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Structural characterization of 3,4-disubstituted γ-peptide and 2,3-disubstituted β-peptide foldamers derived from common multifaceted β-nitromethyl γ-amino acids and the chemical transformation of the β-nitromethyl group in γ-peptides into various functional derivatives are reported. The γ(3,4)-oligomers and α,γ-hybrid peptides showed characteristic C14-and C12-helical conformations in single crystals. Further, the new 2,3-disubstituted acyclic β-peptide showed the C6-helical conformation despite the poor geometry of H-bonds.

The synthesis and utilization of novel thiostatines (β-SH-substituted γ-amino acids) in the design of backbone-disulfide-stabilized β-hairpin mimetics, solution conformations of hybrid β-hairpins and Cys-disulfide-stabilized α-peptide analogue, their thiol exchange, and proteolytic stability are investigated. The results suggest that thiostatines can be used to design proteolytically stable water-soluble β-hairpin mimetics without deviating from overall β-hairpin conformation.

HBTU is a standard coupling agent commonly used for the activation of free carboxylic acids during the solution and solid phase peptide synthesis. 1-Hydroxybenzotriazole (HOBt) plays a significant role in reducing the racemization during peptide synthesis; hence it is regularly used as a coupling additive. Here, we are reporting the mild and facile conjugate addition of HOBt to E-vinylogous γ-amino acids mediated by the HBTU. The reaction is moderately diastereoselective and novel β-benzotriazole N-oxide (β-BtO) substituted γ-amino acids were isolated in moderate to good yields. The single crystal analysis of methyl esters of major (anti) and minor (syn) conjugate addition products infers the formation of exclusively N-alkylated benzotriazole N-oxides instead of O-alkylation of HOBt. In addition, we showed the utilization of β-BtO substituted γ-amino acids in peptide synthesis and studied their conformations in single crystals.

The high diastereoselectivity in the Michael addition of nitromethane to α,β-unsaturated γ-amino esters, crystal conformations of β-nitromethane substituted γ-amino acids and peptides are studied. Results suggest that the N-Boc protected amide NH, conformations of α,β-unsaturated γ-amino esters and alkyl side chains play a crucial role in dictating the high diastereoselectivity of nitromethane addition to E-vinylogous amino esters. Investigation of the crystal conformations of both α,β-unsaturated γ-amino esters and the Michael addition products suggests that an H-C(γ)-C(β)=C(α) eclipsed conformer of the unsaturated amino ester leads to the major (anti) product compared to that of an N-C(γ)-C(β)=C(α) eclipsed conformer. The major diastereomers were separated and subjected to the peptide synthesis. The single crystal analysis of the dipeptide containing β-nitromethane substituted γ-amino acids reveals a helical type of folded conformation with an isolated H-bond involving a nine-atom pseudocycle.

The impact of geometrically constrained cis α,β-unsaturated γ-amino acids on the folding of α,γ-hybrid peptides was investigated. Structure analysis in single crystals and in solution revealed that the cis carbon-carbon double bonds can be accommodated into the 12-helix without deviation from the overall helical conformation. The helical structures are stabilized by 4→1 hydrogen bonding in a similar manner to the 12-helices of β-peptides and the 310 helices of α-peptides. These results show that functional cis carbon-carbon double bonds can be accommodated into the backbone of helical peptides. [ABSTRACT FROM AUTHOR]

A facile synthesis of highly symmetrical tetrasubstituted pyrazines through simple aerial oxidation of β-keto γ-amino esters is reported. The scope of the reaction was examined by use of various amino acid side-chain functional groups and peptides. The mild and efficient transformation of β-keto γ-amino esters into pyrazines may serve as an attractive strategy for self-dimerization of peptides. [ABSTRACT FROM AUTHOR]