Introduction
The headgroups of zwitterionic detergents are hydrophilic and contain both positive and negative charges in equal numbers, resulting in zero net charge. Most zwitterionic detergents have linear structure with the hydrophilic tail of alkyl chain and the positive and negative charges are located in the hydrophilic head. Only a few zwitterionic detergents distribute positive and negative charges at both ends of the molecule. Zwitterionic detergents represent an intermediate step between ionic and non-ionic detergents, combining the properties of these two detergent groups. Similar to non-ionic detergents, zwitterionic detergents have a neutral net charge. However, they are often more efficient at breaking protein-protein bonds, similar to ionic detergents, but the extent to which proteins are affected is less harsh that can maintain the native state and charge of the individual proteins.
Fig. 1. The general structure of zwitterionic detergents.
Common Zwitterionic Detergents
The zwitterionic headgroups such as N-oxides, phosphorylcholines, sulfobetaines, and carboxybetaines appear commonly in zwitterionic detergent structures. The common zwitterionic detergents include laurydimethylamine-N-oxide (LDAO), Fos-choline-12 (FC-12), 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS), and 3-[(3-cholamidopropyl) dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO).
- LDAO: LDAO, also known as dodecyldimethylamine oxide, has a critical micelle concentration of 1-2 mM. LDAO has been used to solubilize and extract colicin A from the plasma membrane and for inactivation of enveloped viruses. It is also suitable to use in crystallization of membrane proteins and to enhance the detection of high molecular weight proteins.
- FC-12: FC-12 belongs to the phosphocholine detergents family. The detergents of this family employ charged amine and phosphate groups in combination with an alkyl chain to produce a zwitterionic surfactant. This unique architecture is water-soluble and capable of both stabilizing and keeping membrane proteins soluble in aqueous solutions, and also produces micelles and is able to extract membrane proteins from cellular membranes. Thus, FC-12 is well suited for the solubilization, stabilization, and purification of membrane.
- CHAPS: CHAPS is sulfobetaine derivative of cholic acid, which combines the useful properties of both sulfobetaine-type and the bile salt detergents. CHAPS is a non-denaturing zwitterionic detergent for solubilizing membrane proteins and breaking protein-protein interactions. It is easily removed from solution by dialysis because it has a high critical micelle concentration, which is very important for the study of membrane proteins.
- CHAPSO: 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.
These detergents are popularly used for membrane proteins research. Among them, LDAO is the most successful in terms of high-resolution membrane protein structure determination [1].
Fig. 2. The chemical structures of common zwitterionic detergents.
Applications
The versatility of zwitterionic detergents is useful for a variety of purposes, such as solubilization of organelles and inclusion bodies and rapid isolation of high multimeric plasmid DNA. However, based on the technologies of chromatography, different types of electrophoresis (including 2D gel electrophoresis), mass spectrometry and NMR, the application of zwitterionic detergents in the study of membrane proteins has received the widest attention. It is worth mentioning that most successful NMR-based structural studies of membrane proteins have been carried out in zwitterionic detergent solutions. Below are some examples.
- Babu et al. used the 1,2-diheptanoyl-sn-glycero-3-phosphatdiyl choline (DHPC) as a detergent to solubilize sarco(endo)plasmic reticulum (SR)-enriched microsomal membrane proteins isolated from rat heart for two-dimensional electrophoresis (2DE). The results resented here demonstrate that DHPC increases the solubility of proteins, especially integral membrane proteins in 2DE, compared to other detergents. The use of DHPC thus appears to increase the analytical power of 2DE with respect to difficult membrane proteins [2].
- Bednarz-Misa et al. used Zwittergent Z 3-14® as a detergent to isolate the outer membrane proteins (OMPs) from Klebsiella pneumoniae cells and resolve them using 2DE. As a result, they identified 134 protein spots. The OMPs identified in this study are possible candidates for the development of a protein-based vaccine against K. pneumoniae infections [3].
Fig. 3. Gel profiles of outer membrane proteins of Klebsiella pneumoniae that extracted from bacteria with Zwittergent Z 3-14® (134 protein spots were identified).
- Henningsen et al. showed that as an effectively tool in 2DE and mass spectrometry, the two zwitterionic detergents of 4-octylbenzol amidosulfobetaine and CHAPS are particularly effective at solubilizing challenging classes of membrane-embedded proteins (an ion channel and a G-protein coupled receptor (GPCR)). The two zwitterionic detergents open up many possibilities for identifying the post-translational modifications and accessory proteins of membrane proteins isolated from a variety of primary tissues or cell lines or under different activation conditions [4].
What Can We Do?
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References
- Sadaf A., et al. Amphipathic agents for membrane protein study[M]. Methods in enzymology. Academic Press, 2015, 557: 57-94.
- Babu G. J., et al. Solubilization of membrane proteins for two-dimensional gel electrophoresis: identification of sarcoplasmic reticulum membrane proteins[J]. Analytical biochemistry, 2004, 325(1): 121-125.
- Bednarz-Misa I., et al. Application of zwitterionic detergent to the solubilization of Klebsiella pneumoniae outer membrane proteins for two-dimensional gel electrophoresis[J]. Journal of microbiological methods, 2014, 107: 74-79.
- Henningsen R., et al. Application of zwitterionic detergents to the solubilization of integral membrane proteins for two‐dimensional gel electrophoresis and mass spectrometry[J]. Proteomics, 2002, 2(11): 1479-1488.
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