Differential Scanning Calorimetry (4.6) Differential scanning calorimetry (DSC) is a way of measuring energy changes associated with physical transitions – Phase transitions in compounds, metals, polymers – Denaturation of biopolymers can be considered a phase transition DSC uses differences in heating of a sample and a standard to determine thermodynamic parameters DSC – Sample and standard temperatures are ramped up over time, but are kept equal – The difference in the amount of heat needed to maintain temp. is the difference in heat capacities (assuming no chemical or physical changes) When a chemical or physical change occurs, the heating of the sample changes drastically – During a phase change, heat goes towards changing physical state, not temp. increasephase change – For protein denaturation, the heat capacity of the natural protein (C N ) is significantly different than the denatured protein (C D )protein denaturation – The melting temp. of the protein is the temp. at the peak maximum – The area under the curve is the enthalpy associated with the corresponding transition

Entropy ( ) Entropy (S) is a measure of disorder in a system – Nature likes to create disorder (i.e., ΔS > 0) – Larger entropies mean that more energy states are available to the system (e.g., vibrational modes) Tables report standard entropies rather than entropy changes due to the third law of thermodynamics – A pure, crystalline substance at zero Kelvin has no entropy (i.e., perfect order) – Disruption of this ordering increases the entropy – For enthalpy, we can only measure changes in heat content, not absolute heat Entropy is obviously temperature dependent since increasing temp. increases molecular motion (and thus disorder) – Dependence differs from enthalpy – Entropy differences can be measured for chemical reactions (ΔS rxn )

Gibbs Free Energy and Spontaneity (6.1) Spontaneity is determined by whether a process will occur without work being done on the system – Though a process is spontaneous, that does not mean it occurs quickly (kinetics determines rate) – Spontaneous processes are processes that don’t require work (e.g., release of heat, increase in disorder) Gibbs free energy (G) determines whether a process at constant pressure and temperature is spontaneous – If the change in Gibbs free energy (ΔG) is negative, the process is spontaneous (differential form: dG < 0) – If the Gibbs free energy change is equal to zero, the process is at equilibrium (e.g., ice/water at freezing point) Gibbs free energy change is related to enthalpy and entropy – Temperature changes can cause some processes to occur spontaneously (e.g., ice melting)

Differential Scanning Calorimeter

Phase Change in DSC

Protein Denaturation in DSC