Controlling the sign and magnitude of screw-sense preference from the C-terminus of an achiral helical foldamer

The global screw-sense preference of an achiral helical oligomer may be controlled by a single chiral monomer located at one terminus. Remarkably, maximal control is induced in oligomers of the achiral quaternary amino acid Aib by a single C-terminal alaninamide residue, probably because the Ala side chain, though small, is compatible with a 3₁₀ helical conformation. The presence or absence of a C-terminal hydrogen bond donor determines the screw sense of the entire oligomer.