Apr 22, 2014

DNA Damage by Histone Radicals in Nucleosome Core Particles

DNA Damage by Histone Radicals in Nucleosome Core Particles

Publication Date (Web): April 22, 2014 (Communication)
DOI: 10.1021/ja501285s
 
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Abstract Image
Although DNA binding proteins shield the genetic material from diffusible reactive oxygen species by reacting with them, the resulting protein (peroxyl) radicals can oxidize the bound DNA. To explore this possible DNA damage by protein radicals, histone H4 proteins containing an azoalkane radical precursor at defined sites were prepared. Photolysis of a nucleosome core particle containing the modified protein produces DNA damage that is consistent with selective C4′-oxidation. The nucleotide(s) damaged is highly dependent on proximity to the protein radical. These experiments provide insight into the effects of oxidative stress on protein-bound DNA, revealing an additional layer of complexity concerning nucleic acid damage.
 

Carbon Nanotube-Templated Synthesis of Covalent Porphyrin Network for Oxygen Reduction Reaction

Carbon Nanotube-Templated Synthesis of Covalent Porphyrin Network for Oxygen Reduction Reaction

Publication Date (Web): April 9, 2014 (Article)
DOI: 10.1021/ja500984k
 
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The development of innovative techniques for the functionalization of carbon nanotubes that preserve their exceptional quality, while robustly enriching their properties, is a central issue for their integration in applications. In this work, we describe the formation of a covalent network of porphyrins around MWNT surfaces. The approach is based on the adsorption of cobalt(II) meso-tetraethynylporphyrins on the nanotube sidewalls followed by the dimerization of the triple bonds via Hay-coupling; during the reaction, the nanotube acts as a template for the formation of the polymeric layer. The material shows an increased stability resulting from the cooperative effect of the multiple π-stacking interactions between the porphyrins and the nanotube and by the covalent links between the porphyrins. The nanotube hybrids were fully characterized and tested as the supported catalyst for the oxygen reduction reaction (ORR) in a series of electrochemical measurements under acidic conditions. Compared to similar systems in which monomeric porphyrins are simply physisorbed, MWNT–CoP hybrids showed a higher ORR activity associated with a number of exchanged electrons close to four, corresponding to the complete reduction of oxygen into water.
 

Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals

Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals

Publication Date (Web): April 18, 2014 (Communication)
DOI: 10.1021/ja5006288
 
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Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.
 

Apr 21, 2014

Complete Photocatalytic Reduction of CO2 to Methane by H2 under Solar Light Irradiation

Complete Photocatalytic Reduction of CO2 to Methane by H2 under Solar Light Irradiation

Publication Date (Web): April 11, 2014 (Communication)
DOI: 10.1021/ja500924t
 
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Nickel supported on silica–alumina is an efficient and reusable photocatalyst for the reduction of CO2 to methane by H2, reaching selectivity above 95% at CO2 conversion over 90%. Although NiO behaves similarly, it undergoes a gradual deactivation upon reuse. About 26% of the photocatalytic activity of Ni/silica–alumina under solar light derives from the visible light photoresponse.
 

Selective molecular recognition by nanoscale environments in a supported iridium cluster catalyst

Received
 
Accepted
 
Published online
 
The active sites of enzymes are contained within nanoscale environments that exhibit exquisite levels of specificity to particular molecules. The development of such nanoscale environments on synthetic surfaces, which would be capable of discriminating between molecules that would nominally bind in a similar way to the surface, could be of use in nanosensing, selective catalysis and gas separation. However, mimicking such subtle behaviour, even crudely, with a synthetic system remains a significant challenge. Here, we show that the reactive sites on the surface of a tetrairidium cluster can be controlled by using three calixarene–phosphine ligands to create a selective nanoscale environment at the metal surface. Each ligand is 1.4 nm in length and envelopes the cluster core in a manner that discriminates between the reactivities of the basal-plane and apical iridium atoms. CO ligands are initially present on the clusters and can be selectively removed from the basal-plane sites by thermal dissociation and from the apical sites by reactive decarbonylation with the bulky reactant trimethylamine-N-oxide. Both steps lead to the creation of metal sites that can bind CO molecules, but only the reactive decarbonylation step creates vacancies that are also able to bond to ethylene, and catalyse its hydrogenation.


Photoinduced doping in heterostructures of graphene and boron nitride

Received
 
Accepted
 
Published online
 
The design of stacks of layered materials in which adjacent layers interact by van der Waals forces1 has enabled the combination of various two-dimensional crystals with different electrical, optical and mechanical properties as well as the emergence of novel physical phenomena and device functionality2345678. Here, we report photoinduced doping in van der Waals heterostructures consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photoinduced doping maintains the high carrier mobility of the graphene/boron nitride heterostructure, thus resembling the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially varying doping profiles such as p–n junctions. We show that this photoinduced doping arises from microscopically coupled optical and electrical responses of graphene/boron nitride heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.

