The synthesis and the structural characterization of a cyclic hexapeptoid with four methoxyethyl and two propargyl side chains have disclosed the presence of a hydrate crystal form [form (I)] and an anhydrous crystal form [form (II)]. The relative amounts of form (I) and form (II) in the as-purified product were determined by Rietveld refinement and depend on the purification procedures. In crystal form (I), peptoid molecules assemble in a columnar arrangement by means of side-chain-to-backbone C=CH...OC hydrogen bonds. In the anhydrous crystal form (II), cyclopeptoid molecules form ribbons by means of backbone-to-backbone CH2...OC hydrogen bonds, thus mimicking β-sheet secondary structures in proteins. In both crystal forms side chains act as joints among the columns or the ribbons and contribute to the stability of the whole solid-state assembly. Water molecules in the hydrate crystal form (I) bridge columns of cyclic peptoid molecules, providing a more efficient packing.
Keywords: Rietveld refinement; cyclic peptoids; single-crystal X-ray diffraction; solid-state assembly.