Redetermined crystal structure of α-dl-me-thio-nine at 340 K.

Two forms, α and β, are known for the racemic amino acid dl-me-thio-nine, C5H11NO2S. The phase transition between them, taking place around 326 K, is associated with sliding at the central inter-faces of the hydro-phobic regions in the crystal, leaving the hydrogen-bonding pattern unperturbed. For the high-temperature α phase, only a structure of rather low quality has been available [R factor = 0.118, no H-atom coordinates; Taniguchi et al. (1980 ▶). Bull. Chem. Soc. Jpn, 53, 803-804]. We here present accurate structural data for this polymorph [R(F) = 0.049], which are compared with other related amino acid structures with similar properties. We report for the first time that the side chain of this phase has a minor disorder component [occupancy 0.0491 (18)] with a gauche+ rather than a gauche- conformation for the N-C-C-C group. In the crystal of the title compound, N-H⋯O hydrogen bonds link the mol-ecules into (100) sheets.

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 = –CH2CH3; Görbitz et al., 2012 ▶), dl-norvaline (dl-Nva, –CH2CH2CH3; Görbitz, 2011 ▶), dl-norleucine (dl-Nle, –CH2CH2CH2CH3; Coles et al., 2009 ▶) and dl-me­thio­nine (dl-Met, –CH2CH2SCH3). 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 β (low T) to the α 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 β 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 ▶). α-dl-Met, (I), however, remained one of the few structures of the standard amino acids for which no high-precision experimental data were available (Görbitz, 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 χ1(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.

Figure 1

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 gauche− orientation (Table 1 ▶), is shown in a lighter colour.

Table 1

Selected torsion angles ()

N1C2C3C4	59.3(4)	N1C2BC3BC4B	73(8)
C2C3C4S1	176.7(2)	C2BC3BC4BS1B	178(5)
C3C4S1C5	69.4(3)	C3BC4BS1BC5B	60(3)

The crystal packing of (I) is shown in Fig. 2 ▶(a) and may be compared with the structure of β-dl-Met in Fig. 2 ▶(b) (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 β 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, N1⋯O2 distances in Table 1 ▶ are 0.03 Å longer, while N1⋯O1 is 0.01 Å shorter. All H⋯A distances surprisingly appear to get shorter at 340 K, but this is an artefact resulting from different ways of handling the amino group (Görbitz, 2014 ▶). In the refinement of β-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.

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 β-dl-Met at 105 K (Alagar et al., 2005 ▶).

Supra­molecular features

Hydrogen-bond geometries are listed in Table 2 ▶. The hydrogen-bonding patterns of all compounds discussed here belong to the ld–ld type (Görbitz et al., 2009 ▶), normally 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 (Görbitz et al., 2009 ▶). Apart from a weak Cα—H⋯O contact along the b axis, all inter­molecular inter­actions within a single sheet involve amino acids of opposite chirality (Fig. 3 ▶); two N—H⋯O inter­actions between amino acids of the same chirality serve to link the adjacent anti­parallel sheets that form a double-sheet hydrogen-bonded layer.

Table 2

Hydrogen-bond geometry (, )

DHA	DH	HA	D A	DHA
N1H1O2i	0.88(3)	1.95(3)	2.812(2)	164(2)
N1H2O2ii	0.92(3)	1.94(3)	2.843(2)	168(2)
N1H3O1iii	0.93(3)	1.86(3)	2.785(2)	171(2)
C2H21O1iv	0.98	2.46	3.264(3)	140

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 3

Hydrogen-bonded sheet of (I). Colour coding as in Fig. 2 ▶, except that H3 atoms connecting sheets appear in yellow. The side chains are shown as small spheres. A single l-Met mol­ecule of the adjacent sheet is shown in black wireframe representation. O2i is at (x, −y + , z − ), O2ii at (x, −y + , z − ) and O1iii at (−x + 1, y − , −z + ) (Table 2 ▶). The blue arrow has the same meaning as in Fig. 2 ▶.

Synthesis and crystallization

From a saturated solution of dl-Met in water (approximately 30 mg ml−1) 50 µl was pipetted into a 40 × 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.

