Parallel assembly of dipolar columns composed of a stacked cyclic tri-β-peptide

Abstract

A novel cyclic trimer of a beta-amino acid, trans-2-aminocyclohexylcarboxylic acid, was synthesized and its conformation and ability to form assemblies investigated. FT-IR and NMR measurements and computational calculations showed that this cyclic tri-beta-peptide has a C3-symmetric conformation with trans amide groups. A notable feature of the conformation is a vertical and parallel orientation of the three amide groups to the cyclic skeleton. The cyclic tri-beta-peptide was crystallized from a solution in trifluoroacetic acid-methanol (or trifluoroacetic acid-water) to yield a rod-shaped molecular assembly, as observed by TEM. The electron crystallography of the rod-shaped assembly both in suspension and in ultrathin cross-section revealed that the cyclic tri-beta-peptides were stacked up to form molecular columns, and that a two-fold screw symmetry operation along the column direction was present in the unit cell, which contained two cyclic tri-beta-peptides. This indicates that all the amide groups are oriented in the same direction. Since any two molecular columns are staggered by a quarter of a c-axis length and aligned parallel to each other, the dipole moments of the columns are aligned to enhance the strength additively in the whole assembly.