Formation of Supramolecular Single and Double Helix-like Structures by Designed Tripeptides
The conformation and self-assembly of an N- and C- protected tripeptide, Boc-Gly-L-Phg-D-Phe-OMe (1, Phg: Phenylglycine) and Boc-Gly-L-Phg-D-Phg-OMe (2) have been investigated. The effect of the insertion of two non-coded amino acid residues, L-Phg/D-Phe and L-Phg/D-Phg, consecutively on the structure of two tripepetides has been investigated. The single-crystal X-ray diffraction analysis of 1 and 2 suggested that both peptides adopted anti-parallel β-sheet structure but they further self-assembled to form a supramolecular single helix and double helix-like architectures, respectively, by various non-covalent interactions in the crystalline state. To the best of our knowledge, this is the first crystallographic report where alternating D/L unnatural amino acid containing small tripeptide exhibited double helix-like architecture. The conformation of these peptides was examined by 2D NMR, solvent dependent NMR titration, and CD spectroscopic studies in solution. Peptide 1 and 2 self-aggregated to form a bunch of flower-petals-like and flower-like architecture, respectively, in an acetonitrile-water medium under field emission scanning electron microscope (FESEM).