Conformational Behavior of 1:1 [α/α-Hydrazino]mer, 1:1 [α/Aza-β3-amino]mer and 1:1 [Aza-β3-amino/α]mer Series: Three Series of Foldamers

Design, synthesis, and biological evaluation of 1,2,4-oxadiazole-based derivatives as multitarget anti-Alzheimer agents

A series of novel 1,2,4-oxadiazole-based derivatives were synthesized and evaluated for their potential anti-Alzheimer disease activity. The results revealed that compounds 2b, 2c, 2d, 3a, 4a, 6, 9a, 9b, and 13b showed excellent inhibitory activity against acetylcholinesterase (AChE) with IC50 values in the range of 0.0158 to 0.121 μM. They were 1.01 to 7.78 times more potent than donepezil (IC50 = 0.123 μM). The newly synthesized compounds exhibited lower activity towards butyrylcholinesterase (BuChE) when compared to rivastigmine. Compounds 4b and 13b showed the most prominent inhibitory potential against BuChE with IC50 values of 11.50 and 15 μM, respectively. Moreover, 4b, and 9b were found to be more potent antioxidant agents (IC50 values of 59.25, and 56.69 μM, respectively) in comparison with ascorbic acid (IC50 = 74.55 μM). Compounds 2b and 2c exhibited monoamine oxidase-B (MAO-B) inhibitory activity with IC50 values of 74.68 and 225.48 μM, respectively. They were 3.55 and 1.17 times more potent than biperiden (IC50 = 265.85 μM). The prominent interactions of the compounds with the AChE active site can be used to computationally explain the high AChE inhibitory activity. The results unveiled 1,2,4-oxadiazole derivatives 2c and 3a as multitarget anti-AD agents. The predicted ADME properties for compounds 2b and 4a were satisfactory, and 4a had the highest likelihood of crossing the blood-brain barrier (BBB), making it the optimum compound for future optimization.

α-Hydrazino Acid Insertion Governs Peptide Organization in Solution by Local Structure Ordering

In this work, we have applied the concept of α-hydrazino acid insertion in a peptide sequence as a means of structurally organizing a potential protein-protein interactions (PPI) inhibitor. Hydrazino peptides characterized by the incorporation of an α-hydrazino acid at specific positions introduce an additional nitrogen atom into their backbone. This modification leads to a change in the electrostatic properties of the peptide and induces the restructuring of its hydrogen bonding network, resulting in conformational changes toward more stable structural motifs. Despite the successful use of synthetic hydrazino oligomers in binding to nucleic acids, the structural changes due to the incorporation of α-hydrazino acid into short natural peptides in solution are still poorly understood. Based on NMR data, we report structural models of p53-derived hydrazino peptides with elements of localized peptide structuring in the form of an α-, β-, or γ-turn as a result of hydrazino modification in the peptide backbone. The modifications could potentially lead to the preorganization of a helical secondary peptide structure in a solution that is favorable for binding to a biological receptor. Spectroscopically, we observed that the ensemble averaged rapidly interconverting conformations, including isomerization of the E-Z hydrazide bond. This further increases the adaptability by expanding the conformational space of hydrazine peptides as potential protein-protein interaction antagonists.

Consecutive hydrazino-Ugi-azide reactions: synthesis of acylhydrazines bearing 1,5-disubstituted tetrazoles

Isocyanide-based multicomponent reactions (IMCRs) allow the construction of relatively complex molecules through a one-pot synthesis. The combination of IMCRs in a consecutive or sequential fashion further extends the complexity of the molecules obtained. Herein, we report the efficient application of this approach to the synthesis of acylhydrazines bearing 1,5-disubstituted tetrazoles. Our strategy was accomplished in only three steps: first, a one-pot hydrazino-Ugi-azide four-component reaction; second a hydrazinolysis and finally an additional hydrazino-Ugi-azide reaction. This sequence provides the title compounds in moderate to excellent yields. The products synthesized herein contain functional groups within their structures that can be easily modified to obtain new acylhydrazino 1,5-disubstituted tetrazoles.

Self-Organization Ability of Chiral Nα-Substituted, Nβ-Boc Protected α-Hydrazinoacetamides in the Crystal and Solution States

The limitations of peptides have severely hampered their use in pharmacology, thus prompting the design of new peptidomimetic foldamers. This requires precise knowledge of the secondary structure of new compounds and the ability to predict their folding. Conformational studies of the basic units of these foldamers can be of invaluable assistance in designing new bioactive compounds. To this end, we investigated the conformation of three chiral N^α-substituted, N^β-Boc protected α-hydrazinoacetamide model compounds containing various side chains both on the N^α- and C^α-atoms in both the crystal and solution states. On the basis of IR absorption spectroscopy, NMR, molecular dynamics calculations and X-ray diffraction experiments, we demonstrated that these three models adopt conformational preferences, relying on eight-, six- or five-membered H-bonded pseudocycles (C₈, C₆ or C₅), depending on the steric bulk of both N^α- or C^α-side chains. This study sheds light onto the versatile folding ability of the specific class of α-N^α-hydrazinopeptides and emphasizes the key role of the C^α-side chain on the conformational preference of the folding.

