CYCLO-BETA-PEPTIDES - STRUCTURE AND TUBULAR STACKING OF CYCLIC TETRAMERS OF 3-AMINOBUTANOIC ACID AS DETERMINED FROM POWDER DIFFRACTION DATA

The solid-state structures of three stereoisomers, 1-3, of the cyclic tetramer of 3-aminobutanoic acid are presented. These cyclo-beta-pepti des were found to be highly insoluble materials, and it proved to be i mpossible to grow crystals of sufficient quality for X-ray single-crys tal analysis. The samples of 1-3 were, however, suitable candidates fo r structure determination from powder diffraction data (Fig. 1), and t he application of this method is described. All three isomers have bee n found to adopt tubular structures (Figs. 2-4) in a fashion similar t o those already observed for certain cyclo-alpha-peptides. The stacks of 16-membered rings are held together by four nonlinear C=O ... H-N H -bonds between pairs of molecules (Fig.5).