ABSTRACT
It is common to talk about biological membranes in terms of their “fluidity”. When pressed, each specialist can give you a definition of what he means, but these definitions vary widely from person to person. One might consider the relative ease of effecting transport across the membrane. Another might think in terms of the temperature of the gel to liquid crystal transition of the membrane lipids. Still others might prefer to define fluidity with respect to the ease of lateral diffusion. Physical chemists would probably talk about molecular mobility, whereas specialists in liquid crystals would prefer molecular ordering. Who is right? Properly defined, fluidity is the reciprocal of viscosity. However, both concepts apply only to isotropic liquids, which membrane lipids are not. We must find a way to describe the properties of molecules which are highly organized, most often in lamellar arrays, and can also undergo both fast and slow motions. One way is to deal with each of these aspects, order and mobility, separately. A highly ordered, very immobile system can certainly be considered not to be “fluid”. Similarly, a highly disordered, very dynamic system can be considered as fluid. However, what do we call a system that is disordered and immobile? Clearly a simple single adjective does not suffice. We must measure and consider each aspect separately. One of the best ways to do this, with a minimum of perturbation and relatively few approximations in analysis, is nuclear magnetic resonance (NMR).
