Rheology of block copolypeptide solutions: Hydrogels with tunable properties

Amphiphilic block copolypeptides were prepared through transition-metal-mediated polymerization of amino acid N-carboxyanhydrides. In aqueous solution these materials form strong hydrogels at low concentrations. The self-assembly process that is responsible for gelation was investigated by measuring the rheological properties of the gels for a variety of molecular architectures: poy-L-lysine-b-poly-L-leucine diblock and poly-L-lysine-b-poly-L-leucine-b-poly-L-lysine triblock copolypeptides. Experiments showed that the rodlike helical secondary structure of enantiomerically pure poly-L-leucine blocks was instrumental for gelation at polypeptide concentrations as low as 0.25 wt %. The hydrophilic polyelectrolyte segments have stretched coil configurations and stabilize the twisted fibril assemblies by forming a corona around the hydrophobic core. The self-assembly of hydrophobic blocks is highly specific and sensitive to the chirality of the helices. It was found that mechanical properties of the gels can be tuned through the molecular architecture of the block copolypeptides and also by carefully mixing different polypeptides in solution.