setup.     

Characterization of photochemical processes for H2 production by CdS nanorod-[FeFe] hydrogenase complexes

We have developed complexes of CdS nanorods capped with 3-mercaptopropionic acid (MPA) and Clostridium acetobutylicum [FeFe]-hydrogenase I (CaI) that photocatalyze reduction of H(+) to H(2) at a CaI turnover frequency of 380-900 s(-1) and photon conversion efficiencies of up to 20% under illumination at 405 nm. In this paper, we focus on the compositional and mechanistic aspects of CdS:CaI complexes that control the photochemical conversion of solar energy into H(2). Self-assembly of CdS with CaI was driven by electrostatics, demonstrated as the inhibition of ferredoxin-mediated H(2) evolution by CaI. Production of H(2) by CdS:CaI was observed only under illumination and only in the presence of a sacrificial donor. We explored the effects of the CdS:CaI molar ratio, sacrificial donor concentration, and light intensity on photocatalytic H(2) production, which were interpreted on the basis of contributions to electron transfer, hole transfer, or rate of photon absorption, respectively. Each parameter was found to have pronounced effects on the CdS:CaI photocatalytic activity. Specifically, we found that under 405 nm light at an intensity equivalent to total AM 1.5 solar flux, H(2) production was limited by the rate of photon absorption (~1 ms(-1)) and not by the turnover of CaI. Complexes were capable of H(2) production for up to 4 h with a total turnover number of 10(6) before photocatalytic activity was lost. This loss correlated with inactivation of CaI, resulting from the photo-oxidation of the CdS capping ligand MPA.     

Tunable growth factor delivery from injectable hydrogels for tissue engineering

Current sustained delivery strategies of protein therapeutics are limited by the fragility of the protein, resulting in minimal quantities of bioactive protein delivered. In order to achieve prolonged release of bioactive protein, an affinity-based approach was designed which exploits the specific binding of the Src homology 3 (SH3) domain with short proline-rich peptides. Specifically, methyl cellulose was modified with SH3-binding peptides (MC-peptide) with either a weak affinity or strong affinity for SH3. The release profile of SH3-rhFGF2 fusion protein from hyaluronan MC-SH3 peptide (HAMC-peptide) hydrogels was investigated and compared to unmodified controls. SH3-rhFGF2 release from HAMC-peptide was extended to 10 days using peptides with different binding affinities compared to the 48 h release from unmodified HAMC. This system is capable of delivering additional proteins with tunable rates of release, while maintaining bioactivity, and thus is broadly applicable.