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Amyloid protein (Abeta1-40) aggregation and conformation was examined using native and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and the results compared with those obtained by atomic force microscopy, and with Congo red binding, sedimentation and turbidity assays. The amount of Abeta aggregation measured was different, depending upon the method used. Incubation for 15 min at pH 5.0 or in the presence of Fe2+, Cu2+ or Zn2+ did not alter the level of Abeta oligomers observed on SDS and native gels. However, the slow aggregation of Abeta to form high molecular mass species over 5 days was inhibited. In contrast, when Abeta aggregation was monitored using a Congo red binding assay or sedimentation assay, a rapid increase in Abeta aggregation was observed after incubation for 15 min at pH 5.0, or in the presence of Fe2+, Cu2+ or Zn2+. The low pH-, Zn2+- or Cu2+-induced Abeta aggregation measured in a turbidity assay was reversible. In contrast, a considerable proportion of the Abeta aggregation measured by native and SDS/PAGE was stable. Atomic force microscopy studies showed that Abeta aged at pH 5.0 or in the presence of Zn2+ produced larger looser rod-shaped aggregates than at pH 7.4. Abeta that had been aged at pH 7.4 was more cytotoxic than Abeta aged at pH 5.0. Taken together, the results suggest that Abeta oligomerizes via two mutually exclusive mechanisms to form two different types of aggregates, which differ in their cytotoxic properties.

The remarkable electrocatalytic properties and small size of carbon nanotubes make them ideal for achieving direct electron transfer to proteins, important in understanding their redox properties and in the development of biosensors. Here, we report shortened SWNTs can be aligned normal to an electrode by self-assembly and act as molecular wires to allow electrical communication between the underlying electrode and redox proteins covalently attached to the ends of the SWNTs, in this case, microperoxidase MP-11. The efficiency of the electron transfer through the SWNTs is demonstrated by electrodes modified with tubes cut to different lengths having the same electron-transfer rate constant.