Macromolecule-metal complex interactions as precursors for the catalytic chemical modification of polymers

Chemical modification of polymers via catalysis has recently emerged as an area of increasing importance in macromolecular chemistry. It provides an efficient synthetic route for the production of novel polymers with desirable physical properties and functional groups which are often inaccessible by conventional polymerization techniques. Diene-based polymers and copolymers are ideal for chemical modification because of the technological importance associated with the parent materials and the reactivities of the double bonds in the polymer chain. In employing organometallic catalysts for such modifications, it has been found that the ligand environment of the catalyst as well as the functionality of the polymer has a profound effect on the nature of the macromolecule-metal complex interaction and the resulting polymer modification. The importance of the macromolecule metal complex interactions and the design of appropriate catalyst systems is illustrated for the hydrogenation, hydroformylation/hydroxymethylation and hydrosilylation of a number of polymers.     

Understanding and tuning the catalytic bias of hydrogenase

When enzymes are optimized for biotechnological purposes, the goal often is to increase stability or catalytic efficiency. However, many enzymes reversibly convert their substrate and product, and if one is interested in catalysis in only one direction, it may be necessary to prevent the reverse reaction. In other cases, reversibility may be advantageous because only an enzyme that can operate in both directions can turnover at a high rate even under conditions of low thermodynamic driving force. Therefore, understanding the basic mechanisms of reversibility in complex enzymes should help the rational engineering of these proteins. Here, we focus on NiFe hydrogenase, an enzyme that catalyzes H(2) oxidation and production, and we elucidate the mechanism that governs the catalytic bias (the ratio of maximal rates in the two directions). Unexpectedly, we found that this bias is not mainly determined by redox properties of the active site, but rather by steps which occur on sites of the proteins that are remote from the active site. We evidence a novel strategy for tuning the catalytic bias of an oxidoreductase, which consists in modulating the rate of a step that is limiting only in one direction of the reaction, without modifying the properties of the active site.     

Fine tuning the reactivity of corrole-based catalytic antioxidants

In order to determine the electronic factors that may affect the catalytic antioxidant activity of water-soluble metallocorroles a series of 10-aryl-5,15-pyridinium manganese(III) corroles was prepared. These complexes were examined regarding the effect of the C(10) substituent on the Mn(IV)/Mn(III) redox potentials, catalytic rate constants for decomposition of HOONO, prevention of tyrosine nitration, and superoxide dismutase activity. This structure-activity relationship investigation provides new insight regarding the mechanism by which manganese(III) corroles act as catalytic antioxidants. It also discloses the superiority of the C(10)-anysil-substituted complex in all examined aspects.     

A nanoreactor for tuning the chemical reactivity of a solute

Present results demonstrate that the redox potential and hence the chemical reactivity of a solute dissolved in a polymer-surfactant supramolecular assembly, considered as a nanoreactor, can be tuned substantially by changing the composition of the supramolecular assembly. It is understood from detailed study that, on changing the polymer-surfactant composition of the supramolecular assembly, the probe undergoes a change in its location in these nanoreactors and accordingly its physical and chemical properties can be modulated.     

Two-particle entropy and structural ordering in liquid water

Entropies of simple point charge (SPC) water were calculated over the temperature range 278-363 K using the two-particle correlation function approximation. Then, the total two-particle contribution to the entropy of the system was divided into three parts, which we call translational, configurational, and orientational. The configurational term describes the contribution to entropy, which originates from spatial distribution of surrounding water molecules (treated as points, represented by the center of mass) around the central one. It has been shown that this term can serve as the metric of the overall orientational ordering in liquid water. Analyzing each of these three terms as a function of intermolecular distance, r, we also find a rational definition of the hydration shell around the water molecule; the estimated radii of the first and second hydration shells are 0.35 nm and 0.58 nm, respectively. We find, moreover, that the first hydration shell around the water molecule participates roughly in 70% of the total orientational entropy of water, and this rate is roughly temperature independent.     

Optimization of chemical ordering in AgAu nanoalloys

The determination of optimal chemical ordering in nanoalloys, i.e. of the most stable pattern in which atoms are arranged in bi- or multicomponent metallic clusters, is quite complex due to the enormous number of different possible configurations. This problem is very difficult to tackle by first-principle methods except for very small systems. On the other hand, the treatment at the atomistic potential level is complicated in many cases (such as AgAu) by charge transfer effects between atoms of different species in different coordination environments. Here an empirical atomistic model is developed to take into account such effects. The model is used to determine the optimal chemical ordering in AgAu nanoalloys. Charge transfer between atoms is taken into account by a modification of the charge equilibration method of Goddard and Rappé [J. Phys. Chem., 1991, 95, 3358], in which a coordination-dependent electronegativity and hardness are introduced. The model is applied to the determination of chemical ordering in AgAu nanoalloys. It is shown that the inclusion of charge transfer effects is important for improving the agreement of the atomistic model with density-functional calculations, leading to the determination of lower-energy chemical ordering patterns.     

