Excess accumulation of phenylalanine is the characteristic of untreated Phenylketonuria (PKU), a well-known genetic abnormality, which triggers several neurological, physical and developmental severities. However, the fundamental mechanism behind the origin of such diverse health problems, particularly the issue of how they are related to the build-up of phenylalanine molecules in the body, is largely unknown. Here, we show cross-seeding ability of phenylalanine fibrils that can effectively initiate an aggregation process in proteins under physiological conditions, converting native protein structures to β-sheet assembly. The resultant fibrils were found to cause severe hemolysis, yielding a plethora of deformed erythrocytes that is highly relevant to phenylketonuria. Unique arrangement of zwitterionic phenylalanine molecules in their amyloid-like higher order entities is predicted to promote both hydrophobic and electrostatic interaction, sufficient enough to trap proteins and to preferentially interact with the membrane components of RBCs. Since the prevalence of hemolysis and amyloid related psychoneurological severities are mostly observed in PKU patients, we propose that the inherent property of phenylalanine fibrils to trigger hemolysis and to induce protein aggregation may have direct relevance to the disease mechanism of PKU.

Here, we show that aromatic amino acid tyrosine, under a physiologically mimicking condition, readily forms amyloid-like entities that can effectively drive aggregation of different globular proteins and aromatic residues. Tyrosine self-assembly resulted in the formation of cross-β rich regular fibrils as well as spheroidal oligomers. Computational data suggest intermolecular interaction between specifically oriented tyrosine molecules mediated through π-π stacking and H-bonding interactions, mimicking a cross-β-like architecture. Both individual protein samples and mixed protein samples underwent aggregation in the presence of tyrosine fibrils, confirming the occurrence of amyloid cross-seeding. The surface of the tyrosine's amyloid like entities was predicted to trap native protein structures, preferably through hydrophobic and electrostatic interactions initiating an aggregation event. Because tyrosine is a precursor to vital neuromodulators, the inherent cross-seeding potential of the tyrosine fibrils may have direct relevance to amyloid-linked pathologies.

Recent reports have revealed the intrinsic propensity of single aromatic metabolites to undergo self-assembly and form nanostructures of amyloid nature. Hence, identifying whether aspartame, a universally consumed artificial sweetener, is inherently aggregation prone becomes an important area of investigation. Although the reports on aspartame-linked side effects describe a multitude of metabolic disorders, the mechanistic understanding of such destructive effects is largely mysterious. Since aromaticity, an aggregation-promoting factor, is intrinsic to aspartame’s chemistry, it is important to know whether aspartame can undergo self-association and if such a property can predispose any cytotoxicity to biological systems. Our study finds that aspartame molecules, under mimicked physiological conditions, undergo a spontaneous self-assembly process yielding regular β-sheet-like cytotoxic nanofibrils of amyloid nature. The resultant aspartame fibrils were found to trigger amyloid cross-seeding and become a toxic aggregation trap for globular proteins, Aβ peptides, and aromatic metabolites that convert native structures to β-sheet-like fibrils. Aspartame fibrils were also found to induce hemolysis, causing DNA damage resulting in both apoptosis and necrosis-mediated cell death. Specific spatial arrangement between aspartame molecules is predicted to form a regular amyloid-like architecture with a sticky exterior that is capable of promoting viable H-bonds, electrostatic interactions, and hydrophobic contacts with biomolecules, leading to the onset of protein aggregation and cell death. Results reveal that the aspartame molecule is inherently amyloidogenic, and the self-assembly of aspartame becomes a toxic trap for proteins and cells, exposing the bitter side of such a ubiquitously used artificial sweetener.

Eugenol has attracted considerable attention because of its potential for many pharmaceutical applications including anti-inflammatory, anti-tumorigenic and anti-oxidant properties. Here, we have investigated the effect of eugenol on amyloid formation of selected globular proteins. We find that both spontaneous and seed-induced aggregation processes of insulin and serum albumin (BSA) are significantly suppressed in the presence of eugenol. Isothermal titration calorimetric data predict a single binding site for eugenol-insulin complex confirming the affinity of eugenol for native soluble insulin species. We also find that eugenol suppresses amyloid-induced hemolysis. Our findings reveal the inherent ability of eugenol to stabilize native proteins and to delay the conversion of protein species of native conformation into β-sheet assembled mature fibrils, which seems to be crucial for its inhibitory effect.

We have synthesized capsaicin-coated silver nanoparticles (AgNPs(Cap)) and have tested their anti-amyloid activity, considering serum albumin (BSA) as a model protein. We found that amyloid formation of BSA was strongly suppressed in the presence of AgNPs(Cap). However, isolated capsaicin and uncapped control nanoparticles did not show such an inhibition effect. Bioinformatics analysis reveals CH-π and H-bonding interactions between capsaicin and BSA in the formation of the protein-ligand complex. These results suggest the significance of surface functionalization of nanoparticles with capsaicin, which probably allows capsaicin to effectively interact with the key residues of the amyloidogenic core of BSA.