Traceless Templated Amide-Forming Ligations.

Template assistance allows organic reactions to occur under highly dilute conditions-where intermolecular reactions often fail to proceed-by bringing reactants into close spatial proximity. This strategy has been elegantly applied to numerous systems, but always with the retention of at least one of the templating groups in the product. In this report, we describe a traceless, templated amide-forming ligation that proceeds at low micromolar concentration under aqueous conditions in the presence of biomolecules. We utilized the unique features of an acylboronate-hydroxylamine ligation, in which covalent bonds are broken in each of the reactants as the new amide bond is formed. By using streptavidin as a template and acylboronates and O-acylhydroxylamines bearing desthiobiotins that are cleaved upon amide formation, we demonstrate that traceless, templated ligation occurs rapidly even at submicromolar concentrations. The requirement for a close spatial orientation of the functional groups-achieved upon binding to streptavidin-is critical for the observed enhancement in the rate and quantity of product formed.

Covalently functionalized amide cross-linked hydrogels from primary amines and polyethylene glycol acyltrifluoroborates (PEG-KATs).

A new method for the rapid preparation of chemically cross-linked hydrogels based on a multi-arm polyethylene glycol (PEG) bearing potassium acyl trifluoroborate (KAT) functional groups with multi-dentate amines is described. These scaffolds - prepared in aqueous buffer - give strong, transparent hydrogels. At pH 3, the gel formation is complete within seconds, and the reaction rate can be tuned by modulating the pH. Rheology measurements show that the hydrogel properties can be tuned as a function of both the weight percent of solids in the gel and the denticity of amine cross-linker, allowing for predictable formation of gels with desired traits. This process relies on a rapid amide-forming reaction of KATs and in situ generated N-chloroamines. Numerous commercially available amines, including di-, tri- and tetra-functional amines as well as peptides and carbohydrates serve as effective cross-linkers. Monodentate amines included in the gelation mixture are covalently linked into the gel matrix by amide-bonds, allowing gels containing immobilized molecules including dyes, sensors, or biotin amine, to be prepared in a single step from simple starting materials. The ability to induce gelation only upon addition of equimolar amounts of an inexpensive inducer (N-chlorosuccinimide) allows premixed components to be stored as an aqueous solution for weeks and converted to gels on demand.