The "Al and "Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of A1 in tetru-and pentacoordination in dehydroxylated kaolinite. The 29Si NMR signal analysis of samples heated above 400 "C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the 27Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the mod$ication of the coordination of Al, which changes from the tetra-or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C ,for 36 h produces the same transformation in the coordination of A1 ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystalliniv are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200"C, the 3:2 mullite NMR spectrum.
Progressive myoclonus epilepsy of the Lafora type or Lafora disease (EPM2; McKusick no. 254780) is an autosomal recessive disorder characterized by epilepsy, myoclonus, progressive neurological deterioration and glycogen-like intracellular inclusion bodies (Lafora bodies). A gene for EPM2 previously has been mapped to chromosome 6q23-q25 using linkage analysis and homozygosity mapping. Here we report the positional cloning of the 6q EPM2 gene. A microdeletion within the EPM2 critical region, present inhomozygosis in an affected individual, was found to disrupt a novel gene encoding a putative protein tyrosine phosphatase (PTPase). The gene, denoted EPM2, presents alternative splicing in the 5' and 3' end regions. Mutational analysis revealed that EPM2 patients are homozygous for loss-of-function mutations in EPM2. These findings suggest that Lafora disease results from the mutational inactivation of a PTPase activity that may be important in the control of glycogen metabolism.
Abstract-Fibrous sepiolite crystals derive much of their commercial value from their molecular size channels and grooves. The crystals fold upon drying and these channels and grooves are lost. A model for the folding and unfolding of the crystals is presented. Extensive i.r., X-ray and thermogravimetric evidence shows that folding occurs when approximately half of the water of hydration, which is coordinated to the edge magnesium atoms inside of the channels, is removed. This occurs near 175~ under vacuum and near 300~ in air. When the crystals fold, all remaining water molecules enter a new environment, that of the hexagonal holes of the neighboring silica surface. A true anhydride is produced at about 500~ under vacuum when the final water is lost, but this final dehydration produces no important structural change. Rehydration of the anhydride to the normal hydrated sepiolite does not occur at room temperatures in 100% r.h. However, above, 60~ rehydration does occur.
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