Insights on Metal Ions and Misfolded Proteins in Alzheimer's Disease Using Flavonoid Derivatives and X-ray Fluorescence Microscopy.

Abstract

The incidence of Alzheimer’s disease (AD) worldwide has been accelerating at a staggering rate, demonstrating the need to understand its origins clearly. Misfolded proteins such as amyloid-beta have been believed to contribute to the disease. Additionally, it has been suggested that the normal functions and localization of metal ions such as Fe(II/III), Cu(I/II), and Zn(II) could be disrupted in the AD brain. There has been evidence to support the possible interplay between Cu(II) and/or Zn(II) with amyloid-beta-related pathology (i.e., metal–amyloid hypothesis); however, this relationship has yet been to be fully elucidated. A growing interest this possible association with AD pathology has led to new strategies for interrogating the metal– amyloid-beta relationship using small molecule metal chelators. Flavonoids are abundant plant-derived molecules that have been known to bind metal ions and interact with amyloid-beta peptides; however, the synergism of these properties toward metal-associated amyloid-beta has previously received little attention. In this thesis, selected flavonoid derivatives were investigated to formulate a structure-interaction-reactivity relationship among chemical structures, metal ions, and amyloid-beta/metal–amyloid-beta species. The molecules myricetin and luteolin, found in fruits and vegetables, were initially screened and found to influence the reactivity of metal–amyloid-beta species. Building upon these initial observations, the interaction and reactivity of the well-known green tea polyphenol (−)-epigallocatechin-3-gallate (EGCG) were explored at the molecular level through biochemical and biophysical investigations. These findings suggest that metal–amyloid-beta–flavonoid species can be generated and possibly lead to off-pathway aggregation in vitro. We further identified the specific contributions of structural features by synthetic manipulation of the flavonoid framework to aminoisoflavone derivatives. Appropriate tuning of hydrophilic and hydrophobic properties within the structural backbone can modify the balance of metal binding and amyloid-beta interaction, which may account for their reactivity toward amyloid-beta and/or metal–amyloid-beta. Taken together, the studies presented herein demonstrate the utility of flavonoids as a source of structures for direct use and/or synthetic modification from which a structure-interaction-reactivity relationship can be formulated for metal-free and metal-associated amyloid-beta species, which will provide insight into development of chemical tools to uncover the involvement of metal–amyloid species in AD.