Fields & Applications Sample Preparation, Technology, Data Analysis, Pharma & Biopharma, Environmental, Forensics, Materials

Analysis of demicellization data from isothermal titration calorimetry

Learn why isothermal titration calorimetry is a method of choice for accurate and precise determination of critical micellar concentration of surfactants for biochemical applications and find out how to analyse demicellization data from Isothermal Titration Calorimetry

12/11/2015

Many industrial and biochemical applications depend on the usage of detergents and other surfactants. These compounds are indispensable, in particular, for the extraction, purification, and handling of membrane proteins. Owing to their amphiphilic nature, detergents can provide a membrane-mimetic environment required by integral membrane proteins to retain their native structures and functions in aqueous solution. Oftentimes, different detergents have to be screened to identify one that is suitable for the solubilization, stabilization, reconstitution, and biophysical, biochemical, or structural scrutiny of the protein of interest [1]. In each case, the membrane protein of interest dictates the suitability of a detergent, the choice of which depends on physicochemical properties such as chain length, headgroup size, and headgroup charge or polarity. However, a detergent found to be optimal for the isolation of a membrane protein need not necessarily be the best choice for downstream purification and in vitro studies. Harsh detergents such as SDS might inactivate the protein; some detergents such as Triton-X 100 absorb in the UV range and thus interfere with spectroscopic analysis; ionic detergents are unsuitable for isoelectric focusing or ion-exchange chromatography; and some detergents pose difficulties during membrane-protein reconstitution, that is, the reincorporation of the solubilized membrane protein into a lipid bilayer. Dialysis is a popular method for detergent removal to accomplish reconstitution, but long dialysis durations are required to remove detergents that have a low critical micellar concentration (CMC).

The CMC is one of the most fundamental characteristics of a surfactant. In aqueous solutions, surfactants form colloidal aggregates, so-called micelles, when their concentration exceeds the CMC. As abrupt changes of various physical properties are observable at or around this detergent concentration [2], its experimental determination can be realized with a variety of methods, including surface tension and conductivity measurements, dye binding experiments, and nuclear magnetic resonance spectroscopy [3]. To obtain the CMC in an accurate and precise manner, however, isothermal titration calorimetry (ITC) is the method of choice [4], which is due to its extraordinary sensitivity, unsurpassed resolution, and high reproducibility [5]. Moreover, ITC does not require sample labeling and provides a complete thermodynamic profile for a single experiment including the molar enthalpy and entropy changes accompanying micellization.

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