NMR and vibrational spectroscopic studies on the structure and self-assembly of Two de novo dipeptides in methanol

Short peptides are known to form self-organized nano-architectures useful for advanced biomedical applications. Both hydrogen-bonding and hydrophobic interactions play a pivotal role in molecular assembly, including self-assembly of proteins and peptides. In the current investigation, we reported the synthesis, self-assembly pattern and physicochemical properties of two de novo dipeptides: N-Boc-(O-benzyl)-tyro-(dimethyl)-glutamate and N-Boc-(O-benzyl)-glu-(O-benzyl)-tyrosine-methyl ester, abbreviated as P1 and P2, respectively. These two peptides produced different morphological features under similar conditions due to the presence of a small twist in their molecular structures. In methanol, P1 gave rise to a unique fibrillar network whereas P2 self-assembled into oligomers of 20-30 nm diameters. The UV-Vis, FT-IR and Raman spectra were recorded using experimental methods and compared with the computational spectra obtained by density functional method (B3LYP/6-31G(d)). The electronic absorption spectra of both P1 and P2 measured in methanol showed broad absorption peaks in the UV region with an absorption maximum at ~ 273 nm. FT-IR and Raman spectra of both the dipeptides in solid state showed amide I vibration bands at ~1650 cm(-1) indicating a similar conformation state of the peptide bonds. However, H/D exchange 1 H NMR measurements confirmed that the amide NH of P1 was strongly involved in H bond formation compared to P2 amide NH and rapid H/D exchange occurred upon addition of D2O in the solution. Thioflavin T (ThT) fluorescence assay measurement suggested that incubation of P1 in methanol at room temperature produced fibrillar aggregates enriched with compact ~sheet conformation. Formation of such ordered structure also caused a blue shift in the amide-carbonyl Raman frequency of the peptide to ~1657 cm(-1). P1 fibrils further showed gel formation behaviour in 1:1 methanol/water mixture. The minimum gelation concentration (MGC) of the dipeptide was found to be 0.5% w/v and the calculated gelation temperature (Tgel) of the corresponding MGC was 67 C. On the other hand, the amide carbonyl stretching vibration of P2 appeared at ~ 1650 cm(-1) in the Raman spectra and it was not significantly altered in its oligomeric state (produced in methanol).Moreover, no significant enhancement of ThT fluorescence intensity of P2 indicating absence of compact beta-sheet conformation of the peptide. The P2 oligomers also showed no gel like phase transformation in methanol/water mixtures.(C) 2022 Elsevier B.V. All rights reserved.

Automated summary

The study reported the synthesis, self-assembly patterns, and physicochemical properties of two novel dipeptides, which showed different morphological features due to slight twists in their molecular structures under similar conditions. These dipeptides demonstrated unique self-assembly behaviors and structural characteristics, providing insights into their potential biomedical applications.