Homocysteine thiolactone and H2 O2 induce amino acid modifications and alter the fibrillation propensity of the Aß25-35 peptide.

Of the proteinaceous ß-sheet-rich amyloid fibrillar structures, the Aß25-35 peptide, a component of the full-length Aß involved in Alzheimer's disease, has similar toxicity to the parent peptide. In this study, the effects of homocysteine thiolactone (HCTL) and hydrogen peroxide (H2 O2 ) on the conformation and fibrillation propensity of the Aß25-35 peptide were investigated. Both HCTL and H2 O2 induced amino acid modifications along with alteration in aggregation propensity. Methionine (Met)-35 was oxidized by H2 O2 and aggregation was attenuated following the increased hydrophilicity of the peptide due to sulfoxide/sulfone formation. The HCTL-modified lysine (Lys-28) residue destabilizes the structure of the peptide, which leads to fibrillation. Our studies provide important information regarding the relationship between amino acid modifications and the amyloid fibrillation process.

Differential copper-guided architectures of amyloid ß peptidomimetics modulate oxidation states and catalysis.

Orchestration of differential architectures of designer peptidomimetics that modulate metal oxidation states to perform multiple chemical transformations remains a challenge. Cu-chelation and self-assembly properties of amyloid ß (Aß14-23) peptide were tuned by the incorporation of cyclic dipeptide (CDP) and pyrene (Py) as the assembly directing and reporting units, respectively. We explore the molecular architectonics of Aß14-23 derived peptidomimetics (AkdNMCPy) to form differential architectures that stabilize distinct Cu oxidation states. The fibrillar self-assembly of AkdNMCPy is modulated to form nanosheets by the one-off addition of CuII. Notably, the serial addition of CuII resulted in the formation of micelle-like core-shell architectures. The micelle-like and nanosheet architectures were found to differentially stabilize CuII and CuI states and catalyze tandem oxidative-hydrolysis and alkyne-azide cycloaddition reactions, respectively.

Combating amyloid-induced cellular toxicity and stiffness by designer peptidomimetics.

Amyloid beta (Aß) aggregation species-associated cellular stress instigates cytotoxicity and adverse cellular stiffness in neuronal cells. The study and modulation of these adverse effects demand immediate attention to tackle Alzheimer's disease (AD). We present a de novo design, synthesis and evaluation of Aß14-23 peptidomimetics with cyclic dipeptide (CDP) units at defined positions. Our study identified AkdNMC with CDP units at the middle, N- and C-termini as a potent candidate to understand and ameliorate Aß aggregation-induced cellular toxicity and adverse stiffness.

Effect of Silica Nanoparticles on the Amyloid Fibrillation of Lysozyme.

Protein fibrils are regarded as undesired products as these are associated with numerous neuro- and non-neurodegenerative disorders. Increasing evidence suggests that the mechanism of fibrillation involves the formation of various oligomeric intermediates, which are known to be more toxic than mature fibrils. Here, we report the impact of synthesized silica nanoparticles (SiNPs) of diameters ∼52 nm on the aggregation behavior of hen egg white lysozyme (HEWL) under heat and acidic conditions. Congo red as well as ThT binding assays and AFM imaging studies indicate that SiNPs trigger the amyloid formation of HEWL in a dose-dependent manner. ThT kinetic studies and FTIR studies suggest that the fibrillation kinetics does not involve the formation of toxic oligomeric intermediates at higher concentrations of SiNPs. By measuring fluorescence lifetime values of the bound ThT, SiNP-induced fibrillation of HEWL can easily be realized. CD spectroscopic studies indicate that native HEWL becomes unfolded upon incubation under the experimental conditions and is rapidly converted into the ß-sheet-rich fibrillar aggregates in the presence of SiNPs with increasing concentrations. It has been further revealed that fibrillar aggregates formed at higher concentrations of SiNPs preferably adopt an antiparallel ß-sheet configuration. The enhanced fibrillation in the presence of SiNPs is likely because of preferential adsorption of the non-amyloidogenic regions of HEWL, resulting in the exposure of the aggregation-prone regions of HEWL toward the solvent. The study will provide deeper insights into the evolution of oligomer-free fibrillation that can be useful to demonstrate the underlying mechanism of amyloid fibrillation.

Probing the role of ortho-dihydroxy groups on lysozyme fibrillation.

Changes in the microenvironment of Trp in lysozyme are one of the key factors in the fibrillation process. Gallic acid through the oxidation of o-dhydroxy moiety into quinone was shown to inhibit lysozyme fibrillation by stabilizing the Trp microregions [Konar et al., Int. J. Biol. Macromol. 103 (2017) 1224-1231]. In this article we compare the inhibitory effects of several gallic acid-based phenolic compounds. The results show that pyrogallol, and 3,4-dihydroxy benzoic acid, each containing the o-dihydroxy moiety exhibited a significant inhibitory effect on lysozyme fibrillation which is further supported by docking studies. Interestingly, the inhibitory effect of pyrogallol is almost similar to that observed for gallic acid. The lower inhibitory effect of 3,5-dihydroxy benzoic acid and 4-hydroxy benzoic acid corroborates this finding as neither of the compounds can be transformed into quinone intermediates. The ineffectiveness of benzoic acid towards fibrillation questions the role of the COOH group in the inhibition. The IC20 values determined show the similar trends. Results of the Thioflavin T binding assay and parameters from the docking studies reveal a strong correlation based on which a relation has been obtained that could be used to identify potential polyphenol based inhibitors by considering docking studies alone.

Gallic acid induced dose dependent inhibition of lysozyme fibrillation.

Amyloidosis is primarily characterized by the deposition of misfolded protein aggregates. Although the natural polyphenols have long been known as effective amyloid inhibitors, the mechanistic details of their inhibitory actions still remain unclear. Our present study explores the inhibition mechanism of polyphenols by studying the anti-amyloidogenic property of gallic acid (GA), the smallest structural unit of tea polyphenols, on hen egg white lysozyme (HEWL) at physiological pH. Using various spectroscopic techniques such as UV-vis, fluorescence, circular dichroism and dynamic light scattering, and microscopic techniques such as TEM and FESEM, it has been shown that GA potentially inhibits the self-aggregation process in a concentration dependent manner. Gel electrophoresis studies suggest that the o-dihydroxy moiety of GA is oxidized into the quinone moiety and H2O2 in the system under the experimental conditions. The quinone binds near the hydrophobic region of HEWL and restricts hydrophobic exposure. Cyclic voltammetry studies reveal that the Met residues of HEWL are oxidized by H2O2 to highly polar sulfoxide-modified side chains. The partially unfolded intermediates formed under the denaturing conditions employed remain in contact with the solvent thus preventing further aggregation.