Applications of electron spin resonance spectroscopy to microrheology studies of membranes and cells

One of the most powerful tools for the study of the molecular dynamics and molecular organization of biological membranes is the spin label method which uses stable free radicals (nitroxides) studied by electron spin resonance spectroscopy. The spectra are very dependent on the mobility and orientation of these probes in a magnetic field. They give information on the tumbling rate of molecules in liquids, the membrane microviscosity, the order parameter of the phospholipid fatty acid chains, the activation energy of their rotation and their flexibility. The method has interesting applications in the rheological domain; for example it enables measurement of the internal viscosity of erythrocytes and red blood cell orientation in a Poiseulle flow.