Molecular motion of a water-soluble nitroxyl radical in gelatin gels

Electron spin resonance spectroscopy (ESR) has been used to measure the rotational correlation times and translational diffusion constants of a radical spin probe in gelatin gels. The radical used is NaTMIOS, the sodium salt of sulphonated 1,1,3,3-tetramethylisoindolin-2-yloxyl. It was found that the mobility of the radical was influenced by the gelatin concentration and by temperature. It was concluded that the spin probe was constrained by the polymer chains and, when conditions were such that the distance between the chains was of the order of the size of the probe, rotational motion was severely impeded.