Ring-Opening Polymerization of Hemoglobin

Hemoglobin (Hb), an oxygen-carrying protein, has an α₂β₂ tetrameric structure that dissociates reversibly into two αβ dimers (α₂β₂ ⇄ 2αβ). We synthesized a cyclic Hb-ring monomer with two β subunits bound through a 10 kDa polyethylene glycol (PEG) chain. The monomer induced ring-opening polymerization to produce a supramolecular polymer via intersubunit interaction of αβ dimers of an Hb molecule at the PEG terminals. Both the ring-closed monomer and the ring-opened supramolecular polymer were then fixed covalently by intramolecular cross-linking of two β subunits. Quantification of fixed products at various monomer concentrations revealed the equilibrium constant ( K), a ratio of propagation and depropagation rate constants, as 5.68 mM^-1. The average degree of polymerization ([Formula: see text]) increased proportionally, concomitantly with the initial monomer concentration. Hb polymer with [Formula: see text] = 13.2 ( M_n = ca. 1 MDa) was obtained by cross-linking at 2.33 mM. Our novel strategy of ring-opening polymerization of Hb will eventually realize a highly aligned and efficiently polymerized Hb for creating artificial oxygen carriers for a clinical use.