THE construction of synthetic molecules that fold or assemble predictably into large, well defined structures represents a fertile area of chemistry. Many supramolecular systems have been reported that self-assemble as a result of non-covalent interactions(1-7); and the control of higher-order protein structure by de novo design has also been demonstrated(8,9). Protein secondary structural motifs have also been stabilized by incorporating artificial groups that impose constraints on the folded architecture(10-12). Here we describe the synthesis of molecules that will fold in water into a pleated structure, as a result of interactions between alternating electron-rich donor groups and electron-deficient acceptor groups. We verify the pleated structure using absorption and NMR spectroscopy. Donor-acceptor interactions have been used previously to engineer specific supramolecular geometries(2,13), and are energetically favourable in organic as well as in aqueous solutions. But whereas previously such interactions have been used to effect self-assembly of distinct molecules, our results show that they can also determine the secondary structure of complex synthetic molecules in solution.