An ESR study of spin‐labeled silk fibroin membranes and spin‐labeled glucose oxidase immobilized in silk fibroin membranes

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Abstract
This article describes the characteristics of silk fibroin membranes and glucose oxidase, immobilized in membranes as determined by a variety of physical methods, mainly the spin‐label electron spin resonance (ESR) method. The properties of membranes insolubilized by different methods, i. e., immersion in 80% methanol aqueous solution, uniaxially drawing by placing on a stretcher, and hydration by placing in a desiccator of 96% relative humidity (RH) for 17 h, are compared. The results are also analyzed in relation to ESR spectra of spin‐labeled immobilized glucose oxidase and 4‐hydroxy‐2,2,6,6‐tetramethyl‐1‐piperidinyloxy as a model of the substrate. It is concluded that the heterogeneous structures of the swollen membranes in water differ locally among membranes insolubilized by different methods, but the immobilized state of the enzyme in such membranes is mostly similar. This is correlated to the fact that the thermal or pH stabilities are essentially same among glucose‐oxidase‐immobilized silk fibroin membranes insolubilized by different methods.