Z/E Isomerism in Nα-Nα-Disubstituted Hydrazides and the Amidoxy Bond: Application to the Conformational Analysis of Pseudopeptides Built of Hydrazinoacids and α-Aminoxyacids

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.

Hydrazinolysis Products of Selected Sugar Lactones-Crystal Structure and Microbiological Activity

Commercially available lactones, as well as those synthesized by us, turned out to be good substrates for the synthesis of sugar hydrazides. The exception was L-ascorbic acid, whose hydrazinolysis led to the formation of a hydrazinium salt, not the hydrazide as expected. The structure of all compounds was confirmed by NMR and X-ray analyses. The lower durability of hydrazinium L-ascorbate was additionally confirmed by thermogravimetric tests. All products were tested for biological activity against Gram-negative bacteria strains Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 and against Gram-positive Staphylococcus aureus ATCC 25923 and Staphylococcus aureus ATCC 29213. Their antifungal activity against Candida albicans SC5314, Candida glabrata DSM 11226 SM 11226, Candida krusei DSM 6128, and Candida parapsilosis DSM 5784 was also tested. The most interesting results of microbiological activity were obtained for D-gluconic acid hydrazide and hydrazinium L-ascorbate. The results of the latter encourage more extensive testing.

nN → π*Ar interactions stabilize the E-ac isomers of arylhydrazides and facilitate their SNAr autocyclizations

We describe a novel mechanism of stabilization of the E-ac isomer of an arylhydrazide via n_N → π*_Ar interactions. We further show that when a leaving group (F) is present at the ortho-position of the carbonyl group of such an arylhydrazide, the n_N → π*_Ar interaction facilitates an S_NAr autocyclization reaction to produce indazolone, an important heterocycle with biological activity. Faster autocyclization of arylhydrazide is observed when an electron withdrawing group is present in the aryl ring, which is a characteristic of S_NAr reactions.

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.

α-Aminoxy Peptoids: A Unique Peptoid Backbone with a Preference for cis-Amide Bonds

α-Peptoids, or N-substituted glycine oligomers, are an important class of peptidomimetic foldamers with proteolytic stability. Nevertheless, the presence of cis/trans-amide bond conformers, which contribute to the high flexibility of α-peptoids, is considered as a major drawback. A modified peptoid backbone with an improved control of the amide bond geometry could therefore help to overcome this limitation. Herein, we have performed the first thorough analysis of the folding propensities of α-aminoxy peptoids (or N-substituted 2-aminoxyacetic acid oligomers). To this end, the amide bond geometry and the conformational properties of a series of model α-aminoxy peptoids were investigated by using 1D and 2D NMR experiments, X-ray crystallography, natural bond orbital (NBO) analysis, circular dichroism (CD) spectroscopy, and molecular dynamics (MD) simulations revealing a unique preference for cis-amide bonds even in the absence of cis-directing side chains. The conformational analysis based on the MD simulations revealed that α-aminoxy peptoids can adopt helical conformations that can mimic the spatial arrangement of peptide side chains in a canonical α-helix. Given their ease of synthesis and conformational properties, α-aminoxy peptoids represent a new member of the peptoid family capable of controlling the amide isomerism while maintaining the potential for side-chain diversity.

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.

N–O linkage in carbohydrates and glycoconjugates

The synthesis and chemical and physicochemical properties as well as biological and medical applications of various hydroxylamine-functionalized carbohydrate derivatives are summarized.

Total synthesis of putative montamine and a proposed structural reassignment

The spectroscopic data for synthetic montamine does not match the proposed N,N′-linked structure and closely resembles a known 4,4′-bismoschamine natural product.