Redetermined crystal structure of α-dl-methionine at 340 K

An accurate redetermination of α-dl-methionine provides coordinates for the H atoms, detailed hydrogen-bond geometries and reveals that the side chain is disordered over a major (95%) and a minor (5%) position.


Chemical context
The racemates of amino acids with linear side chains display a series of unique phase transitions that involve sliding of neighboring molecular bilayers compared to each other. Such behavior has been observed for dl-aminobutyric acid (dl-Abu, R = -CH 2 CH 3 ; Gö rbitz et al., 2012), dl-norvaline (dl-Nva, -CH 2 CH 2 CH 3 ; Gö rbitz, 2011), dl-norleucine (dl-Nle, -CH 2 CH 2 CH 2 CH 3 ; Coles et al., 2009) and dl-methionine (dl-Met, -CH 2 CH 2 SCH 3 ). Two phase transitions have been found for each of the three nonstandard amino acids. For dl-Met, only a single transition is known < 400 K, occurring at approximately 326 K from the (low T) to the form (high T). Both phases were originally described by Mathieson (1952), with R factors > 0.20, and were subject to redeterminations by Taniguchi et al. (1980) at room temperature (R = 0.088) and 333 K (R = 0.118). The form was subsequently redetermined at 105 K (R = 0.041; Alagar et al., 2005; refcode DLMETA05 in the Cambridge Structual Database, Version 5.35; Allen, 2002). -dl-Met, (I), however, remained one of the few structures of the standard amino acids for which no high-precision experimental data were available (Gö rbitz, 2015). We here provide a detailed description of this polymorph, obtained from a single-crystal X-ray diffraction investigation at 340 K.

Structural commentary
The molecular structure of (I) is shown in Fig. 1. Despite the above-room-temperature conditions, thermal vibrations are comparatively modest. A previously undetected minor conformation with 1 (N1-C2B-C3B-C4B) in a gauche+ orientation (Table 1) has occupancy 0.0491 (18). If the presence of this rotamer is neglected, the refinement converges at R = 0.0586 rather than 0.0490. Disorder is extensive for all known phases of dl-Abu and dl-Nva, so it is not unexpected that it is observed here for dl-Met.
The crystal packing of (I) is shown in Fig. 2(a) and may be compared with the structure of -dl-Met in Fig. 2 (Alagar et al., 2005). The difference between the two forms is not limited to the obvious conformational change for the C3-C4-S-C5 torsion angle, which is trans for the form, but involves a large shift along the 9.8 Å axis and also the characteristic translation half a unit-cell length along the 4.7 Å axis. Notably, hydrogen bonding is virtually unaffected by these displacements. Compared to the 105 K data, N1Á Á ÁO2 distances in Table 1 are 0.03 Å longer, while N1Á Á ÁO1 is 0.01 Å shorter. All HÁ Á ÁA distances surprisingly appear to get shorter at 340 K, but this is an artefact resulting from different ways of handling the amino group (Gö rbitz, 2014). In the refinement of -dl-Met, this group was fixed with idealized geometry and a perfectly staggered orientation, while we find, upon relaxing the positional parameteres for all three H atoms, a 14 counterclockwise rotation (for the l-enantiomer) that serves to give three shorter and more linear interactions.

Supramolecular features
Hydrogen-bond geometries are listed in Table 2 The molecular structure of (I), with 50% probability displacement ellipsoids and atomic numbering indicated. The l-enantiomer was used as the asymmetric unit, d-enantiomers being generated by symmetry. The minor side-chain orientation [occupancy 0.0491 (18)], with N1-C2B-C3B-C4B in a gauche+ rather than a gaucheÀ orientation (Table 1), is shown in a lighter colour.

Figure 2
(a) The crystal packing of (I), viewed along the monoclinic b axis (top) and the c axis (bottom). The minor side-chain conformation is not shown, and H atoms bonded to C have been omitted for clarity. l-Met and d-Met molecules are shown with light-and dark-grey C atoms, respectively. The blue arrows show the directions of C2-N bond vectors within each of the two sheets constituting a hydrogen-bonded layer. (b) Corresponding views for -dl-Met at 105 K (Alagar et al., 2005). Table 1 Selected torsion angles ( ).
observed for racemates and quasiracemates where at least one of the side chains (for the l-or the d-enantiomer) is linear and leucine, with an isobutyl side chain, is not involved (Gö rbitz et al., 2009). Apart from a weak C -HÁ Á ÁO contact along the b axis, all intermolecular interactions within a single sheet involve amino acids of opposite chirality (Fig. 3); two N-HÁ Á ÁO interactions between amino acids of the same chirality serve to link the adjacent antiparallel sheets that form a double-sheet hydrogen-bonded layer.

Synthesis and crystallization
From a saturated solution of dl-Met in water (approximately 30 mg ml À1 ) 50 ml was pipetted into a 40 Â 8 mm test tube, which was then sealed with parafilm. A small hole was pricked 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 5 d at 293 K. Suitable single crystals in the shape of plates formed as the organic solvent diffused into the aqueous solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. U iso values for CnB atoms (n = 3-5) belonging to the minor side-chain conformation with occupancy 0.0491 (18) were fixed at the U eq values of the corresponding Cn atom of the major conformation, while S1B was constrained to have the same set of anisotropic displacement parameters as S1. A similar procedure was undertaken for C2B and C2. Coordinates were refined for amino H atoms; other H atoms were positioned with idealized geometry with fixed C-H = 0.96 (methyl), 0.97 (methylene) or 0.98 Å (methine). U iso (H) values were set at 1.2U eq of the carrier atom or at 1.5U eq for methyl and amino groups.