Solid-state phase transitions of DL-aminobutyric acid.

Four polymorphs are known for the amino acid racemate DL-aminobutyric acid: one tetragonal form B and three monoclinic forms A, C, and D for which two solid-solid phase transitions take place between 209 and 337 K. The intermediate form A, in space group P2(1)/c, is known to display disorder in the crystalline state with the ethyl side chain occupying three different conformations. The high temperature form D and the low temperature form C have been assumed to display a similar type of disorder in the space group C2/c. The present investigation, on the basis of high quality crystal structures at 100 K (C form), 220 K (A form), 295 K (A form), and 295 K (D form), provides updated results for all three forms and in particular demonstrates that the A → C and A → D transitions in fact involve only a partial space group conversion, so that two monoclinic phases are simultaneously present at high and low temperatures. To the best of our knowledge, such a phenomenon has not been observed previously for an organic molecule. The nonroutine refinement of the crystallographic data was performed with a specially adapted version of the program JANA2006. The observed polymorphs are compared with those found for DL-norvaline, DL-norleucine, and DL-methionine.