Synthesis and gelation of DOPA-modified poly(ethylene glycol) hydrogels

3,4-Dihydroxyphenylalanine (DOPA) residues are known for their ability to impart adhesive and curing properties to mussel adhesive proteins. In this paper, we report the preparation of linear and branched DOPA-modified poly(ethylene glycol)s (PEG-DOPAs) containing one to four DOPA endgroups. Gel permeation chromatography-multiple-angle laser light scattering analysis of methoxy-PEG-DOPA in the presence of oxidizing reagents (sodium periodate, horseradish peroxidase, and mushroom tyrosinase) revealed the formation of oligomers of methoxy-PEG-DOPA, presumably resulting from oxidative polymerization of DOPA endgroups. In the case of PEG-DOPAs containing two or more DOPA endgroups, oxidative polymerization resulted in polymer network formation and rapid gelation. The amount of time required for gelation of aqueous PEG-DOPA solutions was found to be as little as 1 min and was dependent on the polymer architecture as well as the type and concentration of oxidizing reagent used. Analysis of reaction mixtures by UV-vis spectroscopy allowed the identification of reaction intermediates and the elucidation of reaction pathways. On the basis of the observed reaction intermediates, oxidation of the catechol side chain of DOPA resulted in the formation of highly reactive DOPA-quinone, which further reacted to form cross-linked products via one of several pathways, depending on the presence or absence of N-terminal protecting groups on the PEG-DOPA. N-Boc protected PEG-DOPA cross-linked via phenol coupling and quinone methide tanning pathways, whereas PEG-DOPA containing a free amino group cross-linked via a pathway that resembled melanogenesis. Similar differences were observed for the rate of gel formation as well as the molecular weight between cross-links ((-)M(c)), calculated using equilibrium swelling and the Flory-Rehner equation.