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Biomedical subjects

W Allison

Publications and source records attributed to W Allison.

15 recordsLinked to original sources
PubMed2006

Onset of 3D collective surface diffusion in the presence of lateral interactions: Na/Cu(001).

3He spin-echo measurements are used to follow the picosecond motion of sodium atoms on a copper (001) substrate. 2D correlated motion arising from repulsive adsorbate interactions is observed for coverages as low as 0.04 ML. At coverages greater than 0.05 ML there is a pronounced onset of motion perpendicular to the surface. The perpendicular motion is thermally activated and seems related to the basic translational hopping diffusion process. The correlated motion is modeled successfully using a molecular dynamics simulation and a dipolelike lateral interaction. A simple model which relates the apparent height of the atom with its local coverage is shown to reproduce the experimental observations.

Journal Article↗
PubMed2006

Experimental determination of the helium-metal interaction potential by interferometry of nanostructured surfaces.

We present a direct experimental comparison of the helium-surface interaction potential for two unreconstructed metal surfaces. We analyze phase shifts in helium atom scattering from a nanostructured bimetallic surface to yield the relative shape and position of an adsorbate's potential with respect to the reference defined by the substrate. In our prototype system, submonolayer growth of Ni on Cu(100), the He-Ni/Cu(100) potential has an attractive well that is 1.6+/-0.4 meV shallower, and a repulsive wall 0.11+/-0.03 A closer to the ion cores, compared to the He-Cu(100) potential. Our observations provide a ready test of state-of-the-art theoretical calculations.

Journal Article↗
PubMed2004

Observation of microscopic CO dynamics on Cu(001) using 3He spin-echo spectroscopy.

We present momentum resolved measurements of quasielastic helium atom scattering made using a new 3He spin-echo spectrometer. Our data for the dynamics of CO on Cu(001) indicates an activated jump mechanism which we analyze in detail using molecular dynamics simulations. A nearly isotropic potential energy surface is found with an average barrier height of approximately 125 meV, yielding comparable hopping rates along both the <110> and <100> directions. The measurements provide the first rigorous experimental test of state-of-the-art first-principles calculations previously made on this system.

Journal Article↗
PubMed2004

Physisorption of molecular oxygen on C60 thin films.

The interaction of oxygen molecules with a fullerene surface has been studied using high resolution electron energy loss spectroscopy and temperature programmed desorption. Vibrational excitation of the adsorbed oxygen is observed at 190 meV, an energy value comparable with that for molecular oxygen in the gas phase. We take this to indicate physisorption of molecular oxygen on the C(60) surface. Thermal desorption results also show that the bonding of oxygen molecules to the C(60) overlayer is comparable to that on a graphite surface. A detailed study of the energy dependence of the vibrational excitation reveals an inelastic electron resonance scattering process. The angular dependence of the resonant vibrational excitation exhibits features distinctively different from those for molecular oxygen physisorbed on the related graphite surface, at a comparable coverage. One possible reason is that the corrugated surface potential, due to the curvature of the C(60) molecules, promotes the preferential ordering of the physisorbed oxygen molecules perpendicular to the surface plane of the C(60) overlayer.

Journal Article↗
PubMed2004

Effects of resolution and friction in the interpretation of QHAS measurements.

We use Langevin molecular dynamics (MD) simulations to improve the picture of the processes that contribute to QHAS broadening, as a function of momentum transfer at the crystal. We use a simulation of realistic damped vibrational motion in a harmonic well to establish the contributions to QHAS measurements due to both vibrational motion and intracell diffusion (usually referred to as vibrational dephasing). We demonstrate the experimental conditions under which such contributions are likely to be important. These results are compared with those from a simulation of thermal diffusion over a sinusoidally corrugated potential. We show that resolution and atom-surface "friction" play an important role in determining the form of QHAS measurements and we demonstrate that there is no simple relationship between the "activation energy" derived from an Arrhenius plot of QHAS data and the adiabatic potential barrier height. MD simulations are therefore necessary to perform a complete analysis of the data. Finally, we discuss the application of our results to more sophisticated potentials, particularly those with multiple adsorption sites.

