Host-guest Chemistry - Thermodynamic Principles of Host-Guest Interactions

Thermodynamic Principles of Host-Guest Interactions

There is an equilibrium between the unbound state, in which the host and the guest are separate from each other, and the bound state, in which there is a structurally defined host-guest complex:

H ="host", G ="guest", HG ="host-guest complex"

The "host" component can be considered the larger molecule, and it encompasses the smaller, "guest", molecule. In biological systems, the analogous terms of host and guest are commonly referred to as enzyme and substrate respectively.

The thermodynamic benefits of host guest chemistry are derived from the idea that there is a lower overall Gibbs free energy due to the interaction between host and guest molecules. Chemists are exhaustively trying to measure the energy and thermodynamic properties of these non-covalent interactions found throughout supramolecular chemistry; and by doing so hope to gain further insight into the combinatorial outcome of these many, small, non-covalent forces that are used to generate an overall effect on the supramolecular structure.

In order to rationally and confidently design synthetic systems that perform specific functions and tasks, it is very important to understand the thermodynamics of binding between host and guest. Chemists are focusing on the energy exchange of different binding interactions and trying to develop scientific experiments to quantify the fundamental origins of these non-covalent interactions by utilizing various techniques such as NMR spectroscopy, Raman spectroscopy, isothermal titration calorimetry, surface tension, and UV-Vis Spectroscopy. The experimental data is quantified and explained through analysis of binding constants K_a, Gibbs free energy ΔG_o, Enthalpy ΔH_o, and entropy ΔS_o.