In this study we have performed a mass spectrometric analysis of the alpha-particle-induced fragmentation of sublimated dialanine dipeptide (ala-ala, C6H12N2O3) molecules. The analysis has shown that heating of crystalline ala-ala in the temperature range T = 410 – 470 K leads to a phase transition for the ala-ala molecules with formation of heterocyclic molecules of cyclo(ala-ala) dipeptide (C6H10N2O2, 3,6-Dimethylpiperazine-2,5-dione) and of H2O molecules. The dialanine molecule geometry and cyclization energies were calculated in the framework of the density functional theory (DFT). Three different reaction paths of the peptide bond formation were considered. Taking into account the autocatalytic process occurring in the presence of extra water molecules, a decrease in the activation energy of cyclization was revealed. Using the temperature dependence of mass spectra of sublimated cyclo(ala-ala) molecules, sublimation enthalpy ΔHsub(<T>) was estimated based on “the second law of thermodynamics” in the temperature range T = 443 – 459 K.