Can Helical Peptides Unwind One Turn at a Time? - Controlled Conformational Transitions in α,β(2,3)-Hybrid Peptides

Modulating the Structural Properties of α,γ-Hybrid Peptides by α-Amino Acid Residues: Uniform 12-Helix Versus "Mixed" 12/10-Helix

The most important natural α- and 3₁₀ -helices are stabilized by unidirectional intramolecular hydrogen bonds along the helical cylinder. In contrast, we report here on 12/10-helical conformations with alternately changing hydrogen-bond directionality in sequences of α,γ-hybrid peptides P1-P5 [P1: Boc-Ala-Aic-Ala-Aic-COOH; P2: Boc-Leu-Aic-Leu-Aic-OEt; P3: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; P4: Boc-Ala-Aic-Ala-Aic-Ala-Aic-Ala-OMe; P5: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; Aic=4-aminoisocaproic acid, Aib=2-aminoisobutyric acid] composed of natural α-amino acids and the achiral γ^4,4 -dimethyl substituted γ-amino acid Aic in solution and in single crystals. The helical conformations are stabilized by alternating i→i+3 and i→i-1 intramolecular hydrogen bonds. The experimental data are supported by ab initio MO calculations. Surprisingly, replacing the natural α-amino acids of the sequence by the achiral dialkyl amino acid Ac₆ c [P6: Boc-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-CONHMe; Ac₆ c = 1-aminocyclohexane-1-carboxylic acid] led to a 12-helix with unidirectional hydrogen bonds showing an entirely different backbone conformation. The results presented here emphasize the influence of the structure of the α-amino acid residues in dictating the helix types in α,γ-hybrid peptide foldamers and demonstrate the consequences for folding of small structural variations in the monomers.

Sulfamide-Lattice Restructuring To Form Dimensionally Controlled Molecular Arrays and Gel-Forming Systems

A design approach that incorporates structural requirements for the formation of a 1D assembly, fibril stability, and fibril-fibril interactions for gelation was attempted by using amino acid-based sulfamides with the general structure Aa-NH-SO₂ -NH-Aa (Aa=amino acid). A preference for 1D assembly alone was not a sufficient condition for gelation, which became evident from studies involving sulfamide esters 1-5. Reducing the crystallization tendency without hindering unidirectional growth was executed through diacids of the sulfamide precursors with various amines that form an envelope around the sulfamide core through salt bridges. This strategy was fruitful, and gels of a wide variety of solvents could be formed by varying the acid and amine components. The use of dodecylamine or benzylamine, which could stabilize the molecular layers through alkyl-chain segregation or π-π interactions improved the gelation tendency, whereas the nature of the amino acid side chain, especially the rotational freedom and hydrophobicity, had a direct role in dictating the solvent preference. Crystallographic studies of these two-component systems gave molecular-level insight into the assembly and showed the importance of anisotropy in the distribution of secondary interactions in gelation.

Towards a fragment-based approach in gelator design: halogen effects leading to thixotropic, mouldable and self-healing systems in aryl-triazolyl amino acid-based gelators!

A simple replacement of a H atom by Br transformed non-gelating aryl triazolyl amino acid benzyl ester to a versatile gelator, which formed shape-persistent, self-healing and mouldable gels.

Aryl-triazolyl peptides for efficient phase selective gelation and easy removal of dyes from water

Fine-tuning the gelation ability of aryl triazolyl peptide 1 by C-terminal modification led to the identification of 2 with the remarkable ability to form highly transparent gels in a wide range of solvents including oils.

A practical route to β(2,3)-amino acids with alkyl side chains

Enantiopure N(Boc)-β³-amino nitriles, valuable synthetic intermediates in the multistep homologation of α-amino acids, were alkylated using n-BuLi as base. Alkylations afforded easily separable, almost equimolecular mixtures of diastereomeric N(Boc)-protected syn and anti β^2,3-amino nitriles. Suitable manipulations of both cyano and amino groups eventually led to enantiopure N- and/or C-protected β^2,3-amino acids. For example, methanolysis using conc. HCl gas in MeOH, provides C-protected β^2,3 amino acids in excellent yields. This methodology is applied to the synthesis of a series N(Boc)-β^2,3-dialkyl amino nitriles derived from l-phenylalanine, d-phenylalanine, l-valine and one C-protected β^2,3 amino acid. We demonstrate an efficient procedure for the preparation of anti and syn β^2,3-amino acids with alkyl side chains, from α-amino acids in reasonable yields.