Table 3

Experimental details

Crystal data
Chemical formula	C5H11NO2S
M r	149.21
Crystal system, space group	Monoclinic, P21/c
Temperature (K)	340
a, b, c ()	16.811(5), 4.7281(14), 9.886(3)
()	101.950(7)
V (3)	768.7(4)
Z	4
Radiation type	Mo K
(mm1)	0.35
Crystal size (mm)	0.62 0.55 0.13
Data collection
Diffractometer	Bruker D8 Vantage single crystal CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2013 ▶)
T min, T max	0.819, 1.000
No. of measured, independent and observed [I > 2(I)] reflections	15046, 1513, 1332
R int	0.041
(sin /)max (1)	0.617
Refinement
R[F 2 > 2(F 2)], wR(F 2), S	0.049, 0.129, 1.07
No. of reflections	1513
No. of parameters	107
No. of restraints	9
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
max, min (e 3)	0.27, 0.29

Computer programs: APEX2 and SAINT (Bruker, 2013 ▶), SHELXS2013 and SHELXL2013 (Sheldrick, 2008 ▶) and Mercury (Macrae et al., 2008 ▶).

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536814022211/hb7288sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536814022211/hb7288Isup2.hkl

Click here for additional data file.

Supporting information file. DOI: 10.1107/S1600536814022211/hb7288Isup3.cml

CCDC reference: 1028063

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

C5H11NO2S	F(000) = 320
Mr = 149.21	Dx = 1.289 Mg m−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 16.811 (5) Å	Cell parameters from 9952 reflections
b = 4.7281 (14) Å	θ = 2.5–28.3°
c = 9.886 (3) Å	µ = 0.35 mm−1
β = 101.950 (7)°	T = 340 K
V = 768.7 (4) Å3	Plate, colourless
Z = 4	0.62 × 0.55 × 0.13 mm

Bruker D8 Vantage single crystal CCD diffractometer	1513 independent reflections
Radiation source: fine-focus sealed tube	1332 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.041
Detector resolution: 8.3 pixels mm-1	θmax = 26.0°, θmin = 2.5°
Sets of exposures each taken over 0.5° ω rotation scans	h = −20→20
Absorption correction: multi-scan (SADABS; Bruker, 2013)	k = −5→5
Tmin = 0.819, Tmax = 1.000	l = −12→12
15046 measured reflections

Refinement on F2	9 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.049	H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.129	w = 1/[σ2(Fo2) + (0.0499P)2 + 0.5391P] where P = (Fo2 + 2Fc2)/3
S = 1.07	(Δ/σ)max < 0.001
1513 reflections	Δρmax = 0.27 e Å−3
107 parameters	Δρmin = −0.29 e Å−3

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. Disorder, two side chain orientations.

