On the Mobility of Spin-Labeled Hemin Coordinated to Polymer

Abstract

The molecular motion of nitroxide radical covalently attached to hemin (ferric protoporphyrin IX chloride) has been studied by spin-labeling. The addition of polymeric ligand, a copolymer of styrene and 4-vinyl pyridine, suppresses the mobility of the spin-labeled hemin more strongly than the corresponding monomeric ligands. The correlation time of the rotational motion of the nitroxide measured for the polymer-ligand-spin-labeled hemin complex shows a transition from a high to a low value at a certain temperature. The temperature became higher with increasing polymer concentration. This is not caused by the increased bulk viscosity of the polymer solution but by the immobilization of the spin-labeled hemin by the polymer; an inherent low mobility of the polymer and/or polymer side chain is transmitted to the nitroxide. For the same reason, the polymer-ligand lowers the mobility of nitric oxide coordinated to Fe ion in the hemin in trans position of the ligand.