RESUMO
X-ray powder and electron single-crystal diffraction of crystals of Li(2)TiOSiO(4) and Li(2)TiOGeO(4) showed them to be tetragonal, space group P4/nmm unit-cell parameters a = 6.4379(2), c = 4.40032(2) Å for Li(2)TiOSiO(4) and a = 6.6110(8), c = 4.4372(6) Å for Li(2)TiGeO(4). The compounds are isostructural with their sodium analogues but are considerably compressed along the c axis owing to the smaller size of lithium compared with sodium atoms. Square-pyramidal TiO(5) groups are joined in these compounds by tetrahedral SiO(4) and GeO(4) groups, respectively. (17)O nuclear magnetic resonance spectra of the two compounds, isotopically enriched with (17)O, showed peaks due to the apical titanyl, Ti-O, and basal, bridging, Ti-O-Si or Ti-O-Ge, oxygen atoms of the title compounds. By comparison with reference spectra, oxygen K edges and titanium L(2,3) edges of electron energy-loss spectra were tentatively assigned.
RESUMO
The effect of magnetic fields on the orientation and properties of 4,4'-bis(2,3-epoxypropoxy)-alpha-methylstilbene cured with sulfanilamide has been studied. This epoxy system is initially isotropic and forms a smectic A phase upon curing. A magnetic field was applied during the cure reaction, resulting in alignment of the molecules along the direction of the applied field. Measurement of the orientation parameter of the fully cured material by wide-angle X-ray scattering (WAXS) showed that orientation improved with an increase in field strength. The orientation parameters of the smectic layer normals calculated from the inner reflection of the WAXS pattern attained a maximum level of approximately 0.8 at a field strength of approximately 12 T. The orientation parameters calculated from the outer reflection of the WAXS pattern were considerably lower, possibly due to the presence of amorphous regions associated with domain boundaries or the loss of molecular alignment within the smectic layers due to topological restrictions of the cross-linking sites. Orientation resulted in an anisotropic linear thermal expansion coefficient after curing, although the overall volumetric expansion was constant. The elastic tensile modulus increased with the square of the orientation parameter, attaining a maximum value of 8.1 GPa, compared to 3.1 GPa for the unoriented material. The change in modulus with orientation could be fit with a simple model for the modulus of anisotropic materials.
RESUMO
Oxygen radical generation can be measured when blood flow is restored to previously ischemic tissue. Although several studies have suggested oxygen radicals contribute to lethal injury of myocardium after ischemia, other studies have failed to confirm this implication. Antioxidants, such as N-(2-mercaptoptopionyl)-glycine (MPG) and superoxide dismutase, have had inconsistent effects on lethal myocardial injury in animal models of ischemia and reperfusion. Many variables influence lethal myocardial injury in these models: time of ischemia, time of reperfusion, dose of antioxidant, myocardial oxygen demand, area at risk, collateral blood flow, and body core temperature. The purpose of this study is to test the effects of infusion of MPG on lethal reperfusion injury in a canine model of ischemia and reperfusion with these variables tightly controlled. The left anterior descending coronary artery of anesthetized dogs was ligated for 90 minutes and reperfused for 4 hours. MPG was infused (100 mg/kg/h) 15 minutes before the end of ischemia and throughout reperfusion. Core body temperature was closely monitored, and infarct size was adjusted to transmural myocardial blood flow during ischemia. MPG had no effect on infarct size or infarct size adjusted for changes in collateral blood flow. These data reinforce a general difficulty in demonstrating the effects of antioxidant therapies on lethal injury, even when closely monitoring covariates known to impact infarct size.