Templated assembly of photoswitches significantly increases the energy-storage capacity of solar thermal fuels

Nature Chemistry. doi:10.1038/nchem.1918
Authors: Timothy J. Kucharski, Nicola Ferralis, Alexie M. Kolpak, Jennie O. Zheng, Daniel G. Nocera & Jeffrey C. Grossman
Solar thermal fuels have recently attracted an increasing amount of attention as a potential method for solar energy capture, conversion, storage and utilization. Azobenzene-functionalized single-walled carbon nanotubes demonstrate the proof-of-principle for increasing kinetic stability and energy densities by templating photoswitchable molecules on nanostructures to achieve highly (con)strained configurations.

Reactions of xenon with iron and nickel are predicted in the Earth's inner core

Not relevant to Mayer group research, but I just thought it was cool.

Nature Chemistry.
doi:10.1038/nchem.1925
Authors: Li Zhu, Hanyu Liu, Chris J. Pickard, Guangtian Zou & Yanming Ma
Studies of the Earth's atmosphere have shown that more than 90% of xenon is depleted — the so-called missing Xe paradox. Now a theoretical study shows that Xe and Fe/Ni can form inter-metallic compounds of XeFe3 and XeNi3 under conditions found in the Earth's inner core, and could provide a solution to the puzzle.

Apr 18, 2014

Solid-State Magnetic Switching Triggered by Proton-Coupled Electron-Transfer Assisted by Long-Distance Proton-Alkali Cation Transport

Publication Date (Web): April 14, 2014 (Communication)
DOI: 10.1021/ja502294x
 
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Abstract Image
Acidity of water molecules coordinated to Co ions in CoFe Prussian blue analogues (PBA) has been used to reversibly activate the CoIIIFeII ↔ CoIIFeIII electron transfer. The study of the structure and the electronic structure shows that the process implies an original PCET reaction between a solid-state porous coordination polymer and hydroxide ions in solution. The PCET reaction spreads throughout the solid network thanks to a long-range H+ and Rb+ transport within the pore channels of PBA taking advantage of the hydrogen-bonding network of zeolitic water molecules acting as proton wires.
 

Apr 15, 2014

Kinetics and Thermodynamics of H–/H•/H+ Transfer from a Rhodium(III) Hydride

Publication Date (Web): March 25, 2014 (Article)
DOI: 10.1021/ja412309j
 
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The thermodynamics and kinetics of all three cleavage modes for Rh–H, the transfer of H, H+, or H•, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, ΔG°H, for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H2 with Et3N in CH3CN. The transfer of H from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (kH = 3.5(1) × 105 M–1 s–1), making 1a a very efficient catalyst for the ionic hydrogenation of iminium cations. The pKa of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H+ transfer from 1a to 12 has been estimated (kH+ = 5(1) × 10–4 M–1 s–1) from the half-life of the equilibration. Thus, 1a is a poor H+ donor both thermodynamically and kinetically. However, 1a transfers H to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl) (14a) that can activate H2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at −1.85 V vs Fc+/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh–H bond of 1a of 58.2(3) kcal/mol—among the weakest Rh(III)–H bonds reported. The transfer of H from 1a to Ar3C• (Ar = p-tBuC6H4) is fast, with kH• = 1.17(3) × 103 M–1 s–1. Thus, 1a is a good H and H• donor but a poor H+ donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)].
 

Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

Publication Date (Web): April 1, 2014 (Article)
DOI: 10.1021/ja412471u


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Abstract Image
The mechanistic aspects of hydration of guanine radical cations, G•+ in double- and single-stranded oligonucleotides were investigated by direct time-resolved spectroscopic monitoring methods. The G•+ radical one-electron oxidation products were generated by SO4•– radical anions derived from the photolysis of S2O82– anions by 308 nm laser pulses. In neutral aqueous solutions (pH 7.0), after the complete decay of SO4•– radicals (5 μs after the actinic laser flash) the transient absorbance of neutral guanine radicals, G(-H) with maximum at 312 nm, is dominant. The kinetics of decay of G(-H) radicals depend strongly on the DNA secondary structure. In double-stranded DNA, the G(-H) decay is biphasic with one component decaying with a lifetime of 2.2 ms and the other with a lifetime of 0.18 s. By contrast, in single-stranded DNA the G(-H) radicals decay monophasically with a 0.28 s lifetime. The ms decay component in double-stranded DNA is correlated with the enhancement of 8-oxo-7,8-dihydroguanine (8-oxoG) yields which are 7 greater than in single-stranded DNA. In double-stranded DNA, it is proposed that the G(-H) radicals retain radical cation character by sharing the N1-proton with the N3-site of C in the [G•+:C] base pair. This [G(-H):H+C G•+:C] equilibrium allows for the hydration of G•+ followed by formation of 8-oxoG. By contrast, in single-stranded DNA, deprotonation of G•+ and the irreversible escape of the proton into the aqueous phase competes more effectively with the hydration mechanism, thus diminishing the yield of 8-oxoG, as observed experimentally.

