The interplay between antiferromagnetism and superconductivity in disordered ultrathin high-Tc films

A series of ultrathin films of DyBa₂Cu₃O_7−x, with properties spanning the zero-field superconductor-to-insulator transition, were prepared for the purpose of investigating the magnetic field driven superconductor-insulator transition. The resistance of these films was found to exhibit a peak whose magnitude became larger with the weakening of the superconductivity of the films. For a film just on the superconducting side of the zero-field superconductor-to-insulator transition, three successive changes in the sign of the low temperature dR/dT with increasing field were found. These have been taken as evidence of the superconducting state being successively excited, reentered, and finally exited with increasing field as the sign changes from positive to negative, to positive, and finally to negative. This unusual behavior may result from an interaction of the carriers with Cu²⁺ ions on the copper-oxygen sheets. These spins exhibit an increasing tendency to antiferromagnetic ordering as the superconductivity of the films is weakened. The presence of many grain boundaries in the films may be the reason the effect is strikingly large.     

Infrared spectra of silver complexes with 2-aminopyridine and crystal structure of the 4:3 aminopyridine-silver nitrate compound

The 43 complex formed between 2-aminopyridine (ampy) and AgNO₃ belongs to the orthorhombic space groupPccn. The unit cell of dimensionsa=7.992(4),b=16.788(5),c=21.034(8) Å contains four formula units. The structure was refined on 998 nonzero reflections toR=0.038. The crystal contains the dimeric [Ag₃(ampy)₄]³⁺ cation. Two of the three independent Ag atoms are each coordinated to two ampy ligandsvia the heterocyclic nitrogen lone pair (mean Ag-N=2.18 Å), and the N-Ag-N angle in this [ampy-Ag-ampy]⁺ unit is 154.0°. By using the two available amino groups, the [ampy-Ag-ampy]⁺ unit acts as a bidentate ligand, and two such units related by a twofold axis achieve a roughly tetrahedral environment of NH₂ lone pairs about the third Ag atom (mean Ag-N=2.39 Å). The infrared spectra of [Ag₃(ampy)₄](NO₃)₃, [Ag(ampy)₂]ClO₄, [Ag₃(ampy-H)₂]NO₃, and [Ag₃(ampy-H)₂]ClO₄, and those of their amino-deuterated derivatives, are discussed. Diagnostic regions are proposed to identify coordinationvia the amino group and the heterocyclic nitrogen atom.     

Are Protein Force Fields Getting Better? A Systematic Benchmark on 524 Diverse NMR Measurements

Recent hardware and software advances have enabled simulation studies of protein systems on biophysically-relevant timescales, often revealing the need for improved force fields. Although early force field development was limited by the lack of direct comparisons between simulation and experiment, recent work from several labs has demonstrated direct calculation of NMR observables from protein simulations. Here we quantitatively evaluate recent molecular dynamics force fields against a suite of 524 chemical shift and J coupling ((3)JH(N)H(α), (3)JH(N)C(β), (3)JH(α)C', (3)JH(N)C', and (3)JH(α)N) measurements on dipeptides, tripeptides, tetra-alanine, and ubiquitin. Of the force fields examined (ff96, ff99, ff03, ff03*, ff03w, ff99sb*, ff99sb-ildn, ff99sb-ildn-phi, ff99sb-ildn-nmr, CHARMM27, OPLS-AA), two force fields (ff99sb-ildn-phi, ff99sb-ildn-nmr) combining recent side chain and backbone torsion modifications achieve high accuracy in our benchmark. For the two optimal force fields, the calculation error is comparable to the uncertainty in the experimental comparison. This observation suggests that extracting additional force field improvements from NMR data may require increased accuracy in J coupling and chemical shift prediction. To further investigate the limitations of current force fields, we also consider conformational populations of dipeptides, which were recently estimated using vibrational spectroscopy.     

Breath Biomarkers in Diagnostic Applications

The detection of chemical compounds in exhaled human breath presents an opportunity to determine physiological state, diagnose disease or assess environmental exposure. Recent advancements in metabolomics research have led to improved capabilities to explore human metabolic profiles in breath. Despite some notable challenges in sampling and analysis, exhaled breath represents a desirable medium for metabolomics applications, foremost due to its non-invasive, convenient and practically limitless availability. Several breath-based tests that target either endogenous or exogenous gas-phase compounds are currently established and are in practical and/or clinical use. This review outlines the concept of breath analysis in the context of these unique tests and their applications. The respective breath biomarkers targeted in each test are discussed in relation to their physiological production in the human body and the development and implementation of the associated tests. The paper concludes with a brief insight into prospective tests and an outlook of the future direction of breath research.