Columnar Assembly of Cyclic β-Amino Acid Functionalized with Pyranose Rings

Abstract

A novel cyclic trimer and tetramer of protected beta-glycamino acids were synthesized and investigated on conformation and assembly formation. A characteristic point of these cyclic beta-glycamino acids is their better solubility than other cyclic beta-amino acids due to the pyranose rings. Thus, the assembling process of the cyclic molecules could be examined by CD or NMR spectroscopy. FT-IR and NMR measurements and geometry optimization revealed a highly symmetric and planar conformation for each cyclic beta-peptide with all-trans amide groups. The amide groups in the cyclic peptides took a vertical orientation against the cyclic skeleton to be suitably arranged for intermolecular hydrogen bonds, which should promote formation of molecular assembly in a columnar structure through molecular stacking. These cyclic beta-peptides were successfully crystallized to yield rod-shaped molecular assemblies in nanometer sizes. Evidence for the columnar structure in the crystals was obtained by electron diffraction analysis, which showed a layer spacing of ca. 4.8 A. Interestingly, the molecular assembly of the cyclic trimer showed a high aspect ratio, width less than 40 nm, and length more than 2 mum, suggesting stable molecular stacking in the column.