Thermodynamics of the reactions of dipeptides with macrocyclic 18-crown-6 ether in water

The equilibrium constants, Gibbs energies, enthalpies, and entropies of com plexation between the glycyl-l-alpha-alanine, glycyl-l-leucine, glycyl-gamm a-aminobutyric acid, L-alpha-alanyl-glycine, beta-alanyl-beta-alanine, DL-a lpha-alanyl-DL-valine, DL-alpha-alanyl-DL-leucine, DL-alpha-alanyl-DL-serin e, and DL-alpha-alanyl-DL-asparagine dipeptides and 18-crown-6 ether in wat er at 298.15 K are determined by the calorimetric method. It is found that the complexes formed by dipeptides containing large nonpolar groups are ent halpically stabilized. The "guest" methyl group located near the macrocycli c cavity of the "host" molecule weakens specific hydration of peptides and, thus, facilitates their complexation with 18-crown-6. The influence of the structure of dipeptides on their interaction with crown ethers is demonstr ated.