Thermal treatment of oligopeptides is one of the methods for synthesis of organic nanostructures. However, heating may lead not only to self-assembly of the initial molecules, but also to chemical reactions resulting in the formation of new unexpected nanostructures or change in the properties of the existing ones. In the present work, the reaction of cyclization of dipeptide l-leucyl-l-leucine in solid state under heating was studied. The change in morphology of dipeptide thin film and formation of nanostructures after heating was visualized using atomic force microscopy. This method also was used for demonstration of differences in self-assembly of linear and cyclic dipeptides. The chemical structure of reaction product was characterized by NMR spectrometry, FTIR spectroscopy and GC-MS analysis. Kinetic parameters of cyclization were estimated within the approaches of the nonisothermal kinetics ("model-free" kinetics and linear regression methods for detection of topochemical equation). The results of present work are useful for explanation the changes in the properties of nanostructures based on short-chain oligopeptides, notably leucyl-leucine, after thermal treatment, as well as for the synthesis of cyclic oligopeptides.

The non-zeolitic behavior of l-leucyl-l-leucine and its self-organization in solid state and from solutions with the formation of different nanostructures are reported. This dipeptide forms porous crystals, but does not exhibit molecular sieve effects typical of classical zeolites and biozeolites. The specific sorption properties of l-leucyl-l-leucine result from a change in its crystal packing from channel-type to layered-type, when binding strong proton acceptors or proton donors of molecular size greater than 18-20 cm mol. The high sorption capacity of l-leucyl-l-leucine toward dichloromethane results from the self-organization of the dipeptide, by forming nanofibers or web-like structures. The low thermal stability of clathrates of the dipeptide containing large guest molecules and the selectivity of l-leucyl-l-leucine toward alcohols over nitriles can be used to separate organic mixtures such as methanol/n-butanol and methanol/acetonitrile.

Water and cyclo(leucyl–leucyl) organogels: “Water helps cyclo(Leu–Leu) to form organogels. Cyclo(Leu–Leu) helps to purify water”.

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