Chemistry
Measuring Binding Enthalpies with Isothermal Titration Calorimetry
Quick fact
ITC is the only technique that directly measures the binding enthalpy (ΔH) of a biomolecular interaction without needing to make assumptions or use fluorescent labels. It records the tiny amounts of heat released or absorbed as one molecule is titrated into another.
Why this is interesting
You've probably heard that proteins bind their partners — but did you know we can measure the heat of that embrace? ITC gives us a direct thermodynamic 'fingerprint' of molecular recognition.
Read the full explanation
Understanding Measuring Binding Enthalpies with Isothermal Titration Calorimetry
Imagine you're at a coffee shop, adding sugar to your tea and stirring. Each spoonful dissolves, and you might feel the cup slightly cool or warm — that's a heat effect. In biochemistry, when two molecules bind, they also release or absorb heat. ITC literally measures that heat, but with exquisite precision. The experiment works by placing one molecule (say, a protein) in a sample cell and injecting the other (a ligand) in small, controlled aliquots. Each injection causes a binding event, and the calorimeter records the heat change. As more ligand is added, more binding occurs until the protein becomes saturated, and the heat per injection decreases to just the heat of dilution. The pattern of heat against the molar ratio of ligand to protein creates a binding curve. From that curve, scientists can extract the binding constant (Kd), the stoichiometry (how many ligand molecules bind per protein), and crucially, the enthalpy change (ΔH). The instrument keeps everything at a constant temperature, hence 'isothermal'. This ensures that any heat measured is due to the interaction itself, not from temperature drifts.
A deeper explanation
The power of ITC lies in the first law of thermodynamics: the heat measured at constant pressure equals the change in enthalpy (ΔH) of the system. When a ligand binds to a protein, non-covalent bonds form (hydrogen bonds, van der Waals forces, hydrophobic interactions), releasing energy — that's an exothermic ΔH. If binding requires breaking bonds with water, energy is absorbed — endothermic. By integrating the heat of each injection and subtracting the heat of dilution, one obtains the total heat per mole of injected ligand. The shape of the binding isotherm also gives the Kd. Then, using the relation ΔG = -RT ln(Kd), we get the free energy. With ΔH and ΔG in hand, the entropy change (ΔS) follows from ΔG = ΔH - TΔS. This decomposition into enthalpy and entropy is unique to ITC. It reveals whether binding is driven by favorable contacts (large negative ΔH) or by entropy (favorable release of water). This information is incredibly useful in drug design: a tight binder might rely solely on entropy, but understanding the enthalpy helps chemists optimize specific interactions. ITC is label-free, works in solution, and does not require immobilization, making it the gold standard for studying biomolecular binding thermodynamics.