D,L-Alternating Peptides: Chiral Precision and Biological Nanostructures

The fabrication of new materials using natural building blocks has become a subject of major interest. The versatility of the peptides, in conjunction with their ability to form specific secondary structures, provides a unique platform for the design of nanomaterials with controllable structural features. Numerous experimental studies of the conformational characteristics of linear peptides formed by regularly alternating D and L residues have been carried out. These experimental studies have indicated that D,L-alternating peptides are able to assume very specific conformations. ?-Helix structures are of particular interest due to their capacity to form trans-membrane channels with different transport properties. Single-stranded ?-helix are symmetrical structures in which the backbone CO and NH groups are oriented quasi-parallel to the helical axis but in alternating opposite directions. They form intrachain hydrogen bonds equivalent in pairs. The side chains are oriented perpendicular to the helical axis on the outside of the helix. These features give rise to compact channel structures with pore size increasing proportionally with the number of residues per turn. Double-stranded ?-helices should be always preferred if the only operative tendency would be that of maximizing the number of H bonds. 1H-NMR spectra of Boc-XV-Nle(Leu 1,6,15)-OMe in chloroform solution of different concentration, characteristically show multiple signals for the different protons. This multiplicity indicate that in chloroform there are three ?-helical structures: right-handed ???5,6-helices, right- handed ?4,4-helices, and right-handed ?6,3-helices, in a slow interconversion equilibrium. The study of conformational changes involved in such intricate equilibrium permit the development of modular system of different building blocks and netpoints in order to build shape-memory polypeptides. Future developments are aiming at multifunctional and multistimuli-sensitive shape-memory biopolymers.