Synthesis of C-linked carbo-β2-amino acids and β2-peptides: design of new motifs for left-handed 12/10- and 10/12-mixed helices

Abstract

C-linked carbo-β²-amino acids (β²-Caa), a new class of β-amino acid with a carbohydrate side chain having D-xylo configuration, were prepared from D-glucose. The main idea behind the design of the new β-amino acids was to move the steric strain of the bulky carbohydrate side chain from the Cβ- to the Cα-carbon atom and to explore its influence on the folding propensities in peptides with alternating (R)- and (S)-β²-Caas. The tetra- and hexapeptides derived were studied employing NMR (in CDCl₃), CD, and molecular dynamics simulations. The β²-peptides of the present study form left-handed 12/10- and 10/12-mixed helices independent of the order of the alternating chiral amino acids in the sequence and result in a new motif. These results differ from earlier findings on β³-peptides of the same design, containing a carbohydrate side chain with D-xylo configuration, which form exclusively right-handed 12/10-mixed helices. Quantum chemical calculations employing ab initio MO theory suggest the side chain chirality as an important factor for the observed definite left- or right-handedness of the helices in the β²- and β³-peptides.