Apr 12, 2014

Self-Assembly of Tetrahedral CdSe Nanocrystals: Effective “Patchiness” via Anisotropic Steric Interaction

Publication Date (Web): March 22, 2014 (Communication)
DOI: 10.1021/ja501596z


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Abstract Image
Controlling the spontaneous organization of nanoscale objects remains a fundamental challenge of materials design. Here we present the first characterization of self-assembled superlattices (SLs) comprised of tetrahedral nanocrystal (NCs). We observe self-assembly of CdSe nanotetrahedra into an open structure (estimated space-filling fraction φ ≈ 0.59) which has not been anticipated by many recent theoretical studies and simulations of tetrahedron packings. This finding highlights a gap in the understanding of the hierarchy of energy scales acting on colloidal NCs during self-assembly. We propose a strong dependence of ligand–ligand interaction potential on NC surface curvature. This effect favors spatial proximity of vertices in the dense colloidal crystal and may be considered an emergent “patchiness” acting through chemically identical ligand molecules.

Apr 10, 2014

A Student-Made Silver–Silver Chloride Reference Electrode for the General Chemistry Laboratory: 10 min Preparation

http://pubs.acs.org/doi/abs/10.1021/ed400722e

A Student-Made Silver–Silver Chloride Reference Electrode for the General Chemistry Laboratory: 10 min Preparation

Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States
J. Chem. Educ., Article ASAP
DOI: 10.1021/ed400722e
Publication Date (Web): April 10, 2014
Copyright © 2014 The American Chemical Society and Division of Chemical Education, Inc.
*E-mail: barlag@ohio.edu.

Abstract

A student-prepared silver–silver chloride reference electrode is described. The chemical deposition of AgCl(s) onto Ag(s) is accomplished in 30–50 s by placement of a Ag(s) wire in laundry bleach. An autopipettor tip with an agarose gel plug serves as the electrode housing; the agarose gel contains predissolved KNO3. Reference electrode preparation is completed in about 10 min, allowing enough time for laboratory exercises that utilize the electrode. Preparation and operation of the electrode and recovery of the Ag(s) are designed to teach a number of important chemistry principles.

Direct Detection of Key Reaction Intermediates in Photochemical CO2 Reduction Sensitized by a Rhenium Bipyridine Complex

Direct Detection of Key Reaction Intermediates in Photochemical CO2 Reduction Sensitized by a Rhenium Bipyridine Complex

Publication Date (Web): April 1, 2014 (Article)
DOI: 10.1021/ja500403e
 
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Abstract Image
Photochemical CO2 reduction sensitized by rhenium–bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re–bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4′-dimethyl-2,2′-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)3Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)3DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re–bipyridyl complex, [Re(dmbpy)(CO)3(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled 13CO2 provide clear evidence for the incorporation of CO2 into the Re–bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re–bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm–1, which shifts to 1647 cm–1 for the 13CO2-labeled counterpart; this is assigned as the CO2-coordinated intermediate, ReII–COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re–bipyridyl complex.
 

Apr 9, 2014

Electrochemical coating of [trans-L14CoIIICNFeII(CN)5]Na on ITO/Au electrode and its electrocatalytic properties towards nitrite reduction

Link 
Journal of Electroanalytical Chemistry
Volumes 722–723, 1 May 2014, Pages 1–6

César Cáceres, Manuel Martínez, Carlos Rodríguez, Paulina Dreyse, Victoria Ortega, Mauricio Isaacs

The preparation of an ITO (Indium Tin Oxide) modified electrode with a mixed valence CoIII/FeII complex is described. The ITO electrode was initially modified with Au nanoparticles by dip coating in order to obtain and ITO/Au surface over which a film of the [trans-L14CoIIINCFeII(CN)5]Na (L14CoIII–FeII; L14 = 6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) complex was deposited electrochemically, thus obtaining an ITO/Au/L14CoIII–FeII modified electrode. The modified electrode has been characterized by cyclic voltammetry, Raman spectroscopy, AFM and SEM, thus confirming the presence of the mixed valence complex as a modifier species. The electrode prepared shows good stability in aqueous solution and its activity as electrocatalyst in the reduction of nitrites has been evaluated. Cyclic voltammetry experiments run in presence of nitrites show a current enhancement on the ITO/Au/L14CoIII–FeII modified electrodes with respect to the ITO/Au initial electrode. After controlled potential electrolysis experiments, analysis of the products indicate the presence of hydroxylamine as the main nitrite reduction species, although small quantities of hydrazine and ammonia where also obtained.