Synthesis and characterisation of peptide-polymer conjugate hydrogels for biomedical applications.

Abstract

This study involved two separate projects, both of which explored the application of RAFT polymerisation for the synthesis of well-defined star polymer-peptide conjugates and developed hydrogels from synthesised star polymer conjugates. The first project aimed to develop an in situ forming hydrogel from star poly(N-(2hydroxypropyl)methacrylamide) (PHPMA) via covalent cross-linking, catalysed by Sortase A enzyme (SrtA). The use of SrtA as a cross-linking enzyme for hydrogel-based tissue engineering has been only reported previously by Broguiere et al., Arkenberg and Lin. 1,2 Both groups employed mutant enzymes with enhanced kinetics to achieve fast gelation whereas a wild type SrtA was employed in this work. Well defined star PHPMA (Ð<1.30) were successfully functionalised with SrtA-peptide substrates via a two-step synthesis consisting of an aminolysis and followed by a radical thiol-ene addition reaction. Unfortunately, cross-linking of 4-arm star PHPMA conjugations mediated by SrtA did not yield gelation which could be due to the slow kinetics of wild type SrtA. The second project focused on developing thermo-responsive hydrogels of the selfassembling peptide CFEFEFKFKK by doping the hydrogels with star (2-, 3-, and 4- arm) poly(N-isopropylacrylamide) (PNIPAM)/CFEFEFKFKK conjugates (C, cysteine; F, phenylalanine; E, glutamic acid; K, lysine). The work was based on a study by Maslovskis et al. who created the novel composite hydrogels containing FEFEFKFK peptide and linear PNIPAM-FEFEFKFK conjugates.3 Well-defined star PNIPAM (Ð<1.25) were modified via a three step synthesis consisting of an aminolysis, a vinylsulfone functionalisation, and finally Michael thiol-ene addition with CFEFEFKFK. The doping was found to introduce thermoresponsiveness to the peptide hydrogels with a lower critical solution temperature (LCST) around 36 °C. This suggested that the hydrogels a potential application in human body. The hydrogels doped with 3-arm conjugate 46 kDa, 4-arm conjugate 17 kDa, or 2-arm conjugate 4 kDa exhibited higher elasticity. This indicated that the peptides on the conjugates took part in the self-assembly with the free peptides and that the polymer chains anchored and interacted with the peptide fibres through hydrogen bonding.