	x	y	z	Uiso*/Ueq	Occ. (<1)
N1	0.59254 (10)	0.4431 (4)	0.14177 (16)	0.0345 (4)
H1	0.6057 (14)	0.310 (5)	0.088 (2)	0.052*
H2	0.5958 (14)	0.614 (6)	0.099 (2)	0.052*
H3	0.5381 (16)	0.412 (5)	0.144 (2)	0.052*
O1	0.56499 (8)	0.8214 (3)	0.32823 (13)	0.0414 (4)
O2	0.62423 (10)	0.5530 (3)	0.50516 (13)	0.0492 (4)
C1	0.60684 (11)	0.6163 (4)	0.37937 (17)	0.0305 (4)
C2	0.64538 (12)	0.4338 (8)	0.2836 (3)	0.0307 (7)	0.9509 (18)
H21	0.6505	0.2385	0.3175	0.037*	0.9509 (18)
C3	0.73009 (12)	0.5543 (5)	0.2799 (2)	0.0423 (5)	0.9509 (18)
H31	0.7240	0.7509	0.2515	0.051*	0.9509 (18)
H32	0.7631	0.5497	0.3728	0.051*	0.9509 (18)
C4	0.77480 (16)	0.4004 (7)	0.1849 (3)	0.0697 (8)	0.9509 (18)
H41	0.7407	0.3955	0.0929	0.084*	0.9509 (18)
H42	0.7840	0.2067	0.2165	0.084*	0.9509 (18)
S1	0.87076 (5)	0.5570 (3)	0.17503 (10)	0.0907 (4)	0.9509 (18)
C5	0.9285 (2)	0.4833 (15)	0.3418 (5)	0.140 (2)	0.9509 (18)
H51	0.9820	0.5616	0.3507	0.211*	0.9509 (18)
H52	0.9324	0.2823	0.3552	0.211*	0.9509 (18)
H53	0.9025	0.5659	0.4100	0.211*	0.9509 (18)
C2B	0.6365 (12)	0.435 (16)	0.258 (10)	0.0307 (7)	0.0491 (18)
H22B	0.6223	0.2457	0.2854	0.037*	0.0491 (18)
C3B	0.7299 (13)	0.406 (8)	0.293 (4)	0.042*	0.0491 (18)
H33B	0.7467	0.3580	0.3903	0.050*	0.0491 (18)
H34B	0.7450	0.2491	0.2403	0.050*	0.0491 (18)
C4B	0.7757 (10)	0.665 (6)	0.265 (5)	0.069*	0.0491 (18)
H43B	0.7592	0.8227	0.3153	0.083*	0.0491 (18)
H44B	0.7602	0.7089	0.1669	0.083*	0.0491 (18)
S1B	0.8843 (9)	0.632 (5)	0.311 (2)	0.0907 (4)	0.0491 (18)
C5B	0.902 (2)	0.347 (12)	0.205 (7)	0.138*	0.0491 (18)
H54B	0.9590	0.3181	0.2150	0.207*	0.0491 (18)
H55B	0.8781	0.3901	0.1100	0.207*	0.0491 (18)
H56B	0.8770	0.1791	0.2319	0.207*	0.0491 (18)

	U11	U22	U33	U12	U13	U23
N1	0.0413 (9)	0.0351 (9)	0.0287 (8)	−0.0045 (7)	0.0110 (7)	−0.0053 (7)
O1	0.0466 (8)	0.0371 (8)	0.0425 (8)	0.0092 (6)	0.0137 (6)	−0.0001 (6)
O2	0.0812 (11)	0.0411 (8)	0.0276 (7)	0.0022 (7)	0.0165 (7)	0.0011 (6)
C1	0.0359 (9)	0.0274 (8)	0.0306 (9)	−0.0060 (7)	0.0125 (7)	−0.0012 (7)
C2	0.0386 (10)	0.0273 (9)	0.0269 (19)	0.0018 (9)	0.0084 (9)	0.0009 (10)
C3	0.0366 (11)	0.0439 (12)	0.0478 (12)	−0.0009 (9)	0.0122 (9)	−0.0041 (10)
C4	0.0514 (14)	0.080 (2)	0.0857 (19)	−0.0088 (14)	0.0323 (14)	−0.0238 (16)
S1	0.0536 (5)	0.1247 (9)	0.1036 (7)	−0.0132 (5)	0.0393 (4)	−0.0009 (6)
C5	0.057 (2)	0.234 (6)	0.127 (4)	0.021 (3)	0.013 (2)	0.005 (4)
C2B	0.0386 (10)	0.0273 (9)	0.0269 (19)	0.0018 (9)	0.0084 (9)	0.0009 (10)
S1B	0.0536 (5)	0.1247 (9)	0.1036 (7)	−0.0132 (5)	0.0393 (4)	−0.0009 (6)

