Cylindrical Peptide Structures

The handedness or chirality of molecules, organisms, and elementary particles has been widely appreciated, vindicating Pasteur’s perceptive generalization, “L‘univers est dissymktrique.” Ambidexterity is rare not only in biological organisms but also in molecules. One of the most striking chiral features in biopolymers is the twist or handedness of the helices that are frequently formed. Cylindrical helical structures formed by polypeptide chains are an important constituent of peptide and protein structure^.^-^ The most widespread helical structures are the a-helix3 and 310-heli~,4 both of which have a right-handed screw sense in proteins, determined by the L configuration of the component amino acids.6 Peptide sequences with alternating L and D residues occur in the membrane channel-forming I 5-residue peptide gramicidin A, resulting in a large diameter cylindrical structure formed by wrapping &strands about the helix axis.’ Alternating L,D sequences in cyclic peptides have yielded novel tubular stack^.^.^ Helical polymers with alternating