Engineering the structure of an N-terminal β-turn to maximize screw-sense preference in achiral helical peptide chains

Oligomers of α-aminoisobutyric acid (Aib) are achiral peptides that typically adopt 3₁₀ helical conformations in which enantiomeric left- and right-handed conformers are, necessarily, equally populated. Incorporating a single protected chiral residue at the N-terminus of the peptide leads to induction of a screw-sense preference in the helical chain, which may be quantified (in the form of "helical excess") by NMR spectroscopy. Variation of this residue and its N-terminal protecting group leads to the conclusion that maximal levels of screw-sense preference are induced by bulky chiral tertiary amino acids carrying amide protecting groups or by chiral quaternary amino acids carrying carbamate protecting groups. Tertiary l-amino acids at the N-terminus of the oligomer induce a left-handed screw sense, while quaternary l-amino acids induce a right-handed screw sense. A screw-sense preference may also be induced from the second position of the chain, weakly by tertiary amino acids, and much more powerfully by quaternary amino acids. In this position, the l enantiomers of both families induce a right-handed screw sense. Maximal, and essentially quantitative, control is induced by an l-α-methylvaline residue at both positions 1 and 2 of the chain, carrying an N-terminal carbamate protecting group.