Global analysis of composite particles

The theory of vibrations of a composite particle when vibrational amplitudes are not constrained to be small according to the Eckart conditions is developed using the methods of differential topology. A global classical Hamiltonian appropriate for this system is given, and for the case of the molecular vibration–rotation problem, it is transformed into a global quantum Hamiltonian operator. It is shown that the zeroth-order term in the global Hamiltonian operator is identical to the Wilson–Howard Hamiltonian; higher-order terms are shown to give successively better approximations to the large amplitude problem. Generalized Eckart conditions are derived for the global classical Hamiltonian; the quantum equivalent of these conditions along with the quantum equivalent of the Eckart conditions are given. The spectrum of the global Hamiltonian operator is discussed and it is shown that the calculation of the vibration–rotation energy states of the system reduces to the same straight-forward procedure, the solution of a secular determinant, as was carried out for the Wilson–Howard Hamiltonian at a later time by Nielsen.     

Spectroscopic and computational studies of the ATP:corrinoid adenosyltransferase (CobA) from Salmonella enterica: insights into the mechanism of adenosylcobalamin biosynthesis

CobA from Salmonella enterica is a member of an enzymatic system responsible for the de novo biosynthesis of adenosylcobalamin (AdoCbl), catalyzing the formation of the essential Co-C bond by transferring the adenosyl group from a molecule of ATP to a transient Co(1+)corrinoid species generated in the enzyme active site. A particularly fascinating aspect of this reaction is that the flavodoxin in vivo reducing agent that serves as the electron donor to CobA possesses a reduction potential that is considerably more positive than that of the Co(2+/1+) couple of the corrinoid substrate. To explore how CobA may overcome this challenge, we have employed electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance (EPR) spectroscopies to probe the interaction between Co(3+)- and Co(2+)corrinoids and the enzyme active site. Our data reveal that while Co(3+)corrinoids interact only weakly with CobA, Co(2+)corrinoids undergo partial conversion to a new paramagnetic species that can be obtained in nearly quantitative yield when CobA is preincubated with the co-substrate ATP. This "activated" species is characterized by a distinct set of ligand field transitions in the near-IR spectral region and EPR parameters that are unprecedented for Co(2+)corrinoids. Analysis of these data on the basis of qualitative spectral correlations and density functional theory computations reveals that this unique Co(2+)corrinoid species possesses an essentially square-planar Co(2+) center that lacks any significant axial bonding interactions. Possible implications of these findings for the mechanism of Co(2+) --> Co(1+) reduction employed by CobA and Co-C bond-forming enzymes in general are explored.     

On the effect of trace impurities at the air/water- and the mercury/water interface

An aqueous solution of 1 10^–4 Mn-octanoic acid containing 0.005 M hydrochloric acid and 0.1 M sodium chloride was used to demonstrate the influence of surface-active trace impurities on the air/water- and the mercury/water interface. The solution was purified stepwise to obtain states of successively growing surface-chemical purity. The effect of contamination at the air/water- and the mercury/water interface was followed simultaneously by monitoring the dynamic surface tension in the case of ad- and desorption and the differential capacity, respectively, at various stages of purity. The results give evidence that the effect of trace impurities at the air/solution surface is as important as at the mercury/solution interface.     

The spectrum of α‐resolvable designs with block size four

Abstact: An α‐resolvable BIBD is a BIBD with the property that the blocks can be partitioned into disjoint classes such that every class contains each point of the design exactly α times. In this paper, we show that the necessary conditions for the existence of α‐resolvable designs with block size four are sufficient, with the exception of (α, ν, λ) = (2, 10, 2). © 2000 John Wiley & Sons, Inc. J Combin Designs 9: 1–16, 2001     

All Schatten spaces endowed with the Schur product are Q-algebras

We prove that the Banach algebra formed by the space of compact operators on a Hilbert space endowed with the Schur product is a quotient of a uniform algebra (also known as a Q-algebra). Together with a similar result of Pérez-García for the trace class, this completes the answer to a long-standing question of Varopoulos.     

Nonlocal van der Waals density functional: the simpler the better

We devise a nonlocal correlation energy functional that describes the entire range of dispersion interactions in a seamless fashion using only the electron density as input. The new functional is considerably simpler than its predecessors of a similar type. The functional has a tractable and robust analytic form that lends itself to efficient self-consistent implementation. When paired with an appropriate exchange functional, our nonlocal correlation model yields accurate interaction energies of weakly-bound complexes, not only near the energy minima but also far from equilibrium. Our model exhibits an outstanding precision at predicting equilibrium intermonomer separations in van der Waals complexes. It also gives accurate covalent bond lengths and atomization energies. Hence the functional proposed in this work is a computationally inexpensive electronic structure tool of broad applicability.     

Determination of Shikimic Acid in Fruits of Illicium Species and Various Other Plant Samples by LC–UV and LC–ESI–MS

A simple and specific analytical method for the quantitative determination of shikimic acid from the methanol extract of the fruits of Illicium species and from various plant samples was developed. The LC–UV separation was achieved by reversed-phase chromatography on a C18 column with potassium dihydrogen phosphate and methanol as the mobile phase. In the LC–MS method, the separation was achieved by a C12 column using water and acetonitrile, both containing 0.1% acetic acid as the mobile phase. The methods were successfully used to study the percentage compositions of shikimic acid present in nine species of Illicium and various other plant samples. The detector response was linear with concentrations of shikimic acid in the range from 1.0–500.0 μg mL^?1 by LC–UV and 100–1000 ng mL^?1 by LC–MS. Mass spectrometry coupled with electrospray ionization interface is described for the identification of shikimic acid in various plant samples. This method involved the use of the [M-H]^? ions of shikimic acid at m/z 173.0455 (calculated mass) in the negative ion mode with extractive ion monitoring.     

Structural relationships in small molecule interactions governing gas-phase enantioselectivity and zwitterionic formation

Gas-phase zwitterionic amino acids were formed in complexes of underivatized beta-cyclodextrin through reactions with a neutral base, n-propylamine. The reaction was performed in the analyzer cell of an electrospray ionization-Fourier transform mass spectrometer. Most of the natural amino acids were studied with three cyclodextrin hosts including alpha-, beta-, and gamma-cyclodextrin to understand better the structural features that lead to the stabilization of the zwitterionic complexes. Molecular dynamics calculations were performed to provide insight into the structural features of the complexes. The rate constants of the reactions were obtained through kinetic plots. Examination of both L- and D-enantiomers of the amino acid showed that the reaction was enantioselective. The reaction was then employed to analyze mixtures of Glu enantiomers naturally occurring in the bacteria Bacillus licheniformis.     

Electrorheological analysis of nano laden suspensions

The synthesis and characterization of Pb3O2Cl2 nanowires and the electrorheological (ER) properties of carbon nanofiber (CNF), carbon nanotube (CNT) and Pb3O2Cl2 nanowire (NW) laden suspensions is presented. The ER properties were investigated through oscillatory shear experiments. The viscoelastic response in the presence of dc electric fields was analyzed. Actuation behavior for the CNF and NW laden suspensions was observed at low voltages and low concentration of the reinforcements (0.05 wt%). In the case of the CNT laden suspensions, an effect was observed at a concentration of 0.0125 wt%. Positive and negative electrorheological behaviors were observed due to differences in electrical conductivity and polarization mechanisms.