Cyclo‐β‐peptides: Structure and tubular stacking of cyclic tetramers of 3‐aminobutanoic acid as determined from powder diffraction data

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Abstract
The solid‐state structures of three stereoisomer,1–3, of the cyclic tetramer of 3‐aminobutanoic acid are presented. These cyclo‐β‐peptides were found to be highly insoluble materials, and it proved to be impossible to grow crystals of sufficient quality for X‐ray single‐crystal analysis. The samples of 1–3 were, however, suitable candidates for structure determination from powder diffraction data (Fig. 1), and the application of this method is described. All three isomers have been found to adopt tubular structures (Figs. 2–4) in a fashion similar to those already observed for certain cyclo‐α‐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).

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