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.
Supporting information
CCDC reference: 1063335
Key indicators
- Single-crystal X-ray study
- T = 320 K
- R factor = 0.046
- wR factor = 0.116
- Data-to-parameter ratio = 22.4
checkCIF/PLATON results
No syntax errors found
No errors found in this datablock
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.
Coordinates were refined for amino H atoms with Uiso(H) =
1.5Ueq(N). The C-bound H atoms were positioned with idealized
geometry and treated as riding atoms: C—H = 0.96 - 0.98 Å with
Uiso(H) = 1.5Ueq(C) for methyl H atoms, allowing free
rotation for the terminal side-chain methyl group, and with 1.2Ueq(C)
for other H atoms.
Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT-Plus (Bruker, 2014); data reduction: SAINT-Plus (Bruker, 2014); 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 (Sheldrick, 2015).
2-Amino-4-(methylsulfanyl)butanoic acid
top
Crystal data top
C5H11NO2S | F(000) = 640 |
Mr = 149.21 | Dx = 1.343 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
a = 31.774 (2) Å | Cell parameters from 4136 reflections |
b = 4.6969 (3) Å | θ = 2.6–29.5° |
c = 9.8939 (7) Å | µ = 0.37 mm−1 |
β = 91.224 (2)° | T = 320 K |
V = 1476.20 (18) Å3 | Needle, colourless |
Z = 8 | 0.72 × 0.15 × 0.10 mm |
Data collection top
Bruker D8 Advance single-crystal CCD diffractometer | 2060 independent reflections |
Radiation source: fine-focus sealed tube | 1567 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.031 |
Detector resolution: 8.3 pixels mm-1 | θmax = 29.5°, θmin = 2.6° |
Sets of exposures each taken over 0.5° ω rotation scans | h = −44→39 |
Absorption correction: multi-scan (SADABS; Bruker, 2014) | k = −6→6 |
Tmin = 0.924, Tmax = 1.000 | l = −13→13 |
10516 measured reflections | |
Refinement top
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.046 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.116 | w = 1/[σ2(Fo2) + (0.0458P)2 + 1.6546P] where P = (Fo2 + 2Fc2)/3 |
S = 1.03 | (Δ/σ)max = 0.001 |
2060 reflections | Δρmax = 0.34 e Å−3 |
92 parameters | Δρmin = −0.39 e Å−3 |
Crystal data top
C5H11NO2S | V = 1476.20 (18) Å3 |
Mr = 149.21 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 31.774 (2) Å | µ = 0.37 mm−1 |
b = 4.6969 (3) Å | T = 320 K |
c = 9.8939 (7) Å | 0.72 × 0.15 × 0.10 mm |
β = 91.224 (2)° | |
Data collection top
Bruker D8 Advance single-crystal CCD diffractometer | 2060 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2014) | 1567 reflections with I > 2σ(I) |
Tmin = 0.924, Tmax = 1.000 | Rint = 0.031 |
10516 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.046 | 0 restraints |
wR(F2) = 0.116 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.03 | Δρmax = 0.34 e Å−3 |
2060 reflections | Δρmin = −0.39 e Å−3 |
92 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Refinement. Final R-factor is 0.0364 for a refinement based on 1300 reflections with
2 theta < 50 °. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
S1 | 0.44096 (2) | 0.65785 (16) | 0.45693 (7) | 0.0684 (2) | |
O1 | 0.28350 (4) | 0.4246 (2) | 0.19616 (11) | 0.0316 (3) | |
N1 | 0.29732 (4) | 0.7975 (3) | 0.39652 (12) | 0.0252 (3) | |
H1 | 0.2705 (6) | 0.826 (4) | 0.3685 (19) | 0.038* | |
H2 | 0.2998 (6) | 0.627 (5) | 0.441 (2) | 0.038* | |
H3 | 0.3042 (6) | 0.926 (4) | 0.461 (2) | 0.038* | |
C1 | 0.30472 (4) | 0.6339 (3) | 0.16326 (14) | 0.0225 (3) | |
C2 | 0.32477 (5) | 0.8093 (3) | 0.27735 (13) | 0.0229 (3) | |
H21 | 0.3278 | 1.0073 | 0.2479 | 0.028* | |
O2 | 0.31323 (4) | 0.7062 (3) | 0.04532 (10) | 0.0383 (3) | |
C3 | 0.36823 (5) | 0.6847 (4) | 0.31133 (16) | 0.0318 (3) | |
H31 | 0.3651 | 0.4828 | 0.3293 | 0.038* | |
H32 | 0.3858 | 0.7044 | 0.2329 | 0.038* | |
C4 | 0.39054 (5) | 0.8220 (4) | 0.43157 (19) | 0.0406 (4) | |
