17] Determining intracellular viscosity from the rotational motion of spin labels

Abstract

Spin labels have been useful in studies of the intracellular environment. This chapter discusses the use of rotational motion of spin labels to measure the viscosity of internal aqueous space. It is important to verify that the spin label is actually in the internal aqueous space and not in the cell membrane. Spin labels are useful in the studies of the intracellular environment because of their ability to diffuse into the intracellular space. In a mixture of cells, spin labels, and broadening agent, the resultant electron paramagnetic resonance (EPR) signal arises only from the spin labels that are not in close contact with broadening agent; this is usually the intracellular aqueous space. This signal is used to calculate the relative motion of the spin label inside the cell and can be compared to the motion of the spin label in a solvent of known viscosity, usually water, to obtain a measure of intracellular viscosity. The spin label is the probe used to obtain information about viscosity. Intracellular viscosity of red blood cells and mouse TB cells are determined in the chapter.