Synthesis, Characterization, and Metal Coordinating Ability of Multifunctional Carbohydrate-Containing Compounds for Alzheimer's Therapy

Abstract

Dysfunctional interactions of metal ions, especially Cu, Zn, and Fe, with the amyloid-beta (A beta) peptide are hypothesized to play an important role in the etiology of Alzheimer's disease (AD). In addition to direct effects on A beta aggregation, both Cu and Fe catalyze the generation of reactive oxygen species (ROS) in the brain further contributing to neurodegeneration. Disruption of these aberrant metal-peptide interactions via chelation therapy holds considerable promise as a therapeutic strategy to combat this presently incurable disease. To this end, we developed two multifunctional carbohydrate-containing compounds N,N'-bis[(5-beta-D-glucopyranosyloxy-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL1) and N,N'-bis[(5-beta-D-glucopyranosyloxy-3-tert-butyl-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine (H2GL2) for brain-directed metal chelation and redistribution. Acidity constants were determined by potentiometry aided by UV-vis and 1H NMR measurements to identify the protonation sites of H2GL1,2. Intramolecular H bonding between the amine nitrogen atoms and the H atoms of the hydroxyl groups was determined to have an important stabilizing effect in solution for the H2GL1 and H2GL2 species. Both H2GL1 and H2GL2 were found to have significant antioxidant capacity on the basis of an in vitro antioxidant assay. The neutral metal complexes CuGL1, NiGL1, CuGL2, and NiGL2 were synthesized and fully characterized. A square-planar arrangement of the tetradentate ligand around CuGL2 and NiGL2 was determined by X-ray crystallography with the sugar moieties remaining pendant. The coordination properties of H2GL1,2 were also investigated by potentiometry, and as expected, both ligands displayed a higher affinity for Cu2+ over Zn2+ with H2GL1 displaying better coordinating ability at physiological pH. Both H2GL1 and H2GL2 were found to reduce Zn2+- and Cu2+- induced Abeta1-40 aggregation in vitro, further demonstrating the potential of these multifunctional agents as AD therapeutics.