Synthesis and Conformational Analysis of Aminopyrazolonyl Amino Acid (APA)/Peptides

Abstract

Pyrazole, pyrazolone, and aminopyrazolone derived molecules are bioactive molecules and considered as potential therapeutic drug candidates because of their unique structural properties. These molecules have abilities to interact with several bio‐macromolecules via non‐covalent interactions such as hydrogen bonding and π–π interactions. In structural organization of dipeptides, pyrazole containing aromatic amino acid/dipeptides have been explored and considered as potential amino acid residue. In repertoire of unnatural aromatic amino acids, this report describes the synthesis of 4‐aminopyrazolonyl containing amino acids and their crystal structures. The incorporation of 4‐aminopyrazolonyl at N‐terminal of native amino acid/dipeptides influences the conformational changes of respective peptide which induces the formation of distinctive supramolecular self‐assembly structures such as β‐sheet and α‐helices in their solid‐state crystal. The structural conformation of those peptides, here, are also demonstrated in solution phase by 1H‐NMR (1D/2D) and [D6]DMSO titration methods which support the formation of inter‐/intramolecular hydrogen bonding in solution. Hence, these unnatural amino acid analogues can tune the secondary structure of natural amino acid/peptides by introducing at N‐terminal via amide bond.

Unnatural Amino Acid: 4-Aminopyrazolonyl Amino Acid Comprising Tri-Peptides Forms Organogel With Co-Solvent (EtOAc:Hexane)

“…We began the synthesis of the unnatural amino acid ( 1 ), aminopyrazolonyl acid (APA) by following the previously reported procedure ( Scheme 1 ) ( Bollu and Sharma, 2019b ). In the literature, bipolar organic molecules have a higher propensity for organogelation ( Bardelang et al, 2008 ; Loic, 2017 ).…”

“…The formation of organogels was validated by the widely accepted inverted test tube method ( Wang et al, 2003 ; Nagahama et al, 2008 ; Yoshida et al, 2014 ). Importantly, gelation is not observed when the β-Ala residue in APA-peptide is replaced with α-amino acid residues such as Gly, Ilu, and Ala ( Bollu and Sharma, 2019b ), indicating that the presence of β-Ala at that position is crucial for the formation of gels. Possibly, the presence of the β-Ala (extra methylene) group increases the chain length (extra–CH 2 -), which affects the intramolecular H-bonding interactions and flexibility in sol-state that can reorganize easily during the formation of the rigid gel networking aggregates, appearing as physical gels ( Dado and Gellman, 1994 ; Roy et al, 2004 ; Chatterjee et al, 2007 ).…”

“…To study the amide bonds of APA-peptide ( 2c ) involved in the hydrogen bonding network, we performed the 1 H-NMR DMSO-d 6 titration experiment in CDCl 3 ( Supplementary Figure S23 ) . ( Malik et al, 2002 ; Balachandra and Sharma, 2014 ; Bollu and Sharma, 2019b ) Since the amide bond (β-Ala-NH) appeared in the aromatic region, after DMSO-d 6 titration, we again recorded 1H-COSY to confirm the respective amide NH ( Supplementary Figure S22B ). From these titration 1H-NMR spectra, with increasing concentration of DMSO-d 6 (up to 19 μL), Boc-NH and Ile-NH exhibit a significant downfield shift; however, β-Ala-NH (appended at APA moiety) shows a marginal shift ( Supplementary Figure S24 ).…”

“…The experimental procedures for the synthesis of control dipeptides and APA-peptides ( 2a-2e ) were followed from the literature. ( Bollu and Sharma, 2019b ).…”

“…4-Aminopyrazole containing aromatic unnatural amino acids/dipeptides have abilities to interact with several bio-macromolecules such as interaction with specific β-sheet-rich targets in Aβ-protein and serine proteases via non-covalent interactions ( Schrader and Kirsten, 1996 ; Kirsten and Schrader, 1997 ; Gilfillan et al, 2015 ; Hellmert et al, 2015 ). Previously, we have explored the structural and conformational studies of 4-aminopyrazolone amino acids/di-tri-peptide scaffold for non-covalent interactions, which is one of the important criteria for gelation ( Bollu and Sharma, 2019b ; a ). We report the synthesis of 4-aminopyrazolone acid (APA) containing hybrid peptides with α-/β-amino acids and preparation of their organogels ( Figure 1 ).…”