ß-Bracelets: Macrocyclic Cross-ß Epitope Mimics Based on a Tau Conformational Strain.

The aggregation of misfolded tau into neurotoxic fibrils is linked to the progression of Alzheimer's disease (AD) and related tauopathies. Disease-associated conformations of filamentous tau are characterized by hydrophobic interactions between side chains on unique and distant ß-strand modules within each protomer. Here, we report the design and diversity-oriented synthesis of ß-arch peptide macrocycles composed of the aggregation-prone PHF6 hexapeptide of tau and the cross-ß module specific to the AD tau fold. Termed "ß-bracelets", these proteomimetics assemble in a sequence- and macrocycle-dependent fashion, resulting in amyloid-like fibrils that feature in-register parallel ß-sheet structure. Backbone N-amination of a selected ß-bracelet affords soluble inhibitors of tau aggregation. We further demonstrate that the N-aminated macrocycles block the prion-like cellular seeding activity of recombinant tau as well as mature fibrils from AD patient extracts. These studies establish ß-bracelets as a new class of cross-ß epitope mimics and demonstrate their utility in the rational design of molecules targeting amyloid propagation and seeding.

Hydrogen Bond Exchange and Ca2+ Binding of Aqueous N-Methylacetamide Revealed by 2DIR Spectroscopy.

Cation effects on proteins have been a challenge to understand. Herein, we present two-dimensional infrared (2DIR) spectroscopic measurements, coupled with molecular dynamics and spectroscopic calculations, of N-methylacetamide (NMA), a common model of the peptide backbone, in aqueous CaCl2. The 2DIR spectra reveal that the dynamics of the amide carbonyl of NMA is dominated by exchange between two states of varying hydration, one possessing a structure similar to aqueous NMA and one that is dehydrated by one hydrogen bond. In addition, we demonstrate that at high (>5 M) CaCl2 concentrations, direct binding of Ca2+ to the carbonyl of NMA occurs, stabilizing an iminium-type resonance structure of NMA, with a characteristic C═N+ stretch frequency at 1680 cm-1. This species is only marginally populated and is only detectable in 2DIR spectra due to its larger transition strength.

ICE Concentration Linked with Extractive Stirrer (ICECLES).

Trace and ultra-trace analysis can be difficult to achieve, especially for polar, more volatile, and/or thermally unstable analytes. A novel technique, coined ICE Concentration Linked with Extractive Stirrer (ICECLES), may help address this problem. The implementation of ICECLES described here combines stir bar sorptive extraction (SBSE) with freeze concentration (FC), where an aqueous solution is frozen during SBSE in order to concentrate analytes into a polydimethylsiloxane (PDMS) coated stir bar. Five test probe molecules with a range of log Kows (2-butanol, benzyl alcohol, benzaldehyde, dimethyl trisulfide and bromobenzene) were prepared from aqueous solutions using ICECLES. Thermal desorption gas-chromatography mass-spectrometry was then used to quantify these analytes. Parameters affecting the performance of ICECLES (e.g., freeze rate) were evaluated, with extraction at lower speeds resulting in higher extraction efficiencies, whereas the freeze rate and initial analyte concentration only had a minor effect. ICECLES produced much higher extraction efficiencies than SBSE alone, with signal enhancements of up to 474× SBSE. ICECLES also provided excellent reproducibility and lower LODs than SBSE for all compounds tested. ICECLES performed well when used to analyze multiple triazine pesticides and breakdown products in environmental surface waters. Overall, the ICECLES technique was excellent at preparing aqueous samples for trace analysis and shows promise as a novel analytical sample preparation technology.