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Interfacial Properties of Alzheimer Amyloid- Peptides in Various Self-Assembled Forms

John E. Kirkwood, Jaykumar Rajadas, Martin Widenbrant, and Gerald G. Fuller. Chemical Engineering, Stanford University, 381 North South Mall, Dept. Chemical Engineering- Stauffer 3- Room 113, Stanford, CA 94305

Amyloid beta-peptides (A) of 38-43 amino acid residues are the major constituents of neurite plaques involved in Alzheimer's disease (AD). According to amyloid hypothesis the accumulation of A is driven by increased generation of aggregated A peptides over its clearance. An important event in the pathogenesis of AD is the deposition of these peptides in a fibrillar form, with A42 typically having a greater disrupting property in the plasma membranes of the neuronal cells surrounding the senile plaques. The accumulated peptide aggregates at the membrane interface are hypothesized as the main effectors for neuronal dysfunction, microglial cell activation, neuronal death, and clinical dementia in AD. Both the mechanism for entry of the peptides into the membrane interface and the toxicity of the peptide are not well understood. This work looks at various assembled forms of A40 and A42 and its interaction with hydrocarbon surfaces. We developed techniques to measure the interaction and organization of various self-assembled forms of A40 and A42 peptides at the oil-water interface. Using atomic force microscopy we have characterized the assembled forms of the peptides by adsorbing them to an air water interface. The freshly soluble and self-assembled protofibrillar forms of A peptides adsorbed quickly and irreversibly to a decane-water interface. The rheological properties of the proteins adsorbed to the oil-water interface were tested using two different methods with similar results. The first approach was to measure the internal pressure of a water drop coated with protein in a decane continuum. The volume of the drop was controlled using a syringe pump with the drop pressure continually monitored and recorded. In each case, for both the freshly soluble and self assembled forms of A40 and A42, the drop was observed to transition from a fluid to solid-like state when the volume was reduced. The interfacial properties were also probed using the interfacial apparatus of the TA Instruments G2 Rheometer. These measurements are made using a DuNouy ring placed at the interface between the decane and water. The A proteins tested formed a primarily elastic structure at the interface between decane and water with storage modulus values (~0.2 N/m) an order of magnitude greater in value than the loss modulus (~0.02 N/m). The native and protofibrillar forms of Insulin were also tested and did not form strong structures on the interface with modulus values three orders of magnitude less than the A proteins. This suggests the toxicity of A peptides may be related to their ability to adsorb to the hydrophobic membrane interior.