Solvolysis Mechanisms of RNA Phosphodiester Analogues Promoted by Mononuclear Zinc(II) Complexes: Mechanisic Determination upon Solvent Medium and Ligand Effects

The solvolysis mechanisms of RNA phosphodiester model 2-(hydroxypropyl)-4-nitrophenyl phosphate (HpPNP) catalyzed by mononuclear zinc(II) complexes are investigated in the paper via a theoretical approach. The general-base-catalyzed (GBC) and specific-base-catalyzed (SBC) mechanisms are thoroughly discussed in the paper, and the calculations indicate a SBC mechanism (also named as the direct nucleophilic attack mechanism) when the cyclization of HpPNP is promoted by the Zn:[12]aneN(3) complex ([12]aneN(3) = 1,5,9-triazacyclododecane). The ligand effect is considered by involving two different catalysts, and the results show that the increasing size catalyst provides a lower energy barrier and a significant mechanistic preference to the SBC mechanism. The solvent medium effect is also explored, and reduced polarity/dielectric constant solvents, such as light alcohols methanol and ethanol, are more favorable. Ethanol is proven to be a good solvent medium because of its low dielectric constant. The computational results are indicative of concerted pathways. Our theoretical results are consistent with and well interpret the experimental observations and, more importantly, provide practical suggestions on the catalyst design and selection of reaction conditions

A density functional theory study of the hydrolysis mechanism of phosphodiester catalyzed by a mononuclear Zn(II) complex

Density functional theory (DFT) calculations were used to explore the hydrolysis mechanism of the DNA analog BNPP (BNPP= bis(4-nitrophenyl)phosphate) catalyzed by the mononuclear zinc(II):OH- complex of 1,5,9-triazacyclododecane (Zn:([12] aneN(3))). We present a binding mode in which one terminal phosphoryl oxygen atom as well as the nucleophilic group (hydroxyl anion) binds to zinc center. Two potential mechanisms were found as follows: one is a concerted mechanism with a reaction barrier of 18.1 kcal/mol in liquid phase of implicit solvent and 13.8 kcal/mol in liquid phase of explicit solvent of water molecules; the other is a stepwise mechanism with a hydroxylated phosphate reaction intermediate of a quasi-trigonal bipyramid configuration but is less feasible. Both the concerted reaction pathway and stepwise reaction pathway are S(N)2 manner of nucleophilic substitution reactions. Meanwhile polar protic solvents like water, methanol and ethanol are favored in the catalyst-assisted hydrolysis mechanism. We explore the rationality of deprotonation of mono-anionic phosphates in the transient products and find that it is difficult to dissociate a proton and the ultimate product is NPP- rather than NPP2-. These results are consistent with and systematically interpret the experimental observations, more importantly, provide useful suggestions in the catalyst design and solvent selection. (C) 2012 Elsevier B.V. All rights reserved

Mechanism of aquation and nucleobase binding of ruthenium (II) and osmium (II) arene complexes: A systematic comparison DFT study

A systematic mechanistic study is reported for the aquation and nucleobase binding process of a series of Ru II and Os II arene-based anticancer drug complexes using density functional theory and COSMO implicit solvent model. The structures of Ru II and Os II complexes are similar to each other because of lanthanide contraction of osmium. However, the aquation was substantially more facile for Ru II complexes than Os II complexes. As to nucleobase substitution, various possible paths were explored based on considering the initial conformation of ethylenediamine (en) and the orientation of guanine (G) and adenine (A). Both Ru and Os complexes exhibited much lower free energy barrier for G than A. This observed predominance toward G mainly originated from larger stabilization energy for the initially formed complex, compared with A, in combination with favored kinetics and thermodynamics. Moreover, the calculations indicated that pK as of Os-bound water molecules were uniformly much lower than those of Ru-bound water molecules. Analysis of the natural bond orbital (NBO) charge reveals that Os II has a higher net positive charge than Ru II, leading to a stronger electrostatic attractive interaction between Os II and chloride or water, resulted in higher activation barrier for their departure. These differences between Ru II and Os II en complexes discussed in our study may partly explain the inertness of the Os II complexes in biological system. © 2011 Elsevier B.V. All rights reserved

Effect of Material Selection and Surface Texture on Tribological Properties of Key Friction Pairs in Water Hydraulic Axial Piston Pumps: A Review

A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, especially under extreme conditions of high speed and high pressure, limit the engineering application of the water hydraulic axial piston pump. In this review, the research progress for key friction pair materials (such as special corrosion-resistant alloys, engineering plastics, and engineering ceramics) for water hydraulic axial piston pumps is, firstly, summarized. Secondly, inspired by nature, the processing methods, lubrication drag-reduction mechanism, and tribological properties of the biomimetic surface textures are discussed. The effects of the surface texture shape, equivalent diameter, depth, and arrangement on the pump’s tribological properties are reviewed in detail. Finally, the application status of, and problems with, surface texture technology in water hydraulic axial piston pumps are summarized. It is suggested that future studies should focus on the multi-field coupling lubrication anti-friction mechanism of the multi-type composite texture under extreme conditions and mixed lubrication; and the anti-wear performance of the texture coupled with a coating modification, to further promote the surface texture in the field of lubrication antifriction engineering applications

