Thermodynamics and mechanism of complexation of peptides with 18-crown-6 in water

Abstract: Taking into account the unusual crystalline structures of 18-crown-6/peptides/water complexes, the links between their thermokinetic behaviour and the structural peculiarities are discussed. It has been found that the peptide chain lengthening leads to an increase in the temperature and activation energy of decomposition. The thermodynamics of complexation between small peptides and 18-crown-6 in water has been studied as well. It is shown that the influence of the peptide lengthening and the presence of a methyl group in the peptide side chain on the binding constant can be used as a the basis for the separation of peptide mixtures in solution. DISCUSSION 1. Structure of the crystalline compounds of amino acids and peptides with 18-crown-6 The study of complex formation between peptides and macrocyclic ligands is important in the understanding of numerous biochemical processes and for the design of new materials possessing unusual properties. As shown by X-ray diffractometryl, the crystalline compounds consisting of zwittterionic peptides and 18-crown-6 are formed by a set of hydrogen bonds. The charged ammonium group of the peptide are bound to the macrocycle oxygen atoms through three NH.. O hydrogen bonds resulting in the approximately perpendicular orientation of the peptide backbone to the macrocycle plane (Fig. 1). A significant difference in the "depth of penetration " of the ammonium groups into the macrocycle cavity for complexes of 18-crown-6 with diglycine and dialanine was also found In the 18-crown-6/diglycine complex, the ammonium