792 research outputs found
Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a SquareâPlanar Copper(I) Complex
6,6â˛â˛âBis(2,4,6âtrimethylanilido)terpyridine (H2TpyNMes) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H2TpyNMes)copper(I)âhalide (Cl, Br and I) complexes is dictated by the strength of the NHâhalide hydrogen bond. The CuICl and CuIICl complexes are nearly isostructural, the former presenting a highly unusual squareâplanar geometry about CuI. The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid CuI/CuII electronâtransfer selfâexchange rates. Cu(H2TpyNMes)Cl shows similar fast electron transfer (â105â
mâ1âsâ1) which is the same order of magnitude as biological systems.Entatic state: Hydrogen bonds constrain the geometry of CuI and CuII complexes. A highly unusual squareâplanar geometry about CuI (see structure) is shown to be nearly isostructural to the CuII core. The minimal reorganization energy between redox states allows for extremely rapid CuI/CuII electronâtransfer selfâexchange rates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134494/1/anie201511527_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134494/2/anie201511527-sup-0001-misc_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134494/3/anie201511527.pd
The SQM/COSMO filter: reliable native pose identification based on the quantum-mechanical description of proteinâligand interactions and implicit COSMO solvation
Current virtual screening tools are fast, but reliable scoring is elusive. Here, we present the "SQM/COSMO filter", a novel scoring function featuring a quantitative semiempirical quantum mechanical (SQM) description of all types of noncovalent interactions coupled with implicit COSMO solvation. We show unequivocally that it outperforms eight widely used scoring functions. The accuracy and chemical generality of the SQM/COSMO filter make it a perfect tool for late stages of virtual screening
Spectroscopy of Isolated Prebiotic Nucleobases
We use multiphoton ionization and double resonance spectroscopy to study the excited state dynamics of biologically relevant molecules as well as prebiotic nucleobases, isolated in the gas phase. Molecules that are biologically relevant to life today tend to exhibit short excited state lifetimes compared to similar but non-biologically relevant analogs. The mechanism is internal conversion, which may help protect the biologically active molecules from UV damage. This process is governed by conical intersections that depend very strongly on molecular structure. Therefore we have studied purines and pyrimidines with systematic variations of structure, including substitutions, tautomeric forms, and cluster structures that represent different base pair binding motifs. These structural variations also include possible alternate base pairs that may shed light on prebiotic chemistry. With this in mind we have begun to probe the ultrafast dynamics of molecules that exhibit very short excited states and search for evidence of internal conversions
An Exploration of the Ozone Dimer Potential Energy Surface
The (O3)2 dimer potential energy surface is thoroughly explored at the ab initio CCSD(T) computational level. Five minima are characterized with binding energies between 0.35 and 2.24 kcal/mol. The most stable may be characterized as slipped parallel, with the two O3 monomers situated in parallel planes. Partitioning of the interaction energy points to dispersion and exchange as the prime contributors to the stability, with varying contributions from electrostatic energy, which is repulsive in one case. Atoms in Molecules analysis of the wavefunction presents specific OâŻO bonding interactions, whose number is related to the overall stability of each dimer. All internal vibrational frequencies are shifted to the red by dimerization, particularly the antisymmetric stretching mode whose shift is as high as 111 cmâ1. In addition to the five minima, 11 higher-order stationary points are identified
Good vibrations: probing biomolecular structure and interactions through spectroscopy in the gas phase
Photoionization of the fullerene ion C60+
Photoionization cross section of the fullerene ion C60+ has been calculated
within a single-electron approximation and also by using a consistent many-body
theory accounting for many-electron correlations.Comment: 8 pages, 3 figure
Ab-initio study of model guanine assemblies: The role of pi-pi coupling and band transport
Several assemblies of guanine molecules are investigated by means of
first-principle calculations. Such structures include stacked and
hydrogen-bonded dimers, as well as vertical columns and planar ribbons,
respectively, obtained by periodically replicating the dimers. Our results are
in good agreement with experimental data for isolated molecules, isolated
dimers, and periodic ribbons. For stacked dimers and columns, the stability is
affected by the relative charge distribution of the pi orbitals in adjacent
guanine molecules. pi-pi coupling in some stacked columns induces dispersive
energy bands, while no dispersion is identified in the planar ribbons along the
connections of hydrogen bonds. The implications for different materials
comprised of guanine aggregates are discussed. The bandstructure of dispersive
configurations may justify a contribution of band transport (Bloch type) in the
conduction mechanism of deoxyguanosine fibres, while in DNA-like configurations
band transport should be negligible.Comment: 21 pages, 6 figures, 3 tables, to be published in Phys. Rev.
CheapStat: An Open-Source, âDo-It-Yourselfâ Potentiostat for Analytical and Educational Applications
Although potentiostats are the foundation of modern electrochemical research, they have seen relatively little application in resource poor settings, such as undergraduate laboratory courses and the developing world. One reason for the low penetration of potentiostats is their cost, as even the least expensive commercially available laboratory potentiostats sell for more than one thousand dollars. An inexpensive electrochemical workstation could thus prove useful in educational labs, and increase access to electrochemistry-based analytical techniques for food, drug and environmental monitoring. With these motivations in mind, we describe here the CheapStat, an inexpensive (<$80), open-source (software and hardware), hand-held potentiostat that can be constructed by anyone who is proficient at assembling circuits. This device supports a number of potential waveforms necessary to perform cyclic, square wave, linear sweep and anodic stripping voltammetry. As we demonstrate, it is suitable for a wide range of applications ranging from food- and drug-quality testing to environmental monitoring, rapid DNA detection, and educational exercises. The device's schematics, parts lists, circuit board layout files, sample experiments, and detailed assembly instructions are available in the supporting information and are released under an open hardware license
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