A method was developed to characterize and quantify preparations of monomeric beta-amyloid (Abeta) peptide using capillary electrophoresis (CE) with UV absorbance detection. The detection limit for Abeta monomer using this method was 0.5 microM (19 pg). The self-assembly of Abeta to form amyloid fibrils is closely linked to Alzheimer's disease and is the subject of intense investigations. Consistent preparation of Abeta monomer samples at known concentrations and free of aggregates is a significant challenge for researchers studying the mechanism of Abeta fibril formation and searching for small molecules that inhibit Abeta fibril formation. Samples of Abeta monomer are known to sometimes contain pre-existing aggregates that can affect the kinetics and structure of amyloid fibrils. The CE method presented here showed that some of the monomeric Abeta samples prepared for this study contained a species producing a second peak (in addition to the major monomer peak). The aggregation was monitored using a thioflavin T fluorescence assay, and the resulting fibrils were characterized by transmission electron microscopy. Monomer samples containing the additional peak based on CE analysis were shown to aggregate more rapidly than monomer samples that were free of this putative Abeta aggregate peak.

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