Hydrolytic reaction by zinc finger mutant peptides: successful redesign of structural zinc sites into catalytic zinc sites

To redesign a metal site originally required for the stabilization of a folded protein structure into a functional metal site, we constructed a series of zinc finger mutant peptides such as zf(CCHG) and zf(GCHH), in which one zinc-coordinating residue is substituted into a noncoordinating one. The mutant peptides having water bound to the zinc ion catalyzed the hydrolysis of 4-nitrophenyl acetate as well as the enantioselective hydrolysis of amino acid esters. All the zinc complexes of the mutant peptides showed hydrolytic activity, depending on their peptide sequences. In contrast, the zinc complex of the wild-type, zf(CCHH), and zinc ion alone exhibited no hydrolytic ability. These results clearly indicate that the catalytic abilities are predominantly attributed to the zinc center in the zinc complexes of the mutant peptides. Kinetic studies of the mutant peptides demonstrated that the catalytic hydrolysis is affected by the electron-donating ability of the protein ligands and the coordination environment. In addition, the pH dependence of the hydrolysis strongly suggests that the zinc-coordinated hydroxide ion participates the catalytic reaction. This report is the first successful study of catalytically active zinc finger peptides.     

On the possibility of structural and magnetic ordering in magnetic colloids

The aggregation stability of a magnetic colloid at an excess content of a surfactant is studied. The presence of aggregates with a nonzero magnetic moments is revealed; on this basis, magnetic ordering of magnetic particles in them is regarded as possible. The possible mechanisms of the formation of periodic structural lattices appearing under the action of a direct electric field on the magnetic colloid are studied. A fundamental difference between the structurization processes induced by a surfactant and by an electric field is noted: structurization processes occurring at the excess of surfactant may be associated with the flocculation, whereas such processes proceeding under the effect of an electric field may be due to phase separation of the colloid.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 161–166.Original Russian Text Copyright © 2005 by Dikanskii, Vegera, Zakinyan, Nechaeva, Gladkikh.     

Solvent-effects tuning the catalytic reactivity of prolinamides in asymmetric aldol reactions

Novel prolinamides were prepared and applied as organocatalysts in the asymmetric aldol reaction. Stable imidazolidinones were formed between prolinamides and aromatic aldehydes in organic solvents. It was found that aqueous conditions can significantly suppress the formation of the unwanted imidazolidinone intermediate and improve the catalytic activity of the prolinamides. As a consequence, high chemical yields (up to 99%) and good diastereoselectivity (up to >20:1 dr) and enantioselectivity (up to 95% ee) were achieved in 2-Me-THF or brine. This strategy could serve as a general solution to enhance the performance of prolinamides as organocatalysts.     

Factors determining the enzyme catalytic power caused by noncovalent interactions: Charge alterations in enzyme active sites

Understanding the origin of the enormous catalytic power of enzymes is very important. Electrostatic interactions and desolvation are the phenomena that are most proposed to explain the catalysis of enzymes; however, they also decelerate enzymatic reactions. How enzymes catalyze reactions through noncovalent interactions is still not well-understood. In this study, we explored how enzyme-substrate noncovalent interactions affect the free energy barriers (ΔG³s) of reactions by using a theoretical derivation approach. We found that enzymes reduce ΔG³s of reactions by decreasing positive charges and/or increasing negative charges in the electron-donating centers and by decreasing negative charges and/or increasing positive charges in the electron-accepting centers of reactions. Enzyme-substrate noncovalent interactions are essential approaches through which the charge alterations lead to ΔG³ reductions. Validations with reported experimental data demonstrated that this charge alteration mechanism can explain the catalyses caused by diverse types of noncovalent interactions. Electrostatic interactions and desolvation are the most observed noncovalent interactions essential for ΔG³ reductions. This mechanism does not contradict any specific enzymatic catalysis and overcomes the shortages of the electrostatic interaction and desolvation mechanisms. This study can provide useful guidance in exploring enzymatic catalysis and designing catalyst.     

