Thermodynamics of formation for the 18-crown-6-triglycine molecular complex in water-dimethylsulfoxide solvents

The effect of a water-dimethylsulfoxide (DMSO) solvent on the formation of a molecular complex of 18-crown-6 (18C6) with triglycine (diglycylglycine, 3Gly) is studied via calorimetric titration. It is found that switching from water to an H₂O-DMSO mixture with DMSO mole fraction of 0.30 is accompanied by a monotonic increase in the stability of [3Gly18C6] complex, from logK ^° = 1.10 to logK ^° = 2.44, and an increase in the exothermicity of the reaction of its formation, from ?5.9 to ?16.9 kJ/mol. It is shown that the [3Gly18C6] complex exhibits enthalpy stabilization with negative values of enthalpy and entropy over the investigated range of H₂O-DMSO solvents. Analysis of the reagents’ solvation characteristics reveals that the increase in the reaction’s exothermicity of transfer is due to differences in the solvation of [3Gly18C6] and 18C6 with a small solvation contribution from 3Gly. It is concluded that the change in the Gibbs energy of the reaction 3Gly_solv + 18C6_solv ? [3Gly18C6]_solv is due to differences in the change in the solvation state of the complex and the peptide (Δ_tr G ^°([3Gly18C6])-Δ_tr G ^°(3Gly)).