Capillary Differential Scanning Calorimeter and Isothermal Titration Calorimeter are instruments generally used in structural biology to charcterize the purified protein samples thermodynamically.
Biological phenomena in general involve molecular recognition on an intramolecular or intermolecular level. Intramolecular processes involve ligand binding and interaction between different types of macormolecules, as the interaction between proteins or protein and DNA or RNA. Protein folding is an example for intermolecular recognition. Understanding the interplay between structure and the energetics of their stability and the binding of other macromolecules or small compounds. Parameters describing the stability of such a system are the equilibrium constant of the free energy, as a sum of enthalpy and entropy. These parameters are temperature dependent through the heat capacity change.
Modern calorimeters allow to directly measure the enthalpy and the heat capacity change, with high precision and a limited amount of sample material. The instrument measures the heat flux between the sample and reference chamber as a result of chemical reactions or physical changes. At constant pressure or volume these changes can be directly correlated to changes in enthalpy and heat capacity. Recorded thermogramms yield direct information about the stability of the protein sample and the effect of added compounds as ligands. In case of membrane proteins it is suitable to identify bound detergents.
Two basic calorimetric concepts are used in structural biology: Differential Scanning Calorimetry (DSC) and Isothermal Titration Calorimetry (ITC). In DSC during constant heat increase of the sample temperature changes are monitored, whereas in ITC the power necessary in order to maintain an equal temperature between the sample and a reference cell is observed upon e.g. addition of a ligand.
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