Conformational Analysis of Arabinofuranosides: Prediction of (3)JH,H Using MD Simulations with DFT-Derived Spin-Spin Coupling Profiles.

A molecular dynamics (MD) investigation on a series of oligo-α-arabinofuranosides (1-8) using the AMBER force field and the GLYCAM carbohydrate parameter set is reported. The validation of the method was carried out by direct comparison of experimental vicinal proton-proton coupling constants ((3)JH,H) with those obtained by using an empirically determined Karplus equation and density functional theory (DFT)-derived relationships specifically tailored for α-arabinofuranosyl systems. A simple code was developed to implement the determination of (3)JH,H by applying these relationships to the probability distributions of rotamers and ring conformations displayed by the simulations.

The effectiveness of 1H decoupling in the 13C MAS NMR of paramagnetic solids: an experimental case study incorporating copper(II) amino acid complexes.

The use of continuous-wave (CW) 1H decoupling has generally provided little improvement in the 13C MAS NMR spectroscopy of paramagnetic organic solids. Recent solid-state 13C NMR studies have demonstrated that at rapid magic-angle spinning rates CW decoupling can result in reductions in signal-to-noise and that 1H decoupling should be omitted when acquiring 13C MAS NMR spectra of paramagnetic solids. However, studies of the effectiveness of modern 1H decoupling sequences are lacking, and the performance of such sequences over a variety of experimental conditions must be investigated before 1H decoupling is discounted altogether.

Diastereomeric Xe chemical shifts in tethered cryptophane cages.

Cryptophane cages serve as host molecules to a Xe atom. Functionalization of cryptophane-A has permitted the development of Xe as a biosensor. Synthetic routes used to prepare cryptophanes result in racemic mixtures of the chiral cages.

Xe nuclear magnetic resonance line shapes in channels decorated with paramagnetic centers.

To make predictions of the Xe NMR line shapes for Xe in channels decorated with paramagnetic centers, we consider a model system using the O(2) molecule as the paramagnetic center. The previously calculated quantum mechanical Xe@O(2) hyperfine tensor for various configurations of Xe in the presence of O(2) provides a model for the hyperfine response of Xe atom to the presence of a paramagnetic center. The averaging is carried out using the same grand canonical Monte Carlo methodology as for calculating NMR line shapes for Xe in diamagnetic channels, modified to include the effects of the hyperfine tensor response.

Grand canonical Monte Carlo simulations of the 129Xe NMR line shapes of xenon adsorbed in (+/-)-[Co(en)3]Cl3.

The 129Xe NMR line shapes of xenon adsorbed in the nanochannels of the (+/-)-[Co(en)3]Cl3 ionic crystal have been calculated by grand canonical Monte Carlo (GCMC) simulations. The results of our GCMC simulations illustrate their utility in predicting 129Xe NMR chemical shifts in systems containing a transition metal. In particular, the nanochannels of (+/-)-[Co(en)3]Cl3 provide a simple, yet interesting, model system that serves as a building block toward understanding xenon chemical shifts in more complex porous materials containing transition metals.

Xe NMR lineshapes in channels of peptide molecular crystals.

To further an understanding of the nature of information available from Xe chemical shifts in cavities in biological systems, it would be advantageous to start with Xe in regular nanochannels that have well known ordered structures built from amino acid units. In this paper, we report the experimental observation of Xe NMR lineshapes in peptide channels, specifically the self-assembled nanochannels of the dipeptide L-Val-L-Ala and its retroanalog L-Ala-L-Val in the crystalline state. We carry out grand canonical Monte Carlo simulations of Xe in these channels to provide a physical understanding of the observed Xe lineshapes in these two systems.

Molecular dynamics averaging of Xe chemical shifts in liquids.

The Xe nuclear magnetic resonance chemical shift differences that afford the discrimination between various biological environments are of current interest for biosensor applications and medical diagnostic purposes. In many such environments the Xe signal appears close to that in water. We calculate average Xe chemical shifts (relative to the free Xe atom) in solution in eleven liquids: water, isobutane, perfluoro-isobutane, n-butane, n-pentane, neopentane, perfluoroneopentane, n-hexane, n-octane, n-perfluorooctane, and perfluorooctyl bromide.

NMR studies of peptide T, an inhibitor of HIV infectivity, in an aqueous environment.

The synthetic octapeptide peptide T (ASTTTNYT) has been shown to interfere with binding of the HIV-1 envelope glycoprotein gp120 to the chemokine receptor R5, thus preventing viral infection. This study investigated the degree of conformational order of two analogs of peptide T, one biologically active (D-Ala peptide T amide) and one inactive (D-Ala, D-Tyr peptide T amide) using nuclear magnetic resonance (NMR) spectroscopy in an aqueous environment, both in solution and in the frozen solid state. Standard solution NMR techniques such as DQFCOSY, HMQC, ROESY and inversion recovery measurements have been utilized to characterize these peptides.

Nuclear magnetic shielding and chirality IV. The odd and even character of the shielding response to a chiral potential.

We investigate the odd and even character of the shielding response in a chiral molecule (modeled by a Ne8 helix) when subjected to a chiral potential. We establish that the diastereomeric splittings are a measure of odd powers of Vodd. Implications for diastereomeric, splittings of Xe in handed cages with handed tethers are discussed.

The Xe shielding surfaces for Xe interacting with linear molecules and spherical tops.

The 129Xe nuclear magnetic resonance spectrum of xenon in gas mixtures of Xe with other molecules provides a test of the ab initio surfaces for the intermolecular shielding of Xe in the presence of the other molecule. We examine the electron correlation contributions to the Xe-CO2, Xe-N2, Xe-CO, Xe-CH4, and Xe-CF4 shielding surfaces and test the calculations against the experimental temperature dependence of the density coefficients of the Xe chemical shift in the gas mixtures at infinite dilution in Xe. Comparisons with the gas phase data permit the refinement of site-site potential functions for Xe-N2, Xe-CO, and Xe-CF4 especially for atom-Xe distances in the range 3.

Properties of selected S-nitrosothiols compared to nitrosylated WR-1065.

WR-1065 ([N-mercaptoethyl]-1-3-diaminopropane), the active form of the aminothiol drug Ethyol/Amifostine, protects against toxicity caused by radiation, chemotherapy and endotoxin. Because WR-1065 and other thiols readily bind nitric oxide (NO), injurious conditions or therapies that induce the production or mobilization of NO could alter the effects of WR-1065. S-Nitrosothiols were prepared from various thiols by a standard method to compare properties and stability.