The binding of pinacyanol (PIN), a cationic cyanine dye, to beta-amyloid fibrils (Abeta), which are associated with Alzheimer disease, was quantified by absorption spectrophotometry to measure the concentration of PIN bound to Abeta as a function of the Abeta concentration or by means of the separation of free PIN from bound PIN by centrifugation and subsequent analysis of the supernatant by visible-absorption spectrophotometry. Both methods gave equivalent results. The stoichiometry of PIN binding to Abeta was 1, and the curve representing the concentration effect of Abeta on the concentration of a dye-Abeta complex showed a biphasic curve instead of the hyperbolic curve that is characteristic of weak ligand-macromolecule interactions [e.g., as shown by Congo Red (CR)]). This and the fact that a Scatchard plot could not be fitted to the experimental data suggested that PIN binds tightly to Abeta. A comparison to the interaction of CR with Abeta led us to conclude that PIN is more sensitive than CR.
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