Conformation and electrochemical reduction of myoglobin in PEO oligomers

Horse-heart myoglobin (Mb) was modified with poly(ethylene oxide) (PEO) to solubilize it in PEO oligomers. PEO-modified Mb (PEO–Mb) showed a quasi-reversible electrochemical redox reaction in PEO₂₀₀ (molar mass of 200 g). PEO–Mbs, modified with lower molecular weight of PEO chains, were soluble in PEO oligomers with wider range of molecular weight. A conformation of PEO–Mb was studied with circular dichroism spectroscopy in phosphate buffer solution (PBS) or PEO oligomers. The α-helix content of PEO–Mb, determined by the molar ellipticity at 222 nm, decreased from 71% to about 58% after PEO modification. However, the degree of PEO modification did not affect the α-helix content of PEO–Mbs. On the other hand, the α-helix content of PEO–Mbs was reduced by lowering the molecular weight of the modified PEO chains. Since the α-helix content of PEO–Mb in PBS and that in the PEO oligomers were almost identical, the conformation of PEO–Mb in PBS was considered to be maintained even in PEO oligomers. Although the reduction rate constant of PEO–Mb in PEO oligomers depended on the total molecular weight of the PEO–Mbs, their relation did not obey the Stokes–Einstein equation. The reduction of the PEO–Mb was probably affected by the interfacial electron transfer process at the electrode surface rather than by diffusion in the PEO oligomer.