Redetermined crystal structure of β-dl-methionine at 320 K

The structure of β-dl-methionine, C5H11NO2S, in the space group C2/c, is here confirmed to be fully ordered all the way up to the phase transition at approximately 326 K, where displacive sliding of molecular bilayers gives the disordered P21/c α form [data at 340 K; Görbitz (2014). Acta Cryst. E70, 341–343]. The geometry of hydrogen bonds in LD–LD hydrogen-bonding patterns [Görbitz et al. (2009). Acta Cryst. B65, 393–400] at the hydrophilic core of each molecular bilayer are virtually unperturbed by the phase shift, but the C—C—S—C torsion angle of the side chain changes from trans at 320 K to gauche+ for the major conformation at 340 K.


S1. Comment
The two known polymorphs of DL-methionine (DL-Met), called α and β, were originally described by Mathieson (1952). Taniguchi et al. (1980) established the (first and only) transition temperature T 1 to be about 326 K and carried out redeterminations at room temperature (β-form) and 333 K (α-form). β-DL-Met was subsequently redetermined at 105 K (Alagar et al., 2005;Görbitz, 2014) and α-DL-Met at 340 K (Görbitz et al., 2014). The transition between the two forms involves displacive sliding of molecular bilayers in the crystal. Recently, we have shown that such transitions are not limited to regular racemates of amino acids with linear side chains, but occur also for their quasiracemic complexes, including L-norvaline:D-norleucine (Görbitz & Karen, 2015), and for enantiomeric L-norvaline (Görbitz et al., 2015). A special property of just these two structures is that all side chains remain ordered up to T 1 , but that one or more additional low-occupancy conformations appear at higher temperatures.
The side chain of β-DL-Met is ordered at 105 K (Alagar et al., 2005;Görbitz, 2014), while a minor component is found Fig. 1. As the quality of the available room-temperature crystal structure is rather low (R = 0.088; Taniguchi et al., 1980), it was not known, however, if disorder in this case develops gradually between 105 K and 326 K (T 1 ) or is introduced abruptly during the phase transition. The purpose of the present investigation was to settle this matter by collection of accurate experimental data with modern equipment at a temperature just below T 1. This initiative was triggered by new results for L-Met (Görbitz et al., 2015) that unexpectedly revealed disorder for both molecules in the asymmetric unit at room temperature.
The molecular structure of β-DL-Met at 320 K (I), depicted in Fig. 1, proves to be very well defined with no significant residual peaks in the electron density map. This means that the change from ordered to disordered side chain upon heating through a displacive phase transition, as observed for L-norvaline:D-norleucine and L-norvaline (see above), recurs for DL-Met. There is, however, one important tweak: while the dominating, major side-chain conformation of disordered molecules in the two other systems is always inherited from the ordered, lower-temperature polymorph, Fig. 1 shows that the gauche-, trans, trans conformation of β-DL-Met at 320 K (Table 1), is instead replaced by a gauche-, trans, gauche+ conformation in α-DL-Met at 340 K.
Hydrogen-bond parameters are listed in Table 2. N···O distances are marginally shorter at 320 K than at 340 K (Görbitz et al., 2014).

S2. Experimental
From a saturated solution of DL-Met in water (approximately 30 mg mL -1 ) 30 µL was pipetted into a 40 × 8 mm test tube, which was then sealed with parafilm. A needle was used to pierce a small hole in the parafilm and the tube placed inside a larger test tube filled with 2 ml of acetonitrile. The system was ultimately capped and left for one week at 20 °C.
Suitable single crystals in the shape of needles and plates formed as the organic solvent diffused into the aqueous solution.

S3. Refinement
Coordinates were refined for amino H atoms with U iso (H) = 1.5U eq (N). The C-bound H atoms were positioned with idealized geometry and treated as riding atoms: C-H = 0.96 -0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms, allowing free rotation for the terminal side-chain methyl group, and with 1.2U eq (C) for other H atoms.

Figure 1
The Thermal displacement ellipsoids are shown at the 50% probability level. Atoms of the minor side-chain conformation with occupancy 0.0491 (18) at 340 K (with H atoms omitted) are shown in a lighter tone. The side-chain conformation is gauche-, trans, trans (as defined by the N1-C2-C3-C4, C2-C3-C4-S1 and C3-C4-S1-C5 torsion angles of the L-enantiomer shown) at 105 and 320 K, while the major and minor conformations at 340 K are gauche-, trans, gauche+ and gauche+, trans, gauche+, respectively.