Hemoglobin and Red Blood Cells Catalyze Atom Transfer Radical Polymerization

Hb is a promiscuous protein that not only transports oxygen, but also catalyzes several biotransformations. A novel in vitro catalytic activity of Hb is described. Bovine Hb and human erythrocytes were found to display ATRPase activity, i.e., they catalyzed the polymn. of vinyl monomers under conditions typical for atom transfer radical polymn. (ATRP). N-isopropylacrylamide (NIPAAm), poly(ethylene glycol) Me ether acrylate (PEGA), and poly(ethylene glycol) Me ether methacrylate (PEGMA) were polymd. using organobromine initiators and the reducing agent ascorbic acid in acidic aq. soln. In order to avoid chain transfer from polymer radicals to Hb's cysteine residues, the accessible cysteines were blocked by a reaction with a maleimide. The formation of polymers with bromine chain ends, relatively low polydispersity indexes (PDI), first order kinetics and an increase in the mol. wt. of poly(PEGA) and poly(PEGMA) upon conversion indicate that control of the polymn. by Hb occurred via reversible atom transfer between the protein and the growing polymer chain. For poly(PEGA) and poly(PEGMA), the reactions proceeded with a good to moderate degree of control. Sodium dodecyl sulfate (SDS) gel electrophoresis, CD spectroscopy, and time-resolved UV-visible (UV-vis) spectroscopy revealed that the protein was stable during polymn., and only underwent minor conformational changes. As Hb and erythrocytes are readily available, environmentally friendly, and nontoxic, their ATRPase activity is a useful tool for synthetic polymer chem. Moreover, this novel activity enhances the understanding of Hb's redox chem. in the presence of organobromine compds. [on SciFinder(R)]