29] Critical micellar concentrations of detergents

Abstract

Publisher Summary This chapter describes the critical micellar concentrations (CMC) of detergents. Membrane-spanning proteins are amphiphilic macromolecules that are incorporated vectorially in the lipid membrane in a quasi-solid state. On disruption of the membrane and removal of the lipids, such proteins tend to aggregate unspecifically and precipitate in bulk, thus avoiding an entropically unfavorable exposure of their hydrophobic parts to water. The easiest and least disruptive method to achieve this is the replacement of membrane lipids by detergents that do not form membrane-like structures. Solubilized membrane proteins form mixed micelles with the detergent. Under appropriate conditions, their hydrophobic surface parts are covered by a (curved) monolayer film of the detergent. Because this film only forms at an adequate detergent concentration, the final solution after membrane solubilization contains mixed protein–detergent micelles, mixed lipid–detergent micelles, pure detergent micelles, and detergent monomers. However, just below or at the CMC, mixed micelles may contain more than one protein molecule in the same complex. To avoid this situation of incomplete and evasive solubilization, experiments should be performed at a detergent concentration at which the free-to-mixed micelle ratio is at least 10.