Journal Article↗
PubMed1997

Recurrent mutations in the type VII collagen gene (COL7A1) in patients with recessive dystrophic epidermolysis bullosa.

Mutations in the type VII collagen gene (COL7A1) are known to underlie different forms of the inherited blistering skin disease dystrophic epidermolysis bullosa (DEB). Most COL7A1 mutations are unique to individual families, and therefore it is usually necessary to screen all 118 exons of the gene to determine the molecular pathology in a patient with DEB. This study aimed to identify any recurrent mutations in COL7A1 that might be applicable to mutation-detection strategies in these patients. Mutational analysis was undertaken in 23 British patients with autosomal recessive DEB using PCR amplification of genomic DNA followed by heteroduplex analysis, nucleotide sequencing, and restriction site analysis. Two recurrent mutations were identified: R578X (6 of 46 alleles) and 7786delG (7 of 46 alleles). Haplotype analysis revealed that the mutations existed on similar allelic backgrounds in different patients, consistent with propagation of common British ancestral haplotypes, although R578X and 7786delG also have been described in DEB patients from other ethnic backgrounds. Given the high relative frequency of these two COL7A1 mutations, British patients with recessive DEB should be screened initially for these nucleotide changes by PCR amplification of genomic DNA and restriction analysis before more exhaustive screening of COL7A1.

Alleles↗
PubMed1993

Inhibition of nitric oxide formation by guanidines.

Aminoguanidine, N,N'-diaminoguanidine, methylguanidine, and 1,1-dimethylguanidine were compared to NG-monomethyl-L-arginine (L-NMMA) for their ability to inhibit nitric oxide (NO) formation by cytokine-inducible and vascular constitutive isoforms of NO synthase. These comparisons were performed by assessing (1) cytokine-induced production of nitrite by RINm5F cells, (2) vasoconstrictor responses of isolated rat mesenteric arteries, and (3) in vivo blood pressure responses following intravenous bolus injection into anesthetized rats. Aminoguanidine and L-NMMA were the most potent inhibitors of cytokine-induced NO formation in RINm5F cells, while the other guanidine compounds were 10 (1,1-dimethylguanidine) to 100 (methylguanidine) times less potent. L-NMMA and 1,1-dimethylguanidine were the most potent inhibitors of the vascular constitutive isoform of NO synthase in both assay systems, while aminoguanidine and N,N'-diaminoguanidine were the least potent. These results (1) confirm the selective inhibition of the inducible isoform of NO synthase by aminoguanidine, (2) indicate that N,N'-diaminoguanidine, while approximately 30 times less potent than aminoguanidine in inhibiting inducible NO synthase, has very little effect on constitutive NO synthase activity, and (3) 1,1-dimethylguanidine, like L-NMMA, is a relatively potent inhibitor of both isoforms of NO synthase.

Amino Acid Oxidoreductases↗
PubMed1990

Glucose-induced microvascular functional changes in nondiabetic rats are stereospecific and are prevented by an aldose reductase inhibitor.

Exposure of skin chamber granulation tissue vessels in nondiabetic rats to 11 or 15 mM D-glucose (but not L-glucose or 3-O-methylglucose) twice daily for 10 d induces vascular functional changes (increased albumin permeation and blood flow) identical to those in animals with mild or severe streptozotocin diabetes, respectively. These vascular changes are strongly linked to increased metabolism of glucose via the sorbitol pathway and are independent of nonenzymatic glycosylation as well as systemic metabolic and hormonal imbalances associated with the diabetic milieu. (J. Clin. Invest. 1990. 85:1167-1172.)

Aldehyde Reductase↗
PubMed1976

Complete obstruction of the left main coronary artery associated with congenital pulmonary valvular stenosis.

The surgical treatment of the completely obstructed left main coronary artery is aortocoronary saphenous-vein bypass to the left coronary artery system. Without surgical intervention, this lesion has an exceedingly poor prognosis. This report describes a patient with atherosclerotic obstruction of the left main coronary artery and congenital pulmonary valvular stenosis, an unusual combination of lesions not previously documented together. The patient remains asymptomatic 22 months after aortocoronary saphenous-vein bypass and pulmonary valvotomy.

Coronary Angiography↗