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Curcumin, which can exist in an equilibrium between keto and enol tautomers, binds to β-amyloid (Aβ) fibrils/aggregates. The aim of this study was to assess the relationship between the tautomeric structures of curcumin derivatives and their Aβ-binding activities. Curcumin derivatives with keto-enol tautomerism showed high levels of binding to Aβ aggregates but not to Aβ monomers. The binding activity of the keto form analogue of curcumin to Aβ aggregates was found to be much weaker than that of curcumin derivatives with keto-enol tautomerism. The color of a curcumin derivative with keto-enol tautomerism, which was substituted at the C-4 position, changed from yellow to orange within 30 min of being combined with Aβ aggregates in physiological buffer. This resulted from a remarkable increase in the enol form with extended conjugation of double bonds upon binding. These findings suggest that curcumin …