Why does ITC measure Enthelpy, not Gibbs Energy?

[jjoonathan]

Isothermal Titration Calorimetry measures the enthalpy of (e.g.) protein binding by measuring the amount of heat that must be added/removed to a solution in order to perfectly balance the heat generated/released by the gradual introduction of a protein's binding partner. My understanding is that the experiment operates under constant (well, *almost* constant) temperature and pressure. But don't those conditions (constant T, P) mean that ΔG equals the heat generated/released by binding, as opposed to ΔH? How do we know we should use ΔH instead?

Some references:
[1] http://antisensescienceblog.wordpre...plained-isothermal-titration-calorimetry-itc/
[2] http://www.uic.edu/orgs/ctrstbio/manuals/leavitt.pdf

 

[Andrew Mason]

Isothermal Titration Calorimetry measures the enthalpy of (e.g.) protein binding by measuring the amount of heat that must be added/removed to a solution in order to perfectly balance the heat generated/released by the gradual introduction of a protein's binding partner. My understanding is that the experiment operates under constant (well, *almost* constant) temperature and pressure. But don't those conditions (constant T, P) mean that ΔG equals the heat generated/released by binding, as opposed to ΔH? How do we know we should use ΔH instead?

Some references:
[1] http://antisensescienceblog.wordpre...plained-isothermal-titration-calorimetry-itc/
[2] http://www.uic.edu/orgs/ctrstbio/manuals/leavitt.pdf

The actual heat flow is the change in enthalpy, ΔH (P being constant). The change in Gibbs free energy, ΔG, is the difference between ΔH and Q_rev (the reversible heat flow). The ΔG tells you how close to reversible the reaction is and what direction it will go spontaneously. It is not a measure of the heat flow at constant pressure.

AM