UNUSUAL BETA-SHEET PERIODICITY IN SMALL CYCLIC-PEPTIDES

Cyclic peptide homologs of gramicidin S containing 6, 8, 10, 12, 14 an d 16 residues were synthesized and characterized using circular dichro ism (CD) and H-1 NMR spectroscopy. Based on the three-dimensional stru ctures generated from these data we have found strong evidence of a pe riodic sequence-length dependence on beta-sheet content. In particular , peptides of length 6, 10 and 14 residues exhibit a high beta-sheet c ontent, while peptides of 8, 12 and 16 residues appear to exist as ran dom coils. This unusual beta-sheet periodicity may have important impl ications in our understanding of beta-sheet formation and in the desig n of constrained beta-sheet and beta-hairpin mimics.