AMORPHOUS HYDROXYALUMINIUM SILICATES FORMED UNDER PHYSIOLOGICAL SALINE CONDITIONS, AND IN CACO3-BUFFERED SOLUTIONS - STABILITY AND SIGNIFICANCE FOR ALZHEIMER PLAQUE PRECIPITATES

In an attempt to determine the nature and conditions of formation of the aluminium silicate precipitates present at the core of senile plaque in the brains of patients with Alzheimer's disease, aluminosilicate precipitates have been prepared and equilibrated for up to one year in physiological saline conditions at 40-degrees-C and a pH near 7.4. The structure and composition of these precipitates, and the associated concentrations of free silicic acid, were similar to those found in systems buffered with CaCO3 at 20-25-degrees-C. Imogolite-like products with Si: Al ratios near 0.5 were obtained at silicic acid concentrations < 100-mu-M, and hydrous feldspathoid products with ratios greater-than-or-equal-to 1.0 became dominant at silicic acid concentrations greater than approximately 150-mu-M. These products do not match senile plaque precipitates, which have Si: Al ratios greater-than-or-equal-to 1.0, but form in an environment with < 100-mu-M Si. The relative stability of aluminium hydroxides, proto-imogolite allophane and hydrous feldspathoids are estimated from equilibrated silicic acid concentrations.