Biomedical subjects

Michael B Hay

Publications and source records attributed to Michael B Hay.

5 recordsLinked to original sources

PubMed2006

Quantitative determination of absolute organohalogen concentrations in environmental samples by X-ray absorption spectroscopy.

An in situ procedure for quantifying total organic and inorganic Cl concentrations in environmental samples based on X-ray absorption near-edge structure (XANES) spectroscopy has been developed. Cl 1s XANES spectra reflect contributions from all Cl species present in a sample, providing a definitive measure of total Cl concentration in chemically heterogeneous samples. Spectral features near the Cl K-absorption edge provide detailed information about the bonding state of Cl, whereas the absolute fluorescence intensity of the spectra is directly proportional to total Cl concentration, allowing for simultaneous determination of Cl speciation and concentration in plant, soil, and natural water samples. Absolute Cl concentrations are obtained from Cl 1s XANES spectra using a series of Cl standards in a matrix of uniform bulk density. With the high sensitivity of synchrotron-based X-ray absorption spectroscopy, Cl concentration can be reliably measured down to the 5-10 ppm range in solid and liquid samples. Referencing the characteristic near-edge features of Cl in various model compounds, we can distinguish between inorganic chloride (Cl(inorg)) and organochlorine (Cl(org)), as well as between aliphatic Cl(org) and aromatic Cl(org), with uncertainties in the range of approximately 6%. In addition, total organic and inorganic Br concentrations in sediment samples are quantified using a combination of Br 1s XANES and X-ray fluorescence (XRF) spectroscopy. Br concentration is detected down to approximately 1 ppm by XRF, and Br 1s XANES spectra allow quantification of the Br(inorg) and Br(org) fractions. These procedures provide nondestructive, element-specific techniques for quantification of Cl and Br concentrations that preclude extensive sample preparation.

Bromine↗

PubMed2005

Palladium-catalyzed synthesis of 2,1'-disubstituted tetrahydrofurans from gamma-hydroxy internal alkenes. Evidence for alkene insertion into a Pd-O bond and stereochemical scrambling via beta-hydride elimination.

Palladium-catalyzed reactions of gamma-hydroxy internal acyclic alkenes with aryl bromides afford 2,1'-disubstituted tetrahydrofurans in good yields with diastereoselectivities of 3-5:1. The analogous transformations of substrates bearing internal cyclic alkenes afford fused bicyclic and spirocyclic tetrahydrofuran derivatives in good yields with excellent diastereoselectivities (>20:1). A series of deuterium labeling experiments indicate that the origin of the modest diastereoselectivity in reactions of acyclic internal alkene substrates likely derives from a series of reversible beta-hydride elimination and sigma-bond rotation processes that occur following a rare intramolecular alkene syn-insertion into an intermediate Pd(Ar)(OR) complex. In addition, these studies shed light on the chemoselectivity of insertion, suggesting that the alkene inserts into the Pd-O bond in preference to the Pd-C bond.

Alkenes↗

PubMed2005

Palladium-catalyzed synthesis of tetrahydrofurans from gamma-hydroxy terminal alkenes: scope, limitations, and stereoselectivity.

A new, stereoselective synthesis of substituted tetrahydrofurans via Pd-catalyzed reactions of aryl and vinyl bromides with gamma-hydroxy terminal alkenes is described. This transformation affords trans-2,5- and trans-2,3-disubstituted tetrahydrofurans with up to >20:1 dr. This methodology also provides access to bicyclic and spirocyclic tetrahydrofuran derivatives in good yield with 10-20:1 dr. The scope and limitations of these transformations are discussed in detail, as are the effect of substrate sterics and electronics on yield and stereoselectivity. A proposed mechanism of these transformations is presented along with a model that rationalizes the stereochemical outcome of the reactions.

Journal Article↗

PubMed2003

Two-dimensional condensed phases from particles with tunable interactions.

We present a conceptually simple experimental model for condensed phases, consisting of an ensemble of identical magnetic dipoles on a vibrating bed. The model combines tunable and accurately known pair potentials, equilibration times of seconds, and lattice structure and dynamics visible to the naked eye. Fundamental ensemble properties--specifically phonon propagation, edge relaxation, and binary condensation--are directly observed and quantitatively linked to the underlying pair potential.

Journal Article↗

PubMed2002

Evaluation of the film-coating properties of a hydroxyethyl cellulose/hydroxypropyl methylcellulose polymer system.

The effect of different grades of hydroxyethyl cellulose (HEC) and hydroxypropyl methllcellulose (HPMC) on the film-formation and taste-masking ability for ibuprofen granules was evaluated. Three batches of coated ibuprofen granules were prepared using a roto-granulator, each with a different coating composition. Two grades of HEC [MW300,000 (H) and MW90,000 (L)] were combined with three different grades of HPMC [MW 11,000 (L), MW 25,000 (M) and MW 35,000 (H)] to prepare the coating solutions. Mechanical strength and physical properties of the polymer films were evaluated. Films made from HPMC (L)/HEC (H), HPMC (M)/HEC (H), and HPMC (H)/HEC (H) were stronger and more flexible than the HPMC (L) HEC (L) films. The assay, dissolution, particle size distribution, and environmental scanning electron microscopy (ESEM) data of the three batches of the coated ibuprofen granules were similar.

Cellulose↗