Synthesis of polyarbutin by oxidative polymerization using PEGylated hematin as a biomimetic catalyst

INTRODUCTION Enzymatic polymerization has been extensively developed over the past two decades.1–7 In particular, horseradish peroxidase (HRP) has been used for enzymatic oxidative polymerization of phenolic compounds. Dordick and co-workers8,9 reported HRP-catalyzed polymerization of arbutin, a glucose-bearing phenol derivative. Water-soluble polyarbutins with molecular weight (Mn) in the range of 1600–3200 were synthesized. Acidic deglycosylation resulted in polyhydroquinone, which is a redox-active polymer useful for sensing materials. Nakano et al.10 also reported HRP-catalyzed polymerization of arbutin and its derivatives, such as glucosyl, galactosyl and mannosyl arbutins. The resulting polymers exhibited high adsorbability to lectins. Many of these polymers, however, are of low molecular weight, restricting their use as thin films and other forms of practical interest. Peroxidases are hemoenzymes having ferric complexes of protoporphyrin IX bound to them by a coordinate bond to the imidazole ring of a histidine residue.11 Hematin, a hydroxylated protoporphyrin IX complex of ferric ion (Fe3+), has been examined as a biomimetic catalyst substituting HRP.12–15 Hematin, however, has a low solubility in both organic and aqueous solvents. To improve the solubility, Kumar and co-workers16,17 prepared polyethylene glycol-containing hematin (PEGylated hematin) using N,N ¢-carbonyldiimidazole/1,8diazabicyclo[5.4.0]undec-7-ene as a condensing agent. PEGylated hematin was shown to catalyze the radical polymerization of sodium styrene sulfonate and the oxidative polymerization of aniline. However, neither the characterization of PEGylated hematin nor the details of the polymerization were reported. In the present study, we prepared PEGylated hematin by the Mitsunobu reaction using hematin and polyethylene glycol (PEG) monomethyl ether and examined it as a catalyst for polymerization of arbutin in a buffer solution of pH 7.0 (Scheme 1). PEGylated hematin was characterized by 1H nuclear magnetic resonance (NMR), and the polymerization process was examined by gel permeation chromatography (GPC). The polyarbutin obtained was characterized by 1H NMR and infrared spectroscopy. We compared PEGylated hematin, hematin and HRP as a catalyst for arbutin polymerization.