Complexation of peptides with crown ethers. Structure and thermal behaviour of ternary compounds: 18-crown-6/glycylglycine/water and 18-crown-6/L-α-alanyl-L-α-alanine/water

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Abstract
X-ray structures of ternary complexes composed of small dipeptides, i.e. glycylglycine or L-α-alanyl-L-α-alanine, 18-crown-6 and water molecules are determined based on single-crystal studies. It has been concluded that both dipeptides exist in the complex as zwitterions and are bonded to the 18-crown-6 macrocycle by the -NH+ 3 end group through N-H…O hydrogen bonds. The carboxylic groups at the opposite side of the peptide molecules take part in hydrogen bonding with water molecules. This scheme of water-peptide hydrogen bonding is particularly interesting in the glycylglycine compound where the water content is rather high (approximately 16 moles per mole of crown/dipeptide, 6 of which are found in the X-ray study) and may have interesting relevance with respect to the structure of gel solutions which easily form in these systems. The DSC and TGA thermograms of different hydrated forms of the complexes were obtained. Two types of water, i.e. weakly and strongly bonded, were observed upon heating of the hydrated complex. On further heating the anhydrous complexes have at least two transitions in the range 320–470 K: melting of complexes at 373–383 K and decomposition above 410 K.

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