Insights into the secondary structures of lactam N-substituted stapled peptides.

Stapled peptides derived from the Ugi macrocyclization comprise a special class of cyclopeptides with an N-substituted lactam bridge cross-linking two amino acid side chains. Herein we report a comprehensive analysis of the structural factors influencing the secondary structure of these cyclic peptides in solution. Novel insights into the s-cis/s-trans isomerism and the effect of N-functionalization on the conformation are revealed.

Improved Stability and Tunable Functionalization of Parallel ß-Sheets via Multicomponent N-Alkylation of the Turn Moiety.

In contrast to the myriad of methods available to produce α-helices and antiparallel ß-sheets in synthetic peptides, just a few are known for the construction of stable, non-cyclic parallel ß-sheets. Herein, we report an efficient on-resin approach for the assembly of parallel ß-sheet peptides in which the N-alkylated turn moiety enhances the stability and gives access to a variety of functionalizations without modifying the parallel strands. The key synthetic step of this strategy is the multicomponent construction of an N-alkylated turn using the Ugi reaction on varied isocyano-resins. This four-component process assembles the orthogonally protected turn fragment and incorporates handles serving for labeling/conjugation purposes or for reducing peptide aggregation. NMR and circular dichroism analyses confirm the better-structured and more stable parallel ß-sheets in the N-alkylated peptides compared to the non-functionalized variants.