Enzymatic and Non-enzymatic Pathways to Formation of Dopa-modified Peg Hydrogels

Introduction Marine mussels are known to secrete unique adhesive proteins that form strong bonds with various substrates in the presence of water.1 These mussel adhesive proteins (MAPs) are secreted as fluids and can undergo rapid curing to form adhesive plaques in a wet environment. The presence of the unusual amino acid 3,4-dihydroxyphenylalanine (DOPA) in MAPs is believed to impart strong adhesive characteristics and cross-link formation.2 Although the mechanisms of these processes are not fully understood, it is believed that the unoxidized catechol form of DOPA is responsible for adhesion while the oxidized o-DOPA quinone is responsible for the cross-linking of the adhesive proteins.2 We are investigating the preparation of conjugates between poly(ethylene glycol) (PEG) and DOPA for potential use as bioadhesives. PEG was chosen because of its nontoxic, non-immunogenic, and nonantigenic properties.3 When immobilized chemically or physically on polymer surfaces, it renders the surfaces highly resistant to biological fouling due to protein and cell adhesion. Conjugation of PEG to biomolecules is known to increase the solubility and molecular weight while having little effect on the chemical and biological properties of the conjugated biomolecules.3 In a previous report,4 we investigated the synthesis and metal ion complexation properties of simple PEG-DOPA constructs. In this study, PEG-DOPA molecules of varying molecular weight and architecture were synthesized using standard peptide coupling chemistry and investigated for their ability to form cross-inked hydrogels under oxidative conditions. Chemical and enzymatic oxidizing reagents such as sodium periodate (NaIO4), horseradish peroxidase/hydrogen peroxide (HRP/H2O2), and mushroom tyrosinase/oxygen (MT/O2) were added to aqueous solutions of PEG-DOPA to induce gelation. Reaction conditions were studied in an effort to understand the cross-linking mechanisms and to develop rapidly hardening adhesive hydrogels. The process of cross-linking was monitored using UV/vis spectroscopy and gel permeation chromatography in concert with multi-angle laser light scattering (GPC-MALLS).