Theory-Driven Synthesis of Surface-Modified Nanoparticles for Biochemical Applications

Alzheimerâ??s disease (AD) is the sixth-leading cause of death in the United States and accounts for over 60% of cases of dementia. Aggregation of amyloid-β protein (Aβ) in the brain parenchyma and cerebrovasculature has been indicated as a major etiological factor in AD, in accordance with the â??amyloid hypothesis.â? Protein monomers assemble to first yield particularly toxic soluble speciesâ??oligomers and protofibrilsâ??then larger insoluble fibrils, which deposit as the senile plaques characteristic of Alzheimerâ??s pathology. Current AD treatment attempts only to alleviate symptoms of memory loss and problems with logic and reasoning; much of Alzheimerâ??s research at the present seeks to obtain a greater understanding of the Aβ aggregation process, and evaluate compounds that inhibit it.

In previous research, gold nanoparticles (AuNPs) with weakly-associated electrostatic layers have demonstrated potent modulation of amyloid aggregation, at stoichiometric ratios as low as 1:2,000,000 nanoparticles per protein. AuNPs featuring a poly(acrylic acid) (PAA) outer coating, in particular, demonstrated inhibition of Aβ aggregation in environments with low ionic strength, while strongly promoting aggregation in aqueous salt solutions. This unique dual behavior makes these structures especially attractive as therapeutic candidates. From the effects of salt presence, we hypothesize that poly(acrylic acid)-coated gold nanoparticles alter Aβ aggregation by destabilizing the local solution environment. However, questionable stability of the electrostatic layers and complexity in modeling approaches suggest a simpler design of PAA-AuNPs with greater structural and experimental integrity.

This project describes the modeling and synthesis of neutral gold nanoparticles functionalized with end-tethered poly(acrylic acid) chains. Citrate-stabilized nanoparticles were synthesized using a modified Turkevich method procedure, and sized using TEM imaging. Acrylic acid polymers were synthesized via RAFT polymerization from a specialized thiol-containing chain transfer agent (CTA). The identity of the PAA sample was confirmed through NMR and FTIR spectroscopy, and average polymer size was measured via size exclusion chromatography. Polymers were then grafted to the nanoparticle surface, replacing citrate ions in a ligand exchange process. Functionalization to yield PAA-AuNPs was confirmed via UV-Vis spectroscopy.

The previous hypothesis is confirmed in part by study of such nanoparticle systems with a self-consistent mean field theory model for molecular dynamics. This approach performs a statistical thermodynamic analysis of a randomly-generated protein ensemble on a gold surface, as well as all species in the surrounding solution. Calculations of equilibria reveal that the polyelectrolyte brush â??pullsâ? species in toward the surface to relieve negative charge density, affecting qualities such as pH of the local solution environment. However, in the presence of salt species, small sodium ions easily screen the effects of charge, resulting in a totally different solution profile.

Ultimately, the PAA-AuNPs synthesized in this project will be directly introduced into an aggregation mixture with Aβ monomer. Formation of larger structures will be measured via fluorescence spectroscopy. The combined theory and experiment will yield a better understanding of amyloid aggregation mechanics, and a process by which to optimize the design of an aggregation inhibitor.