Reversible polypeptide hydrogels from asymmetric telechelics with temperature-dependent and Ni2+-dependent connectors

Abstract

An asymmetric (‘hybrid’) triblock polypeptide TR₄H with two different, orthogonally self-assembling end blocks has been constructed by conjugating a long (37 kDa) random coil block (R₄) with a triple helix former T = (Pro-Gly-Pro)₉ at the N terminus, and a histidine hexamer (‘Histag’, H) at the C terminus. This molecule can form trimers at room temperature by assembly of the T blocks, which can in turn assemble upon addition of Ni²⁺, by association of Ni complexes involving the H block. This results in reversible hydrogels with dual responsiveness. We have studied mechanical properties of these gels, and compared them to gels formed by the symmetric triblock TR₈T which is equivalent to a dimer of TR₄H, but can only form triple helix-based networks. We find that there is an optimum mole ratio for Ni²⁺ with respect to the polypeptide of about 1; gels are weaker at both lower and higher Ni²⁺ dose. At the optimum dose, the high-frequency storage modulus is in between the value expected for nickel-induced dimerization and trimerization of the H blocks. We also find that the gels relax on time scales of about 50 s, which is two orders of magnitude faster than for TR₈T gels, implying that relaxation is dominated by the dynamics of the Ni²⁺ complex.