Chemical behavior of peptides in their solid, crystalline state differs from that in solution or gas. In the crystal, peptide molecules are usually constrained to a single conformation with similar intermolecular surrounding, and a specific H-bonding pattern. These factors affect thermal stability of peptides as well as the pathways of their thermally-induced transformations. Thermoanalytical methods, combined with spectroscopic techniques, present a unique opportunity to control heat supply to the sample and to monitor the induced reactions in situ. Our group utilizes short peptides in the design and preparation of crystals, co-crystals and inclusion compounds with a specific molecular arrangement, and investigates thermally-induced transformations in these solids [1,2]. We use TGA (coupled with FTIR) and DSC instruments simultaneously as both reaction vessels and analytical tools to monitor the process. Further analysis is completed using a range of other techniques either to scale the process or to compare the course of the reaction in the solid and gaseous phase. We hope that these studies will lead to the development of new synthetic approaches conforming to the principles of green chemistry. Several series of glycine- and leucine-containing peptides have been studied, containing from two to six amino acid residues in homo- and hetero-sequences. The formation of polymeric materials and the products of intramolecular condensation was observed, in addition to other chemical transformations typical for peptides reacting in solution or gas. These reactions were studied in greater detail as they yield synthetically valuable peptide products and materials.
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