A double-stranded beta-helix with antiparallel chains in a crystalline oligo-L-D-peptide.

Abstract

There is much interest in the ionophore-mediated and channel transmembrane transport of alkali metal ions. Valinomycin and gramicidin A, which is a linear pentadecapeptide with alternating L-D-residues, are classic representatives of carrier and channel-type ionophores, respectively, and their structure–function relationships are being intensively studied1–4. Helical structures for alternating D,L peptides have been proposed from conformational energy calculations5–7, and from experimental spectroscopic data single-helical structures have been postulated for gramicidin A (ref. 8). Stereochemical and energy considerations7,9 have suggested that polypeptides with alternating L- and D-amino acid residues may take up coaxial double-helical β-structures, both parallel and antiparallel, which are stabilised by systematic interchain NH….OC hydrogen bonds. Experimental results supporting the occurrence of structures of this kind have been obtained by Veatch, Fossei and Blout2 for gramicidin A in nonpolar solvents, and by Lotz, Colonna-Cesari, Heitz and Spach10 for poly-(γ-benzyl-D,L-glutamate) in the solid state. However, there has previously been no experimental determination of the detailed structural and conformational parameters characterising these structures. We now provide detailed conformational parameters of a double-stranded β- helix with antiparallel chains obtained by single crystal X-ray analysis of the model L-D peptide Boc-(L-Val-D-Val)4-OMe (ref. 11).