Entropy Change Associated with the of Folding and Unfolding of Barnase Project

Entropy Change Associated with the of Folding and Unfolding of Barnase Project

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Thermo Project Fall 2013 ChBE 2130 - B Yoel Cortes-Pena Siri Kore Weiyi Huang
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Differential Scanning Calorimetry Calorimetry is one of the most popular techniques used to measure thermal properties of materials such as enthalpy and entropy. Of all the different calorimeters that researchers use, differential scanning calorimeter (DSC) is the most common. The apparatus measures the amount of heat absorbed or released for a certain material to make a molecular transition from one conformation to another as a function of time and temperature. DSC can be broken down into two different categories of operation, either heat flux DSC or power compensated DSC. In a heat flux DSC, the starting material is enclosed in a pan along with an empty pan, used as a reference state, inside a furnace. The temperature within the furnace is increased at a linear heating rate to observe the heat transfer into the sample. Because of the difference in heat capacity between the pan with the material and the empty pan, a temperature difference is noticed and measured. Thus heat flow is determined by Ohm’s law: (1) Where q is heat flow, ΔT is the temperature difference between the two pan, and R is the resistance of the heating device. This is a powerful tool to decipher factors that contribute to folding and stability of molecules (Gill, Moghadam, & Ranjbar, 2010). In a power-compensated DSC, the sample and reference pan are placed in separate furnaces and kept at constant temperature. A measurement is taken on the amount of thermal power required to maintain that temperature and plotted. In an experiment, thermodynamic parameters are associated with heat-induced macromolecular transition (Gill, Moghadam, & Ranjbar, 2010). Typically, molar heat capacity is
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measured as a function of temperature. At constant pressure, the derivative of the enthalpy function in terms of temperature gives the heat capacity of the material. (2) dH/dt is the heat flow rate, m is the sample mass, Cp is the specific heat capacity, and dT/dt is the rate that the temperature is being raised for the purpose of the experiment. For any molecule in aqueous solution, there exists an equilibrium between the native conformation (folded state) and its denatured state (unfolded state). During the unfolding process of a protein, forces that are used to stabilize the structure is broken. At high temperatures, conformational entropy disrupt these forces causing the unfolding of the protein. Change in specific heat occurs when changes in hydration of side chains become exposed in its new conformation, becoming exposed to the solvent in the denatured state (Gill, Moghadam, & Ranjbar, 2010). Cp for the transitional state can be obtained through the difference between pre- transitional and post-transitional baselines of a DSC process. The transition enthalpy (ΔH) is the integration of the plot of Cp as a function of T. Where Cp is the heat capacity during the transition minus the baseline heat capacity of the protein as if there was no transition.
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