Hydrogen-bonded self-assembled peptide nanotubes from cystine-based macrocyclic bisureas

The design, synthesis, and characterization of amino acid-based cyclic bisureas, a new class of macrocyclic peptides, are described. These cystine-based macrocycles are constructed by a single-step procedure involving condensation of 1,omega-alkane diisocyanate[(CH2)(n)(NCO)(2); n = 4,6, 12] with either the simple L-cystine dimethyl ester to provide simple cyclic bisureas of 16-, 18-, and 24-membered rings through 1 + 1 cyclization or with extended cystine bispeptides leading to a variety of cystine-based macrocyclic peptide bisureas. The potential of cyclic bisureas to serve as artificial receptors for 1,omega-alkane dicarboxylic acids has been demonstrated with 18- and 24-membered macrocycles which show specific binding with dianions of oxalic and succinic acid, respectively. Single-crystal X-ray studies have shown these eyelid bisureas to possess an inherent property of self-assembling into vertical stacks of tube-like structures. The hollow, open-ended tubes offer enormous scope as models for studying biological phenomena or for designing materials with novel electronic and optical properties.