H41 | 0.3739 | 0.7986 | 0.5118 | 0.049* | |
H42 | 0.3940 | 1.0242 | 0.4153 | 0.049* | |
C5 | 0.45831 (8) | 0.8257 (7) | 0.6094 (3) | 0.0750 (8) | |
H51 | 0.4861 | 0.7596 | 0.6334 | 0.113* | |
H52 | 0.4393 | 0.7799 | 0.6804 | 0.113* | |
H53 | 0.4589 | 1.0282 | 0.5965 | 0.113* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
S1 | 0.0405 (3) | 0.0825 (5) | 0.0809 (4) | 0.0201 (3) | −0.0269 (3) | −0.0255 (4) |
O1 | 0.0365 (6) | 0.0287 (6) | 0.0295 (6) | −0.0081 (5) | −0.0036 (4) | −0.0017 (4) |
N1 | 0.0289 (6) | 0.0278 (6) | 0.0189 (6) | 0.0044 (5) | −0.0031 (5) | −0.0035 (5) |
C1 | 0.0270 (7) | 0.0198 (6) | 0.0204 (6) | 0.0041 (5) | −0.0051 (5) | −0.0015 (5) |
C2 | 0.0308 (7) | 0.0192 (6) | 0.0186 (6) | −0.0018 (5) | −0.0028 (5) | 0.0008 (5) |
O2 | 0.0657 (8) | 0.0314 (6) | 0.0176 (5) | −0.0024 (6) | −0.0001 (5) | 0.0011 (4) |
C3 | 0.0278 (7) | 0.0356 (8) | 0.0318 (8) | 0.0003 (7) | −0.0033 (6) | −0.0050 (7) |
C4 | 0.0336 (8) | 0.0417 (10) | 0.0458 (10) | 0.0047 (7) | −0.0144 (7) | −0.0078 (8) |
C5 | 0.0579 (14) | 0.104 (2) | 0.0620 (14) | 0.0004 (14) | −0.0292 (12) | −0.0068 (15) |
Geometric parameters (Å, º) top
S1—C5 | 1.779 (2) | C2—H21 | 0.9800 |
S1—C4 | 1.7907 (17) | C3—C4 | 1.516 (2) |
O1—C1 | 1.2398 (18) | C3—H31 | 0.9700 |
N1—C2 | 1.4825 (18) | C3—H32 | 0.9700 |
N1—H1 | 0.90 (2) | C4—H41 | 0.9700 |
N1—H2 | 0.92 (2) | C4—H42 | 0.9700 |
N1—H3 | 0.90 (2) | C5—H51 | 0.9600 |
C1—O2 | 1.2504 (18) | C5—H52 | 0.9600 |
C1—C2 | 1.5258 (19) | C5—H53 | 0.9600 |
C2—C3 | 1.530 (2) | | |
| | | |
C5—S1—C4 | 100.84 (11) | C4—C3—H31 | 108.6 |
C2—N1—H1 | 108.6 (12) | C2—C3—H31 | 108.6 |
C2—N1—H2 | 111.8 (12) | C4—C3—H32 | 108.6 |
H1—N1—H2 | 110.3 (17) | C2—C3—H32 | 108.6 |
C2—N1—H3 | 113.4 (12) | H31—C3—H32 | 107.6 |
H1—N1—H3 | 109.4 (17) | C3—C4—S1 | 109.19 (12) |
H2—N1—H3 | 103.2 (16) | C3—C4—H41 | 109.8 |
O1—C1—O2 | 126.16 (13) | S1—C4—H41 | 109.8 |
O1—C1—C2 | 117.06 (12) | C3—C4—H42 | 109.8 |
O2—C1—C2 | 116.66 (13) | S1—C4—H42 | 109.8 |
N1—C2—C1 | 108.95 (12) | H41—C4—H42 | 108.3 |
N1—C2—C3 | 110.76 (11) | S1—C5—H51 | 109.5 |
C1—C2—C3 | 108.45 (12) | S1—C5—H52 | 109.5 |
N1—C2—H21 | 109.6 | H51—C5—H52 | 109.5 |
C1—C2—H21 | 109.6 | S1—C5—H53 | 109.5 |
C3—C2—H21 | 109.6 | H51—C5—H53 | 109.5 |
C4—C3—C2 | 114.47 (13) | H52—C5—H53 | 109.5 |
| | | |
O1—C1—C2—N1 | −31.43 (17) | C2—C3—C4—S1 | −179.16 (12) |
O2—C1—C2—N1 | 152.41 (13) | C3—C4—S1—C5 | −174.55 (16) |
O1—C1—C2—C3 | 89.21 (16) | H1—N1—C2—C1 | −44.4 (13) |
O2—C1—C2—C3 | −86.95 (16) | H2—N1—C2—C1 | 76.5 (13) |
N1—C2—C3—C4 | −55.52 (18) | H3—N1—C2—C1 | −167.3 (14) |
C1—C2—C3—C4 | −175.03 (14) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.90 (2) | 1.88 (2) | 2.7732 (17) | 173.7 (19) |
N1—H2···O2ii | 0.92 (2) | 1.92 (2) | 2.8264 (18) | 171.0 (18) |
N1—H3···O2iii | 0.90 (2) | 1.94 (2) | 2.7973 (18) | 159.2 (18) |
Symmetry codes: (i) −x+1/2, y+1/2, −z+1/2; (ii) x, −y+1, z+1/2; (iii) x, −y+2, z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O1i | 0.90 (2) | 1.88 (2) | 2.7732 (17) | 173.7 (19) |
N1—H2···O2ii | 0.92 (2) | 1.92 (2) | 2.8264 (18) | 171.0 (18) |
N1—H3···O2iii | 0.90 (2) | 1.94 (2) | 2.7973 (18) | 159.2 (18) |
Symmetry codes: (i) −x+1/2, y+1/2, −z+1/2; (ii) x, −y+1, z+1/2; (iii) x, −y+2, z+1/2. |
Selected torsion angles (°) topTorsion angle | β-DL-Met, 320 K | α-DL-Met, 340 Ka | α-DL-Met, 340 Kb |
N1—C2—C3—C4 | -55.52 (18) | -59.3 (4) | 73 (8) |
C1—C2—C3—C4 | -175.03(14 | -178.0 (2) | -78 (5) |
C2—C3—C4—S1 | -179.16 (12) | 176.7 (2) | 178 (5) |
C3—C4—S1—C5 | -174.55 (16) | 69.4 (3) | 60 (3) |
Notes: (a) major conformation, occupancy 0.9509 (18) (Görbitz et
al.,
2014); (a) minor conformation, occupancy 0.0491 (18). |
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 T1. 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).