N1—C2B	1.23 (8)	S1—C5	1.766 (5)
N1—C2	1.497 (3)	C5—H51	0.9600
N1—H1	0.88 (3)	C5—H52	0.9600
N1—H2	0.92 (3)	C5—H53	0.9600
N1—H3	0.93 (3)	C2B—C3B	1.542 (6)
O1—C1	1.242 (2)	C2B—H22B	0.9800
O2—C1	1.253 (2)	C3B—C4B	1.505 (6)
C1—C2	1.521 (4)	C3B—H33B	0.9700
C1—C2B	1.63 (10)	C3B—H34B	0.9700
C2—C3	1.541 (3)	C4B—S1B	1.794 (6)
C2—H21	0.9800	C4B—H43B	0.9700
C3—C4	1.506 (3)	C4B—H44B	0.9700
C3—H31	0.9700	S1B—C5B	1.765 (7)
C3—H32	0.9700	C5B—H54B	0.9600
C4—S1	1.796 (3)	C5B—H55B	0.9600
C4—H41	0.9700	C5B—H56B	0.9600
C4—H42	0.9700
C2B—N1—H1	112 (4)	C5—S1—C4	101.2 (2)
C2—N1—H1	111.8 (15)	S1—C5—H51	109.5
C2B—N1—H2	112 (3)	S1—C5—H52	109.5
C2—N1—H2	112.0 (14)	H51—C5—H52	109.5
H1—N1—H2	108 (2)	S1—C5—H53	109.5
C2B—N1—H3	112 (3)	H51—C5—H53	109.5
C2—N1—H3	111.9 (14)	H52—C5—H53	109.5
H1—N1—H3	106 (2)	N1—C2B—C3B	127 (6)
H2—N1—H3	107 (2)	N1—C2B—C1	117 (4)
O1—C1—O2	125.88 (17)	C3B—C2B—C1	110 (5)
O1—C1—C2	117.89 (17)	N1—C2B—H22B	98.6
O2—C1—C2	116.11 (18)	C3B—C2B—H22B	98.6
O1—C1—C2B	110 (2)	C1—C2B—H22B	98.6
O2—C1—C2B	124 (2)	C4B—C3B—C2B	114.8 (7)
N1—C2—C1	108.6 (2)	C4B—C3B—H33B	108.6
N1—C2—C3	109.8 (2)	C2B—C3B—H33B	108.6
C1—C2—C3	108.7 (2)	C4B—C3B—H34B	108.6
N1—C2—H21	109.9	C2B—C3B—H34B	108.6
C1—C2—H21	109.9	H33B—C3B—H34B	107.5
C3—C2—H21	109.9	C3B—C4B—S1B	114.5 (6)
C4—C3—C2	114.8 (3)	C3B—C4B—H43B	108.6
C4—C3—H31	108.6	S1B—C4B—H43B	108.6
C2—C3—H31	108.6	C3B—C4B—H44B	108.6
C4—C3—H32	108.6	S1B—C4B—H44B	108.6
C2—C3—H32	108.6	H43B—C4B—H44B	107.6
H31—C3—H32	107.5	C5B—S1B—C4B	101.5 (5)
C3—C4—S1	113.9 (2)	S1B—C5B—H54B	109.5
C3—C4—H41	108.8	S1B—C5B—H55B	109.5
S1—C4—H41	108.8	H54B—C5B—H55B	109.5
C3—C4—H42	108.8	S1B—C5B—H56B	109.5
S1—C4—H42	108.8	H54B—C5B—H56B	109.5
H41—C4—H42	107.7	H55B—C5B—H56B	109.5
N1—C2—C3—C4	−59.3 (4)	O1—C1—C2—N1	−29.4 (3)
C2—C3—C4—S1	176.7 (2)	O2—C1—C2—N1	154.35 (18)
C3—C4—S1—C5	69.4 (3)	O1—C1—C2—C3	90.0 (2)
N1—C2B—C3B—C4B	73 (8)	O2—C1—C2—C3	−86.2 (2)
C2B—C3B—C4B—S1B	178 (5)	C1—C2—C3—C4	−178.0 (2)
C3B—C4B—S1B—C5B	60 (3)	C1—C2B—C3B—C4B	−78 (5)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2i	0.88 (3)	1.95 (3)	2.812 (2)	164 (2)
N1—H2···O2ii	0.92 (3)	1.94 (3)	2.843 (2)	168 (2)
N1—H3···O1iii	0.93 (3)	1.86 (3)	2.785 (2)	171 (2)
C2—H21···O1iv	0.98	2.46	3.264 (3)	140