Detergents - more than foam!

Detergents are widely used in biochemistry, cell biology or molecular biology. Cell lysis, protein solubilization, protein crystallization or reduction of background staining in blotting experiments are just a few of numerous applications. The goal of this brochure is to summarize the important facts from the literature, which may help the reader to select a detergent for his special application. Detergents can be classified for instance according to their chemical structure stating their constituent polar and nonpolar group (glucosides, alkyl ionic detergents, polyoxyethylene alcohols, bile salts, sulphonates etc.), the charge character (anionic, cationic, zwitterionic = amphoteric and non-ionic) or simply whether they are mild or strong in terms of their ability to solubilize and / or to denature proteins. They all have in common that they are soluble amphipathic (amphiphilic) compounds, with both lipophilic (hydrophobic, nonpolar) and lipophobic (hydrophilic, polar) sites within one molecule. Therefore, they will form stable aggregates (micelles) above a critical concentration, called CMC value (Critical Micellar Concentration). This value is specific to each detergent and different factors, like temperature, chemical structure, salt concentration and pH value may influence it.

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Choosing the best detergent for a special application is not trivial
. In many cases, a set of detergents has to be tested to select the one with the best properties. If the goal of the isolation of a protein is to preserve its structural and functional state, one should consider the temperature, the pH and the ionic strength of the system or interference with assays. Several other parameters may influence the choice too. Assuming that the factors mentioned above are dictated by the protein and are held constant, the factors are left to be optimised are detergent head group, detergent tail group, and detergent concentration (Neugebauer 1990, Tanford & Reynolds 1976).

In short, the criteria for selecting a detergent are:
Solubility: Many detergents have limited water-solubility at elevated temperatures. On warming a solution, separation into an aqueous phase containing little detergent, and a heavier, detergent-rich phase will occur.
Denaturing or inactivation of proteins: If the native biological structure and activity of a protein has to be preserved, mild detergents have to be applied. Most of the strong (effective) detergents, such as SDS or CTAB, will denature the protein, often irreversibly.
Removal from samples: When detergents have to be removed from a sample, dialysis is the method of choice.
Interference with protein activities or assays: The chemical nature should be considered e. g. when used in enzyme activity assays, protein determination etc. They should not serve as an enzyme substrate. Absorption of the detergent in the UV region (280 nm) will interfere with protein determination. Interference with colorimetric assays is unwanted too. Charged detergents are less used in chromatography.
Efficiency of extracting the protein: In most cases, the detergent will be selected according to its solubilization capability of the desired protein. Several considerations will influence the choice, such as keeping the detergent concentration as low as possible. Keep in mind that not all detergents will separate subunits of proteins. This may lead to wrong estimations of the molecular weight.
CMC value and micelle size: Detergents are quite small molecules, but form micelles which may be of similar size to proteins. The micelle size increases and the CMC decreases with increasing size of the lipophilic part of the detergent and, to a lesser extent, with decreasing size and polarity of the polar groups (Helenius et al. 1979). Detergents with a high CMC form smaller micelles. Unfortunately, they have the disadvantage of being less efficient as detergents.