Experimental and computational investigation of heteroatom substitution in nucleolytic Cu(ii) cyclen complexes for balancing stability and redox activity

Cu(II) complexes of cyclen-based ligands CuL1–CuL6 were synthesized and characterized. The corresponding ligands L1–L6 comprise different donor sets including S and O atoms. Whereas cyclen (L1) is commercially available, L2–L6 were synthesized according to protocols available in the literature. Cleavage activity of the complexes towards plasmid DNA was tested in the presence and absence of ascorbate as a reducing agent (oxidative vs. hydrolytic cleavage). As previously shown, the substitution of N donor atoms with hard donor O atoms leads to efficient oxidative nucleases, but dissociation of the complex upon reduction. We thus opted for S substitution (soft donors) to stabilize the reduced Cu(I) species. Increasing the S content, however, leads to species that are difficult to reoxidize in order to ensure efficient oxidative DNA cleavage. We are showing by experimental (cyclic voltammetry) and computational means (DFT) that the rational combination of O and S atoms next to two nitrogen donors within the macrocycle (oxathiacyclen complex CuL6) leads to the stabilization of both redox states. The complex thus exhibits the highest oxidative DNA cleavage activity within this family of cyclen-based Cu(II) complexes – without leaching of the metal ion during reduction

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

Effect of Surface Shape and Content of Steel Fiber on Mechanical Properties of Concrete

Steel fiber reinforced concrete (SFRC) has gained popularity in the last decades attributed to the improvement of brittleness and low tensile strength of concrete. This study investigates the effect of three shapes of steel fibers (straight, hooked end, and corrugated) with four contents (0.5%, 1%, 1.5%, and 2%) on the mechanical properties (compression, splitting tension, shear, and flexure) of concrete. Thirteen groups of concrete were prepared and investigated experimentally. Test results indicated that steel fiber had significant reinforcement on mechanical properties of concrete. When the steel fiber content increases from 0.5% to 2.0%, the compressive strengths increase about 4–24%, splitting tensile strengths increase about 33–122%, shear strengths increase about 31–79%, and flexural strengths increase about 25–111%. Corrugated steel fiber has the best reinforced effect on strength of SFRC, hooked end steel fiber takes the second place, and straight steel fiber is the least. Calculated formulas of compressive, splitting tensile, shear, and flexural strengths were established with consideration of the bonding properties between concrete and steel fiber. Influence factors of steel fiber αf and concrete matrix strength αc were put forward and determined by regression analysis of experimental data. Calculated results agree well with the experimental results

Theoretical Study of O-2 Molecular Adsorption and Dissociation on Silicon Carbide Nanotubes

The adsorption/dissociation of the O-2 molecule on the Surface of silicon carbide nanotubes (SiCNTs) was investigated by density functional theory. We found several adsorption configurations, including chemisorption and cycloaddition configurations, for triplet and singlet O-2. Unlike the case for carbon nanotubes, the chemisorption of triplet O-2 oil SiCNTs is exothermic with remarkable charge transfer from nanotubes to the O-2 molecule. Singlet O-2 adsorption oil the Surface of SiCNTs can yield cycloaddition structures with large binding energies and sizable charge transfer. The reaction mechanism studies show that for triplet O-2, the chemisorption configuration is favorable, but the cycloaddition configuration is preferred for singlet O-2. For singlet O-2, we also Studied the dissociation of the O-2 molecule, and a two-step mechanism was presented. The dissociation of molecular O-2 results in formation of two three-membered rings with large binding energies. The key to the dissociation process of singlet O-2 on the SiCNT surface is the first step with a barrier energy of 0.40 eV. Finally, the electronic properties of SiCNTs with adsorbed triplet and singlet O-2 are shown to be dramatically influenced

Mechanistic Investigation of Dirhodium-Catalyzed Intramolecular Allylic C–H Amination versus Alkene Aziridination

The reaction mechanisms and chemoselectivity on the intramolecular allylic C–H amination versus alkene aziridination of 4-pentenylsulfamate promoted by four elaborately selected dirhodium paddlewheel complexes are investigated by a DFT approach. A predominant singlet concerted, highly asynchronous pathway and an alternative triplet stepwise pathway are obtained in either C–H amination or alkene aziridination reactions when mediated by weak electron-donating catalysts. A singlet stepwise C–H amination pathway is obtained under strongly donating catalysts. The rate-determining step in the C–H amination is the H-abstraction process. The subsequent diradical-rebound C–N formation in the triplet pathway or the combination of the allylic carbocation and the negative changed N center in the singlet pathway require an identical energy barrier. A mixed singlet–triplet pathway is preferred in either the C–H insertion or alkene aziridination in the Rh₂(NCH₃CHO)₄ entry that the triplet pathway is initially favorable in the rate-determining steps, and the resultant triplet intermediates would convert to a singlet reaction coordinate. The nature of C–H amination or alkene aziridination is estimated to be a stepwise process. The theoretical observations presented in the paper are consistent with the experimental results and, more importantly, provide a thorough understanding of the nature of the reaction mechanisms and the minimum-energy crossing points

Boron nitride nanotubes functionalized by a series of carbenes

We systematically studied the structural, energetic and electronic properties of zigzag boron nitride nanotubes (BNNTs) functionalized by a class of substituted carbenes (CR2) where R = H, F, Cl, CH3, CN and NO2 on different absorption sites using density functional theory. For R = H, F and Cl, the open structure is preferred with a BNNT sidewall bond cleavage, while for R = CH3 and CN, a competition between the open and closed cyclopropane-like three-membered ring (3MR) structure occurs. Interestingly, for R = NO2 we find a novel double five-membered ring (5MR) structure with high reaction stability. This new structure cannot be found in BNNTs' alternative carbon nanotubes (CNTs). In addition, the electronic properties of BNNTs functionalized with carbenes are hardly changed for R = H, F, Cl, CH3 and CN, but are significantly affected when R = NO2 due to the heterocyclic double 5MR structure