Unusual Aggregates Formed by the Self-Assembly of Proline, Hydroxyproline, and Lysine

There is a plethora of significant research that illustrates toxic self-assemblies formed by the aggregation of single amino acids, such as phenylalanine, tyrosine, tryptophan, cysteine, and methionine, and their implication on the etiology of inborn errors of metabolisms (IEMs), such as phenylketonuria, tyrosinemia, hypertryptophanemia, cystinuria, and hypermethioninemia, respectively. Hence, studying the aggregation behavior of single amino acids is very crucial from the chemical neuroscience perspective to understanding the common etiology between single amino acid metabolite disorders and amyloid diseases like Alzheimer's and Parkinson's. Herein we report the aggregation properties of nonaromatic single amino acids l-proline (Pro), l-hydroxyproline (Hyp), and l-lysine hydrochloride (Lys). The morphologies of the self-assembled structures formed by Pro, Hyp, and Lys were extensively studied by various microscopic techniques, and controlled morphological transitions were observed under varied concentrations and aging times. The mechanism of structure formation was deciphered by concentration-dependent ¹H NMR analysis, which revealed the crucial role of hydrogen bonding and hydrophobic interactions in the structure formation of Pro, Hyp, and Lys. MTT assays on neural (SHSY5Y) cell lines revealed that aggregates formed by Pro, Hyp, and Lys reduced cell viability in a dose-dependent manner. These results may have important implications in the understanding of the patho-physiology of disorders such as hyperprolinemia, hyperhydroxyprolinemia, and hyperlysinemia since all these IEMs are associated with severe neurodegenerative symptoms, including intellectual disability, seizures, and psychiatric problems. Our future studies will endeavor to study these biomolecular assemblies in greater detail by immuno-histochemical analysis and advanced biophysical assays.