Discovery and exploration of new solid forms of dipeptide: l-alanyl-l-glutamine

l-Alanyl-l-glutamine (Ala-Gln) is a dipeptide which is widely used in medical and nutritional fields. Two new crystalline forms (form III and form IV) and a DMSO solvate form (S_DMSO) of Ala-Gln were discovered in this study, which were distinguished from the known forms by Raman spectroscopy and powder X-ray diffraction (PXRD). Single crystal structures of the three new solid forms were obtained. Moreover, the crystal structure data of known forms: form II, dihydrate form (2H) and ethanol solvate form (S_EtOH), which have not been reported to the best of our knowledge, were solved. The results showed that there were four kinds of molecular arrangements and frameworks in the Ala-Gln solid forms. The comparison of the molecular arrangements and cell parameters indicated that the forms III, 2H and S_EtOH were isomorphic. The hydrogen bonding network of the DMSO solvate was different from those of forms II, III and IV. By comparing the bulk density, lattice energy and thermal analysis results of the three polymorphs of Ala-Gln, the order of bulk density and lattice energy was consistent with the decomposition enthalpy. Besides that, it was found that the crystal shape of the solvates can be retained after solvent removal.