Spontaneous Self-Assembly of Fully Protected Ester 1:1 [α/α-Nα-Bn-hydrazino] Pseudodipeptides into a Twisted Parallel β-Sheet in the Crystal State

Previous studies have demonstrated that amidic α/β-pseudodipeptides, 1:1 [α/α-N^α-Bn-hydrazino], have the ability to fold via a succession of γ-turn (C₇ pseudocycle) and hydrazinoturn in CDCl₃ solution, their amide terminals enabling the formation of an intramolecular H-bond network. Despite their lack of a primary amide terminals allowing the formation of the hydrazinoturn, their ester counterparts 1-4 were proven to self-assemble into C₆ and C₇ pseudocycles by intramolecular H-bonds in solution state and into an uncommon twisted parallel β-sheet through intermolecular H-bonding in the crystal state to form a supramolecular helix, with eight molecules needed to complete a full 360° rotation. Such self-organization (with eight molecules) has only been observed in a specific α/α-pseudodipeptide, depsipeptide (Boc-Leu-Lac-OEt). Relying on IR absorption, NMR, X-ray diffraction, and CD analyses, the aim of this study was to demonstrate that stereoisomers of ester 1:1 [α/α-N^α-Bn-hydrazino] pseudodipeptides 1-4 are able to self-assemble into this β-helical structure. The absolute configuration of the asymmetric C^α-atom of the α-amino acid residue influences the left- or right-handed twist without changing the pitch of the formed helix.

The impact of α-hydrazino acids embedded in short fluorescent peptides on peptide interactions with DNA and RNA

A series of novel hydrazino-based peptidomimetics and analogues comprising N-terminal lysine and C-terminal phenanthridinyl-l-alanine were prepared. The presented results demonstrate the up to now unknown possibility to finely modulate peptide interactions with DNA/RNA by α-hydrazino group insertion and how the different positioning of two α-hydrazino groups in peptides controls binding to various double stranded and single stranded DNA and RNA. All peptidomimetics bind with 1-10 micromolar affinity to ds-DNA/RNA, whereby the binding mode is a combination of electrostatic interactions and hydrophobic interactions within DNA/RNA grooves. Insertion of the α-hydrazino group into the peptide systematically decreased its fluorimetric response to DNA/RNA binding in the order: mono-hydrazino < alternating-hydrazino < sequential-hydrazino group. Binding studies of ss-polynucleotides suggest intercalation of phenanthridine between polynucleotide bases, whereby affinity and fluorimetric response decrease with the number of α-hydrazino groups in the peptide sequence. Particularly interesting was the interaction of two sequential α-hydrazino acids-peptidomimetic with poly rG, characterised by a specific strong increase of CD bands, while all other peptide/ssRNA combinations gave only a CD-band decrease. All mentioned interactions could also be reversibly controlled by adjusting the pH, due to the protonation of the fluorophore.

Synthesis of hybrid hydrazino peptides: protected vs unprotected chiral α-hydrazino acids

Peptidomimetics based on hydrazino derivatives of α-amino acids represent an important class of peptidic foldamers with promising biological activities, like protease inhibition and antimicrobial activity. However, the lack of straightforward method for the synthesis of optically pure hydrazino acids and efficient incorporation of hydrazino building blocks into peptide sequence hamper wider exploitation of hydrazino peptidomimetics. Here we described the utility of N^α-benzyl protected and unprotected hydrazino derivatives of natural α-amino acids in synthesis of peptidomimetics. While incorporation of N^α-benzyl-hydrazino acids into peptide chain and deprotection of benzyl moiety proceeded with difficulties, unprotected hydrazino acids allowed fast and simple construction of hybrid peptidomimetics.

Solid-phase synthesis of tetrahydropyridazinedione-constrained peptides

The design and solid-phase synthesis of tetrahydropyridazine-3,6-dione (Tpd) peptidomimetics derived from backbone-aminated peptides is reported. The described protocol features the synthesis of chiral α-hydrazino acids suitable for chemoselective incorporation into growing peptide chains. Acid-catalyzed cyclization to form the Tpd ring during cleavage affords the target peptidomimetics in good yield and purity. The scope of Tpd incorporation is demonstrated through the synthesis of constrained peptides featuring nucleophilic/electrophilic side chains and sterically encumbered α-substituted hydrazino acid residues.