Curcumin tautomerization in the mechanism of pentameric amyloid- ß42 oligomers disassembly.

Alzheimer's disease is a neurologic disorder characterized by the accumulation of extracellular deposits of amyloid-ß (Aß) fibrils in the brain of patients. The key etiologic agent in Alzheimer's disease is not known; however oligomeric Aß appears detrimental to neuronal functions and increases Aß fibrils deposition. Previous research has shown that curcumin, a phenolic pigment of turmeric, has an effect on Aß assemblies, although the mechanism remains unclear. In this study, we demonstrate that curcumin disassembles pentameric oligomers made from synthetic Aß42 peptides (pentameric oAß42), using atomic force microscopy imaging followed by Gaussian analysis. Since curcumin shows keto-enol structural isomerism (tautomerism), the effect of keto-enol tautomerism on its disassembly was investigated. We have found that curcumin derivatives capable of keto-enol tautomerization also disassemble pentameric oAß42, while, a curcumin derivative incapable of tautomerization did not affect the integrity of pentameric oAß42. These experimental findings indicate that keto-enol tautomerism plays an essential role in the disassembly. We propose a mechanism for oAß42 disassembly by curcumin based on molecular dynamics calculations of the tautomerism. When curcumin and its derivatives bind to the hydrophobic regions of oAß42, the keto-form changes predominantly to the enol-form; this transition is associated with structural (twisting, planarization and rigidification) and potential energy changes that give curcumin enough force to act as a torsion molecular-spring that eventually disassembles pentameric oAß42. This proposed mechanism sheds new light on keto-enol tautomerism as a relevant chemical feature for designing such novel therapeutic drugs that target protein aggregation.

The Effect of Ethanol on Disassembly of Amyloid-ß1-42 Pentamer Revealed by Atomic Force Microscopy and Gel Electrophoresis.

The most common type of dementia, Alzheimer's disease, is associated with senile plaques formed by the filamentous aggregation of hydrophobic amyloid-ß (Aß) in the brains of patients. Small oligomeric assemblies also occur and drugs and chemical compounds that can interact with such assemblies have attracted much attention. However, these compounds need to be solubilized in appropriate solvents, such as ethanol, which may also destabilize their protein structures. As the impact of ethanol on oligomeric Aß assembly is unknown, we investigated the effect of various concentrations of ethanol (0 to 7.2 M) on Aß pentameric assemblies (Aßp) by combining blue native-PAGE (BN-PAGE) and ambient air atomic force microscopy (AFM). This approach was proven to be very convenient and reliable for the quantitative analysis of Aß assembly. The Gaussian analysis of the height histogram obtained from the AFM images was correlated with band intensity on BN-PAGE for the quantitative estimation of Aßp. Our observations indicated up to 1.4 M (8.3%) of added ethanol can be used as a solvent/vehicle without quantitatively affecting Aß pentamer stability. Higher concentration induced significant destabilization of Aßp and eventually resulted in the complete disassembly of Aßp.