Repetitive polypeptides comprising 16 repeats of the sequence -(AlaGly)_3ProGluGly- (1a) have been prepared from Escherichia coli as overexpressed recombinant proteins. Partial in vivo replacement of the proline (Pro) residues in sequence 1a with L-azetidine-2-carboxylic acid (Aze) was achieved by expression of the target protein in medium containing Aze and lacking Pro. NMR and amino acid analysis for residual proline in the polymer indicated 25-40% replacement of Pro by Aze. While polymers of 1a form conformationally disordered solids, incorporation of Aze allows this material to adopt a â-sheet structure in the solid state.