Which process of gas entrapment by β-cyclodextrin (β-CD) is more favorable—insertion into the central cavity without hydration water displacement or guest binding accompanied by water molecule displacement?

β-cyclodextrin (β-CD) can form complexes with gaseous substances either by inserting the guest into its central cavity without displacing the hydration water molecules or by displacing some water molecules to accommodate the guest. The study evaluates the thermodynamic favorability of these processes for various gases.

From: "Inclusion Complexes between β-Cyclodextrin and Gaseous Substances—N2O, CO2, HCN, NO2, SO2, CH4 and CH3CH2CH3: Role of the Host’s Cavity Hydration", Inorganics 2024, 12(4), 110

The study found that the displacement of a few water molecules (1, 2, or 3) by the guest molecule is generally more favorable than insertion without water displacement. For example, the complexation of N2O with β-CD is most favorable when one water molecule is displaced (∆G = −10.9 kcal/mol), and less favorable as more water molecules are displaced. This suggests that the presence of some water molecules in the cavity helps stabilize the complex through hydrogen bonding, while displacing too many water molecules reduces the stability of the complex.