The Dual-Stereocontrol Mechanism: Heteroselective Polymerization of rac-Lactide and Syndioselective Polymerization of meso-Lactide by Chiral Aluminum Salan Catalysts.

We describe alkoxo-aluminum catalysts of chiral bipyrrolidine-based salan ligands that follow the dual-stereocontrol mechanism wherein a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl ("match") is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities ("mismatch") is inactive towards any lactide. Polymerization of rac-LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer. The racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. A succession of match diastereomers in selective meso-LA insertions led to syndiotactic PLAs reaching a syndiotacticity degree of α=0.96. This polymer featured a Tm of 153 °C matching the highest reported value, and the highest crystallinity (ΔHm =56 J g-1 ) ever reported for syndiotactic PLA.

Formation of functional super-helical assemblies by constrained single heptad repeat.

Inspired by the key role of super-helical motifs in molecular self-organization, several tandem heptad repeat peptides were used as building blocks to form well-ordered supramolecular nano-assemblies. However, the need for stable helical structures limits the length of the smallest described units to three heptad repeats. Here we describe the first-ever self-assembling single heptad repeat module, based on the ability of the non-coded α-aminoisobutyric acid to stabilize very short peptides in helical conformation. A conformationally constrained peptide comprised of aromatic, but not aliphatic, residues, at the first and fourth positions formed helical fibrillar assemblies. Single crystal X-ray analysis of the peptide demonstrates super-helical packing in which phenylalanine residues formed an 'aromatic zipper' arrangement at the molecular interface. The modification of the minimal building block with positively charged residues results in tight DNA binding ascribed to the combined factors of helicity, hydrophobicity and charge. The design of these peptides defines a new direction for assembly of super-helical nanostructures by minimal molecular elements.