This chapter describes the motions of membrane molecules over time scales of 10-12 s to a few seconds and over distance scales from 10-9 to 10-4 cm. The smaller-scale motions are the most readily understood. Flexing of acyl chains, local disorder of membrane lipids, allows the conformational changes in membrane proteins required for their functioning. Motions on this scale are probed directly in magnetic resonance experiments and indirectly when enzyme or receptor activity is changed by changes in membrane lipid composition and apparent viscosity. Membrane lipid viscosity appears to affect movements of membrane proteins on distance scales from 10 nm (a few molecular diameters) to 1000 nm or so. Rotational diffusion of membrane proteins is in fair agreement with expectations from the size of the proteins and the local viscosity of membrane lipids. In contrast, diffusion of membrane proteins laterally over long distances is often much slower than anticipated from lipid viscosity.
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