Relationship between the tautomeric structures of curcumin derivatives and their Aβ-binding activities in the context of therapies for Alzheimer's disease

Curcumin, which can exist in an equilibrium between keto and enol tautomers, binds to beta-amyloid (A beta) fibrils/aggregates. The aim of this study was to assess the relationship between the tautomeric structures of curcumin derivatives and their A beta-binding activities. Curcumin derivatives with keto-enol tautomerism showed high levels of binding to A beta aggregates but not to All monomers. The binding activity of the keto form analogue of curcumin to All 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 All 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 derivatives exist predominantly in the enol form during binding to A beta aggregates, and that the enolization of curcumin derivatives is crucial for binding to A beta aggregates. The keto-enol tautomerism of curcumin derivatives may be a novel target for the design of amyloid-binding agents that can be used both for therapy and for amyloid detection in Alzheimer's disease. (C) 2010 Elsevier Ltd. All rights reserved.