Hemoglobin and Red Blood Cells Catalyze Atom Transfer Radical Polymerization

Silva, Tilana B. and Spulber, Mariana and Kocik, Marzena K. and Seidi, Farzad and Charan, Himanshu and Rother, Martin and Sigg, Severin J. and Renggli, Kasper and Kali, Gergely and Bruns, Nico (2013) Hemoglobin and Red Blood Cells Catalyze Atom Transfer Radical Polymerization. BIOMACROMOLECULES, 14 (8). pp. 2703-2712. ISSN 1525-7797

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Abstract

Hemoglobin (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 polymerization of vinyl monomers under conditions typical for atom transfer radical polymerization (ATRP). N-isopropylacrylamide (NIPAAm), poly(ethylene glycol) methyl ether acrylate (PEGA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) were polymerized using organobromine initiators and the reducing agent ascorbic acid in acidic aqueous solution. 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, relative low polydispersity indices (PDI), first order kinetics and an increase in the molecular weight of poly(PEGA) and poly(PEGMA) upon conversion indicate that control of the polymerization 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. SDS gel electrophoresis, circular dichroism spectroscopy and time-resolved UV-Vis spectroscopy revealed that the protein was stable during polymerization, and only underwent minor conformational changes. As Hb and erythrocytes are readily available, environmentally friendly and non-toxic, their ATRPase activity is a useful tool for synthetic polymer chemistry. Moreover, this novel activity enhances the understanding of Hb’s redox chemistry in the presence of organobromine compounds.

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