Detergents are amphipathic molecules that contain both polar and hydrophobic regions, and structurally diverse. Detergents belong to a class of compounds called surfactants, which can reduce interfacial surface tension in mixtures (i.e., oil and water) by adsorbing to interfaces. The structural properties allow detergents to accumulate in aqueous media. The polar region can form hydrogen bonds with water while the hydrophobic region tends to aggregate into micelles and associates with hydrocarbon and nonpolar domains. The simplest classification of detergents is based on the four basic types of headgroups, namely anionic, cationic, non-ionic and zwitterionic detergents.
Fig. 1. The schematic representation of (A) detergent monomer and (B) detergent micelles.
Main Physical Parameters
Several physical parameters of detergents deserve attention, which can provide important information for the selection of appropriate detergents in relevant scientific research.
- Hydrophilic-lipophilic balance (HLB): The total effect of hydrophilic head group and hydrophobic tail on the properties of the detergent molecule is known as the HLB. The HLB is defined by a number that ranges from 0 to 40. In general, an HLB number<10 indicates that a detergent has low solubility in water while an HLB number between 10 and 20 indicates that the detergent is readily soluble in water [1].
- Critical micelle concentration (CMC): Micellization is a critical phenomenon when considering detergent applications. The minimal detergent concentration at which micelles are observed at a given temperature is called the CMC. At any concentrations lower than the CMC, only detergent monomers are observed; at concentrations higher than CMC both detergent micelles and detergent monomers co-exist, along with other non-micellar phases that are not dissolved in water.
- Aggregation numbers: Another physical property of the micelle is the aggregation numbers. Aggregation numbers refer to the number of detergent monomers present within a micelle. Most detergents used for biochemical applications have aggregation numbers that range from 50 to 100. Detergents with smaller aggregation numbers tend to form more spherical micelles while detergents with larger aggregation numbers tend to form ellipsoid micelles. In general, aggregation numbers increase as the length of the hydrocarbon chains increase [2].
- Cloud point: Cloud point refers to the temperature at which the detergents solution is separated into two phases when the concentration is close to or higher than CMC. At this temperature, detergent micelles begin to aggregate and become insoluble in water, and the detergent solution becomes turbid.
Applications
Detergents are useful in a wide variety of applications. Undoubtedly, the most extensive and well-known application of detergents is in the study of membrane proteins.
- Main applications: The use of detergents for membrane proteins study starts with protein extraction from the native membranes. Next, the target membrane protein solubilized by detergent is isolated via several purification steps where detergent molecules play an essential role in maintaining protein solubility and stability. Then, the membrane protein purified in detergent micelles is directly used for membrane proteins crystallization using the vapor-diffusion method (i.e., in surfo method) or lipidic cubic phase (LCP)-based crystallization method (i.e., in meso method). Thus, detergents are indispensable elements for membrane proteins manipulation including membrane proteins extraction, solubilization, stabilization, purification, and crystallization. It is noteworthy that the optimal detergent to use varies depending on the step of the process. For example, a promising detergent for membrane proteins solubilization is not necessarily suitable for membrane proteins stabilization. A generally accepted detergent order for membrane proteins solubilization efficiency would be ionic > zwitterionic > nonionic detergents. However, this order is typically reversed when considering membrane proteins stabilization efficacy [3].
- Other applications: Other applications of detergents include polyacrylamide gel electrophoresis (PAGE), cell lysis, inclusion body solubilization, lipid raft preparation, DNA/RNA separation. Detergents are also useful as model membranes for in vitro studies and as vehicles for protein/DNA/drug delivery.
Fig. 2. The application of detergents in the purification process of the membrane proteins.
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References
- Kruglyakov P. M. Hydrophile-lipophile balance of surfactants and solid particles: physicochemical aspects and applications[M]. Elsevier, 2000.
- le Maire M., et al. Interaction of membrane proteins and lipids with solubilizing detergents[J]. Biochimica et Biophysica Acta (BBA)-Biomembranes, 2000, 1508(1-2): 86-111.
- Sadaf A., et al. Amphipathic agents for membrane protein study[M]. Methods in enzymology. Academic Press, 2015, 557: 57-94.
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