Cationic surfactants, protein solubilization

Proteins solubilized in aqueous solution interact more or less with hydrophilic groups of surfactants at the oil-water interface. Therefore, the type of hydrophilic group is strongly influenced by the protein extraction efficiency. Anionic and cationic surfactants interact with charged protein surfaces more strongly than non-ionic surfactants. This feature also means that the non-ionic surfactants are favourable for protein stabilization in water droplets because of the not-so-hard interaction between the protein and the surfactant. In protein extraction, such an electrostatic interaction between proteins and surfactants is the main driving force in protein transfer. 

An increase of the relative amount of protein in the organic phase versus pH would be expected to take place when a cationic surfactant is used. Hatton (2) reported results on solubilization of catalase using a cationic surfactant, dodecyltrimethylammonium bromide (DTAB) in n-octane, with hexanol as cosurfactant. At pH values below pi = 5.3 no solubilization occurred, while there was a significant transfer for pH above pi. 

Other results of extraction of proteins using different trialkylammonium salts differ somewhat from this tendency. Jolivalt (36) studied the solubilization of a-chymotrypsin in the Aliquat 336 system using isotridecanol as a cosurfactant. Alpha-chymotrypsin was most significantly extracted for pH above its isoelectric point but as the pH decreased below the pi, the extraction yield decreased very slowly and became negligible at 4 pH units below the pi. This means that a-chymotrypsin can be extracted in significant proportion by a cationic surfactant even when positively charged overall. 

From an analytical chemistry point of view, one important property of micelles is their ability to dissolve different kinds of compounds. Compounds with low solubility in water, or even insoluble, can be dissolved and dispersed by the micelles in aqueous solutions. In analytical chemistry, main applications of cationic surfactants are linked to their solubilization power [1]. In fact, the first applications were mainly focused on the use of micelles to solubilize proteins. The solubilization power of surfactants is obviously observed when working with surfactant concentrations higher than the critical micelle concentration (CMC) value. Micellar media make possible not only to change the solubility of several analytes but also to change their microenvironment and thus to control several physicochemical phenomena. 

Recent interest focuses on the determination of protein molecular weight by electrophoresis with SDS, on the separation and purification of proteins from living tissue by solubilization with surfactants, and on solution behavior of proteins. " For convenience, most of these studies use anionic surfactants, for which abundant reference data are available, and serum albumins, which bind readily with a variety of species. The serum albumins function as carrier proteins for nutrients, metabolites, and drugs in the bloodstream. Only a few investigations have employed cationic surfactants. ... 

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