Solid-State Phase Transitions of dl-Aminobutyric Acid

Abstract

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.

Effect of pressure on methylated glycine derivatives: relative roles of hydrogen bonds and steric repulsion of methyl groups

“…Pressure-induced phase transitions in this case are related to the rotation of the side chains switching over the hydrogen bonds: S-HÁ Á ÁS/S-HÁ Á ÁO in orthorhombic l-cysteine (Moggach, Allan et al, 2006;Minkov, Goryainov et al, 2010) and dl-cysteine (Minkov et al, 2008;Minkov, Tumanov et al, 2010), O-HÁ Á ÁO hydroxyl /O-HÁ Á ÁO carboxyl in l-serine (Kolesnik et al, 2005;Moggach et al, 2005;Moggach, Marshall & Parsons, 2006;Drebushchak et al, 2006). In the structures with hydrophobic side chains, one can expect the phase transitions at high pressure to be related to ordering/disordering of the side chains and changing their conformations, to optimize packing, similar to what has been reported for dl-aminobutyric acid (Gö rbitz et al, 2012), dl-norvaline (Gö rbitz, 2011) and dl-norleucine (Dalhus & Gö rbitz, 1996) on cooling. In glycine, the only amino acid with 'no side chain', the effect of pressure depends strongly on the starting structure.…”

Crystal structures of amino acids: from bond lengths in glycine to metal complexes and high-pressure polymorphs

“…A completely different type of phase transitions occurs for the racemates of Met [35,157] and other, non-standard amino acids with linear side chains, such as Abu, [159] Nva [160] and Nle. [161] These structures are divided into layers, and at a specific temperature (which is different on cooling and on heating, i.e.…”

On the stacking disorder of DL-norleucine

“…According to the classification system of Gö rbitz et al (2009), the hydrogen-bond network corresponds to the type ld-ld/I. This molecular arrangement is very common for racemic amino acids and is also found, for example, in dlisoleucine (Dalhus & Gö rbitz, 2000), dl-norvaline (Gö rbitz, 2011), dl-methionine (Mathieson, 1952) and dl--amino-nbutyric acid (Ichikawa & Iitaka, 1968;Gö rbitz et al, 2012).…”