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Entropy-Driven Supramolecular Ring-Opening Polymerization of a Cyclic Hemoglobin Monomer for Constructing a Hemoglobin–PEG Alternating Polymer with Structural Regularity

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journal contribution
posted on 15.04.2021, 18:03 by Takashi Matsuhira, Hiromi Sakai
Our earlier report described that a cyclic hemoglobin (Hb) monomer with two β subunits of a Hb molecule (α2β2) bound through a flexible polyethylene glycol (PEG) chain undergoes reversible supramolecular ring-opening polymerization (S-ROP) to produce a supramolecular Hb polymer with a Hb–PEG alternating structure. In this work, we polymerized cyclic Hb monomers with different ring sizes (2, 5, 10, or 20 kDa PEG) to evaluate the thermodynamics of S-ROP equilibrium. Quantification of the produced supramolecular Hb polymers and the remaining cyclic Hb monomers in the equilibrium state revealed a negligibly small enthalpy change in S-ROP (ΔHp ≤ 1 kJ·mol–1) and a markedly positive entropy change increasing with the ring size (ΔSp = 26.8–33.2 J·mol–1·K–1). The results suggest an entropy-driven mechanism in S-ROP: a cyclic Hb monomer with the larger ring size prefers to form a supramolecular Hb polymer. The S-ROP used for this study has the potential to construct submicrometer-sized Hb–PEG alternating polymers having structural regularity.