Quantum chemical modelling of ethene epoxidation with hydrogen peroxide: role of catalytic sites

Ethene epoxidation with hydrogen peroxide was studied on hydroxylated binuclear metal sites, using DFT calculations at the B3LYP/6-311+G(d,p) level of theory. A decrease of the activation enthalpy of approximately 100 kJ mol(-1) was observed compared to the gas phase reaction between hydrogen peroxide and ethene. It was previously shown that micro-solvation with water reduces the activation enthalpy by approximately 77 kJ mol(-1) and only the additional 24 kJ mol(-1) can be attributed to the binuclear site. Three different metal centres were tested, Ti(iv), Si(iv) and Ge(iv), in order to investigate any specific role of the metal centre on the activation enthalpy. The results clearly show that the activation enthalpy is independent on the nature of the metal centre. This emphasises the role of the hydrogen bonded network provided by the hydroxylated metal sites, on the stabilisation of the transitions state. In ref. 1 (A. Lundin, I. Panas and E. Ahlberg, J. Phys. Chem. A, 2007, 111, 9080) it was demonstrated that, at the transition state and upon micro-solvation, the hydrogen peroxide entity becomes polarized within the hydrogen bonding network, forming a negatively-charged fragment distant from the ethene molecule and a positively-charged fragment directly involved in the oxygen insertion step. The same mechanism was found to hold also for the reaction at the binuclear catalytic site, since the required hydrogen bonding is effectively provided by the hydroxylated metal centres. This mechanism is compared to the two-step pathway which employs a metal peroxide intermediate. Both reaction channels were found to be plausible in confined environments.     

Role of carboxylate chelating agents on the chemical, structural and textural properties of hydroxyapatite

Organically-modified hydroxyapatite materials were synthesized through the addition of oxalic, succinic, adipic and citric acids to a calcium hydroxide solution before neutralization by ammonium dihydrogenphosphate. All carboxylic acids have a significant influence on apatite crystallinity and nanoparticle size, as indicated by XRD and TEM. Chemical and thermogravimetric analyses as well as FTIR and {(1)H}-(13)C CP MAS NMR spectroscopies indicate that the additives are present in the final material. (1)H, {(1)H}-(31)P HPDec MAS, CP MAS and 2D {(1)H}-(31)P CP-HETCOR MAS NMR experiments suggest that carboxylic acids are localized on the apatite nanocrystallite surface, resulting in the formation of a disordered outer layer. Nitrogen sorption measurements indicate minor modifications of the specific surface area of the resulting mesoporous materials upon carboxylic acid addition but more significant variations in the average dimensions of the pores as well as in the chemical nature of the pore surface. Although these evolutions are mainly in good agreement with the ligand affinity for calcium ions in solution, an unexpected difference was observed between succinic and adipic acid, that may be attributed to steric constraints resulting from the interfacial nature of the calcium-ligand interactions. These data should provide useful guidelines to identify novel efficient additives to control apatite growth.     

Discrimination and ordering of chemical structures by the number of walks

The use of the number of walks for the discrimination of graphs representing chemical structures is discussed. A highly selective graph-theoretical index based on the number of walks is defined. The index values were calculated for 661 acyclic and 376 cyclic structures. The selectivity of the new index is compared to that of some of the most selective previously defined indexes. The ordering of structures induced by the value of the index is also considered.Presented in part at the 7th International Conference on Computers in Chemical Research and Education, held in Garmisch-Partenkirchen, Germany, June 10–14, 1985     

Targeted acetylation of wood: a tool for tuning wood-water interactions

Cellulose - Wood is an increasingly important material in the sustainable transition of societies worldwide. The performance of wood in structures is intimately tied to the presence of moisture in...     

Tracking the structural evolution at atomic-scale in the spinel Mn3O4 induced by electrochemical cycling

Mn₃O₄ is a possible candidate for use as an electrode material and has been found to undergo structural transformation during electrochemical cycling. Clarifying the transformation process is important in developing methods for improving electrochemical performance. Here, using scanning transmission electron microscopy (STEM) combined with the electron energy loss spectroscopy (EELS) technique in aberration-corrected TEM, we succeeded in tracking the structural evolution at an atomic-scale and identified the intermediate stage as rock-salt-structured MnO. A reasonable route was deduced via which the spinel Mn₃O₄ was transformed firstly into MnO and then into MnO₂.     

Engineering active sites for enhancing synergy in heterogeneous catalytic oxidations

The simultaneous isomorphous substitution of Al(III) and P(V) ions, in an aluminophosphate framework, with redox active Co(III) and Ti(IV) metal ions, generates highly active single-site heterogeneous catalysts that exhibit considerable synergy, compared to their corresponding monometallic analogues, in the catalytic epoxidation of olefins.     

Vibrational spectra of spinels with cation ordering on the octahedral sites

Infrared and Raman spectra are reported for compounds with 1 : 1, 1 : 3, and 1 : 2 mixed ordering on the octahedral sites of the spinel structure. The spectra of the superstructures are complex with many more bands observed than occur in the parent spinel structure, although less than were predicted by group theoretical analysis. A qualitative interpretation of the spectra can be made through the large Brillouin zone concept.