Star polypeptides by NCA polymerisation from dendritic initiators: synthesis and enzyme controlled payload release

Well-defined star polypeptides were successfully synthesised by initiation of γ-benzyl-L-glutamate N-carboxyanhydride (NCA) from polypropylene imine (PPI) dendrimers. The dendrimer generation and the dendrimer to NCA ratio were systematically varied to afford a range of star shaped architectures with a maximum of 8 to 64 poly(γ-benzyl-L-glutamate) (PBLG) arms. High molar masses up to 500 000 g mol−1 were achieved that were otherwise unobtainable for the analogous linear polypeptides in the absence of the dendrimer core. By deprotection the PBLG star polypeptides were converted into poly(L-glutamic acid) (PGA) star polypeptides. Various concentrations of rhodamine B could be loaded into the polypeptide star architectures dependent on the number of PGA arms and the length of the grafted polypeptide chain produced. Furthermore, the polypeptidic nature of PGA-grafted dendrimers ensures their responsiveness, through controlled degradation, to the target enzyme thermolysin. An enzyme-responsive release mechanism was devised and demonstrated in which rhodamine B payload was released upon incubation with thermolysin but not the control enzyme chymotrypsin. The rate and extent of rhodamine B release was dependent on the composition of the hybrid material, which can be readily tuned to provide highly specific temporal and spatial controlled release.