Solid-state stability studies of cholecystokinin (CCK-4) peptide under nonisothermal conditions using thermal analysis, chromatography and mass spectrometry

The solid-state stability of cholecystokinin (CCK-4) peptide under nonisothermal conditions was studied by differential scanning calorimetry (DSC), chromatography and mass spectrometry, identifying and schematizing the degradation products. To model the degradation mechanism of the peptide using the combined Kissinger and direct-differential methods, the observed degradation process was characterized by decomposition temperature (T_m), reacted fraction (α_m), activation energy (E_a), and pre-exponential factor (A). Results obtained by the two calculation methods were similar. The cleavage reaction on both N- and C-terminal sides of aspartic acid was the principal degradation pathway, although the reaction can occur consecutively and/or in parallel. Therefore to determine the relative importance of the different degradation pathways, a system of differential equations relevant to each degradation reaction was analysed using the R^® statistical program. The results obtained show that the consecutive reaction was the less plausible, whereas a slightly better fit was obtained for the reaction with both processes than for the in-parallel reaction. In this situation, the F-test was applied to discriminate between the models, indicating that the simpler model is the most probable. In conclusion, the results demonstrate for the first time that, in solid-state, n - 1 cleavage occurs in parallel to n + 1 cleavage at aspartic acid residues and not consecutively.