Facile Synthesis of FeCoNiCuIr High Entropy Alloy Nanoparticles for Efficient Oxygen Evolution Electrocatalysis

The lack of an efficient and stable electrocatalyst for oxygen evolution reaction (OER) greatly hinders the development of various electrochemical energy conversion and storage techniques. In this study, we report a facile synthesis of FeCoNiCuIr high-entropy alloy nanoparticles (HEA NPs) by a one-step heat-up method. The involvement of glucose made the NPs grow uniformly and increased the valence of Ir. The resulting FeCoNiCuIr NPs exhibit excellent OER performance in alkaline solution, with a low overpotential of 360 mV to achieve a current density of 10 mA cm−2 at a Tafel slope of as low as 70.1 mV dec−1. In addition, high stability has also been observed, which remained at 94.2% of the current density after 10 h constant electrolysis, with a constant current of 10 mA cm−2. The high electrocatalytic activity and stability are ascribed to the cocktail effect and synergistic effect between the constituent elements. Our work holds the potential to be extended to the design and synthesis of high-performance electrocatalysts

Simulation Study on Bearing Lubrication Mechanism and Friction Characteristics of the Biomimetic Non-Smooth Surface of a Cross-Scale, Second-Order Compound Microstructure

The reasonable design of biomimetic non-smooth surfaces is a novel and effective way to solve problems such as the poor lubricity and serious friction and wear of friction pairs of seawater axial piston pumps. Inspired by cross-scale, second-order compound microstructures on the surfaces of some living organisms, a hydrodynamic lubrication model of a slipper pair with a surface featuring spherical pits containing spherical convex hulls was built. This study analyzed the bearing lubrication mechanism and friction characteristics of cross-scale, second-order compound microstructure from the microflow perspective via the CFD method and optimized the working and geometric parameters using a hybrid orthogonal test scheme. The study’s results show that the cross-scale, second-order compound microstructure can produce a superimposed hydrodynamic pressure effect to improve the bearing capacity of the lubrication film of a slipper pair, reducing the friction coefficient. The orders of factors (the working parameter and geometric parameters) under multiple indices (the total pressure-bearing capacity and the friction coefficient) were found. The optimal combination is a spherical pit with a first order diameter of 0.7 mm, a first order depth-to-diameter ratio of 0.1, an area rate of 20%, an arrangement angle of α/3 and a spherical convex hull with a second order diameter of 0.13 mm, and a second order depth-to-diameter ratio of 0.3. Compared to a smooth surface and a first-order, non-smooth microstructure, the cross-scale, second-order compound microstructure has an 11.0% and 8.9% higher total pressure-bearing capacity, respectively, and the friction coefficient decreased by 9.5% and 5.4%, respectively

Facile Synthesis of FeCoNiCuIr High Entropy Alloy Nanoparticles for Efficient Oxygen Evolution Electrocatalysis

The lack of an efficient and stable electrocatalyst for oxygen evolution reaction (OER) greatly hinders the development of various electrochemical energy conversion and storage techniques. In this study, we report a facile synthesis of FeCoNiCuIr high-entropy alloy nanoparticles (HEA NPs) by a one-step heat-up method. The involvement of glucose made the NPs grow uniformly and increased the valence of Ir. The resulting FeCoNiCuIr NPs exhibit excellent OER performance in alkaline solution, with a low overpotential of 360 mV to achieve a current density of 10 mA cm−2 at a Tafel slope of as low as 70.1 mV dec−1. In addition, high stability has also been observed, which remained at 94.2% of the current density after 10 h constant electrolysis, with a constant current of 10 mA cm−2. The high electrocatalytic activity and stability are ascribed to the cocktail effect and synergistic effect between the constituent elements. Our work holds the potential to be extended to the design and synthesis of high-performance electrocatalysts

Aquation and dimerization of osmium(II) anticancer complexes: a density functional theory study

In this paper, the hydrolytic and aqueous solution chemistry of two half-sandwich Os-II arene complexes [(eta(6)-p-cym)Os(pic)Cl] (1) and [(eta(6)-p-cym)Os(mal)Cl] (2) (pic = 2-picolinic acid and mal = maltolate) have been investigated using density functional theory (DFT). For aquation (substitution of chloride by H2O) of the complexes, three attacking models were explored, including two forms of side attack (A and B) and back attack C. Side attack A required the lowest free energy of activation of the three, both in the gas phase and in aqueous solution, suggesting that it best describes the hydrolysis of the complexes. Both the activation and reaction energies indicated faster aquation for 2 than 1, which was in accordance with previous experimental observations. With the side attack model of the complexes, it was found that the conformations of complexes had little effect on the aquation process. Moreover, mechanistic pathways have been obtained for the dimerization of aqua adducts. As for 1a, the ligand departure was the rate-determining step with an activation free energy of 26.1 kcal mol(-1), while for 2a, the first step of ring opening and protonation is rate-determining with a free energy of activation of 24.8 kcal mol(-1), suggesting that 1a was kinetically more stable toward dimerization. There were three factors presented to explain the stability of 1a: differences in HOMO/LUMO densities, the large activation energy of 1a, and stabilization of Os-pic bonding. This study assists in understanding the aqueous solution chemistry of the anticancer complexes and in the design of novel anticancer drugs