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

Carl Henrik Görbitz; Jan Christian Paulsen; Jon Borgersen

doi:10.1107/S2056989015008749

. 2015 May 13;71(Pt 6):o398–o399. doi: 10.1107/S2056989015008749

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

Carl Henrik Görbitz ^a,^*, Jan Christian Paulsen ^a, Jon Borgersen ^b

PMCID: PMC4459326 PMID: 26090187

Abstract

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.

Keywords: crystal structure, amino acid, phase transition, disorder

Related literature

For previous investigations of dl-methionine (dl-Met), see: Mathieson (1952 ▸); Taniguchi et al. (1980 ▸); Alagar et al. (2005 ▸); Görbitz (2014 ▸); Görbitz et al. (2014 ▸). For a discussion of displacive phase transitions of amino acids with linear side chains and structures of quasiracemic complexes, see: Görbitz & Karen (2015 ▸). For the phase behaviour of the corresponding enantiomeric substances, including l-Met and l-norvaline, see: Görbitz et al. (2015 ▸). For a discussion of hydrogen-bonding patterns in the crystal structures of hydrophobic amino acids, see: Görbitz et al. (2009 ▸). graphic file with name e-71-0o398-scheme1.jpg

Experimental

Crystal data

C₅H₁₁NO₂S
M _r = 149.21
Monoclinic,
a = 31.774 (2) Å
b = 4.6969 (3) Å
c = 9.8939 (7) Å
β = 91.224 (2)°
V = 1476.20 (18) Å³
Z = 8
Mo Kα radiation
μ = 0.37 mm⁻¹
T = 320 K
0.72 × 0.15 × 0.10 mm

Data collection

Bruker D8 Advance single-crystal CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2014 ▸) T _min = 0.924, T _max = 1.000
10516 measured reflections
2060 independent reflections
1567 reflections with I > 2σ(I)
R _int = 0.031

Refinement

R[F ² > 2σ(F ²)] = 0.046
wR(F ²) = 0.116
S = 1.03
2060 reflections
92 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.34 e Å⁻³
Δρ_min = −0.39 e Å⁻³

Data collection: APEX2 (Bruker, 2014 ▸); cell refinement: SAINT-Plus (Bruker, 2014 ▸); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS2014 (Sheldrick, 2008 ▸); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015 ▸); molecular graphics: Mercury (Macrae et al., 2008 ▸); software used to prepare material for publication: SHELXL2014.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2056989015008749/su5129sup1.cif

e-71-0o398-sup1.cif^{(335.9KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015008749/su5129Isup2.hkl

e-71-0o398-Isup2.hkl^{(113.5KB, hkl)}

Click here for additional data file.^{(3KB, cml)}

Supporting information file. DOI: 10.1107/S2056989015008749/su5129Isup3.cml

Click here for additional data file.^{(1.3MB, tif)}

dl I a et al. et al. gauche trans trans l gauche trans gauche+ gaucsu5129.cif he trans gauche+ . . DOI: 10.1107/S2056989015008749/su5129fig1.tif

The molecular structure of β-dl-methionine at 320 K, with atom labelling, flanked by the structures at 105 K (published with the alternative space group setting I2/a, Alagar et al., 2005; Görbitz, 2014) and 340 K (Görbitz et al., 2014). 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 gaucsu5129.cif he+, trans, gauche+, respectively.

CCDC reference: 1063335

Additional supporting information: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (, ).

DHA	DH	HA	D A	DHA
N1H1O1ⁱ	0.90(2)	1.88(2)	2.7732(17)	173.7(19)
N1H2O2ⁱⁱ	0.92(2)	1.92(2)	2.8264(18)	171.0(18)
N1H3O2ⁱⁱⁱ	0.90(2)	1.94(2)	2.7973(18)	159.2(18)

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Table 2. Selected torsion angles ().

Torsion angle	-DL-Met, 320K	-DL-Met, 340K^a	-DL-Met, 340K^b
N1C2C3C4	55.52(18)	59.3(4)	73(8)
C1C2C3C4	175.03 (14)	178.0(2)	78(5)
C2C3C4S1	179.16(12)	176.7(2)	178(5)
C3C4S1C5	174.55(16)	69.4(3)	60(3)

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Notes: (a) major conformation, occupancy 0.9509(18) (Grbitz et al., 2014 ▸); (a) minor conformation, occupancy 0.0491(18).

supplementary crystallographic information

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 T₁ 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 T₁, 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 for α-DL-Met at 340 K (Görbitz et al., 2014), 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 (T₁) 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.