Scalable synthesis and coupling of quaternary α-arylated amino acids: α-aryl substituents are tolerated in α-helical peptides.

Quaternary amino acids are important tools for the modification and stabilisation of peptide secondary structures. Here we describe a practical and scalable synthesis applicable to quaternary alpha-arylated amino acids (Q4As), and the development of solid-phase synthesis conditions for their incorporation into peptides. Monomeric and dimeric α-helical peptides are synthesised with varying degrees of Q4A substitution and their structures examined using biophysical methods. Both enantiomers of the Q4As are tolerated in folded monomeric and oligomeric α-helical peptides, with the (R)-enantiomer slightly more so than the (S).

Increasing the Functional Group Diversity in Helical ß-Peptoids: Achievement of Solvent- and pH-Dependent Folding.

We report the synthesis of a series of bis-functionalized ß-peptoid oligomers of the hexamer length. This was achieved by synthesizing and incorporating protected amino- or azido-functionalized chiral building blocks into precursor oligomers by a trimer segment coupling strategy. The resulting hexamers were readily elaborated to provide target compounds displaying amino groups, carboxy groups, hydroxy groups, or triazolo-pyridines, which should enable metal ion binding. Analysis of the novel hexamers by circular dichroism (CD) spectroscopy and 1H-13C heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR) spectroscopy revealed robust helical folding propensity in acetonitrile. CD analysis showed a solvent-dependent degree of helical content in the structural ensembles when adding different ratios of protic solvents including an aqueous buffer. These studies were enabled by a substantial increase in solubility compared to previously analyzed ß-peptoid oligomers. This also allowed for the investigation of the effect of pH on the folding propensity of the amino- and carboxy-functionalized oligomers, respectively. Interestingly, we could show a reversible effect of sequentially adding acid and base, resulting in a switching between compositions of folded ensembles with varying helical content. We envision that the present discoveries can form the basis for the development of functional peptidomimetic materials responsive to external stimuli.

Functionalized Helical ß-Peptoids.

Peptidomimetic foldamers adopting well-defined three-dimensional structures while being stable toward proteolysis are of interest in biomedical research, chemical biology, and biomimetic materials science. Despite their backbone flexibility, ß-peptoids containing N-( S)-1-(1-naphthyl)ethyl ( Ns1npe) side chains can fold into unique triangular prism-shaped helices. We report herein the successful introduction of amino groups onto robustly folded ß-peptoid helices by construction and incorporation of novel chiral building blocks. This is the first example of an X-ray crystal structure of a linear ß-peptoid containing more than one type of side chain. We thus present a unique foldamer design comprising a robustly folded core with functionalized side chains protruding perpendicular to the helical axis to provide a highly predictable display of functional groups. This work paves the way for development of ß-peptoid foldamers with a desired function, such as catalytic properties or as scaffolds enabling polyvalent display.

Backbone-Fluorinated 1,2,3-Triazole-Containing Dipeptide Surrogates.

The 1,2,3-triazole moiety can be incorporated as a peptide bond bioisostere to provide protease resistance in peptidomimetics. Herein, we report the synthesis of peptidomimetic building blocks containing backbone-fluorinated 1,4-disubstituted 1,2,3-triazole moieties. Synthetic protocols for the preparation of various Xaa-Gly dipeptide surrogates in the form of Xaa-ψ[triazole]-F2Gly building blocks were established, and selected examples were introduced into the endogenous peptide opioid receptor ligand Leu-enkephalin as a model compound.

Functionalized triazolopeptoids--a novel class for mitochondrial targeted delivery.

Here we introduce linear 1,4-triazolopeptoids as a novel class of cell penetrating peptidomimetics suitable as organ targeting molecular transporters of bioactive cargo. Repetitive triazole moieties with up to three residues were assembled on solid supports using copper-catalyzed alkyne-azide cycloadditions (CuAAC) in a submonomer approach. Depending on the lipophilicity of their side chain appendages the 1,4-triazolopeptoids showed either endosomal localization or a strong colocalization with the mitochondria of HeLa cells with moderate toxicity. While the basic triazolopeptoids mainly target the neuromast cells in zebrafish embryos, the lipophilic ones colocalize with either cartilage in the jaws and the blood vessel system.