Zwitterionic surfactant belongs to surfactants. Common surfactants are categorized into three groups based on their characteristics: ionic (anionic or cationic), non-ionic and zwitterionic.
Zwitterionic Surfactant Definition
Definition: Zwitterionic surfactants are compounds that decrease the surface tension between two liquids or liquids and solids by solubilizing hydrophobic molecules. These water-soluble surface-active agents are comprised of a hydrophobic portion, usually, a long alkyl chain, attached to hydrophilic or water solubility-enhancing functional groups. Detergents are commonly used in biochemistry, cell biology, and molecular biology for cell lysis, membrane protein and lipid purification, protein crystallization, and reduction of background staining in blotting experiments.
Zwitterionic Detergent Surfactant
Zwitterionic detergent surfactants can be anionic (negatively charged), cationic (positively charged), or non-ionic (no charge). That depends on the acidity of the solution and the existence of other components. Phospholipids are common zwitterionic detergent surfactants that have Generally Regarded As Safe (GRAS) status, which permits their use in food. However, some phospholipids, such as lecithin, are too lipophilic and have a high critical packing parameter (described later), making their incorporation into MEs difficult. If used, co-surfactants and short-chain alcohols are required in high concentrations to increase the fluidity of the interface.
Lauryldimethylamine oxide and myristamine oxide are two commonly used zwitterionic surfactants of the tertiary amine oxides structural type.
Zwitterionic Surfactant Detergent
The headgroups of zwitterionic surfactant detergent are hydrophilic and contain both positive and negative charges in equal numbers, resulting in zero net charges. They are more harsh surfactants than the non-ionic detergents. A typical zwitterionic surfactant detergent is 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, better known as CHAPS. CHAPS high CMC (6 mM at room temperature) allows efficient removal by dialysis. It is very common in sample preparation at concentrations of 2-4% for isoelectric focusing and 2D electrophoresis. CHAPSO differs with CHAPS in that it contains a more polar headgroup, which makes it more capable of solubilizing hydrophobic molecules. Thus, CHAPSO is mainly used for solubilization of integral membrane proteins.
zwitterionic-type surfactants have both cationic and anionic centers attached to the same molecule. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. The anionic part can be more variable and include sulfonates, as in the sustained CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate) and Cocamidopropyl hydroxysultaine. Betaines such as Cocamidopropyl betaine have a carboxylate with ammonium. The most common biological zwitterionic-type surfactants have a phosphate anion with an amine or ammonium, such as the phospholipids phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelins.
Zwitterionic Surfactant Buffer
Addition of zwitterionic surfactants to an electrophoretic buffer suppress the electroosmotic flow by 50−90%. The onset of suppression occurs at approximately the critical micelle concentration of the surfactant.
The zwitterionic surfactant forms a dynamic wall coating that prevents the adsorption of cationic proteins as well as suppresses the electroosmotic flow (EOF). Addition of polarizable anions to buffers containing this zwitterionic surfactant increases the once suppressed EOF to values nearing +3 × 10-4 cm2/(V s). The retention of the EOF allows for the separation of analytes of widely different mobilities and is demonstrated by the simultaneous separation of cationic and anionic proteins.
High-efficiency separations of some model proteins can be obtained, when a mixture of a cationic and a zwitterionic fluorosurfactant is added to the running buffer solution. By changing concentration proportions between the surfactants, a change in separation selectivity is obtained. This procedure provides an alternative way for selectivity tuning in protein separations by capillary electrophoresis.
A study has been made of the performance of zwitterionic surfactant when used as buffer additives in capillary electrophoresis. Thus, it showed to be possible to change the direction of the electroosmotic flow by changing the pH of the buffer solution. Possible mechanisms for the behaviour of the electroosmotic flow at different pH and when using different surfactant combinations are suggested.
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