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In the title compound, C4H10NO3+C2HO4-, the DL-threonine mol­ecule exists as a cation and the oxalic acid mol­ecule in the mono-ionized state. The mol­ecules aggregate into infinite parallel layers which extend along the diagonal of the ac plane. These layers have no hydrogen-bonded interactions between them, only van der Waals interactions. The semi-oxalate ion deviates far from planarity. No classic head-to-tail hydrogen bonds are observed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536801012193/ci6046sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536801012193/ci6046Isup2.hkl
Contains datablock I

CCDC reference: 170923

Key indicators

  • Single-crystal X-ray study
  • T = 293 K
  • Mean [sigma](C-C) = 0.004 Å
  • R factor = 0.053
  • wR factor = 0.175
  • Data-to-parameter ratio = 9.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


Yellow Alert Alert Level C:
REFLT_03 From the CIF: _diffrn_reflns_theta_max 72.10 From the CIF: _reflns_number_total 1605 TEST2: Reflns within _diffrn_reflns_theta_max Count of symmetry unique reflns 1716 Completeness (_total/calc) 93.53% Alert C: < 95% complete
0 Alert Level A = Potentially serious problem
0 Alert Level B = Potential problem
1 Alert Level C = Please check

Comment top

Threonine, an essential amino acid necessary to maintain nitrogen equilibrium in the adult human, is a significant constituent of many common plant and milk proteins. It does not undergo transamination and is also potentially glucogenic. X-ray (Shoemaker et al., 1950) and neutron (Ramanadham et al., 1973) diffraction investigations on the crystals of the L-isomer have already been carried out. Recently, a precise determination of the crystal structure of L-threonine at 12 K (Janczak et al., 1997) was reported. However, the crystal structure of its racemate is not yet known since, on crystallization, DL-threonine produces a racemic mixture of the crystals of D– and L– forms (Shoemaker et al., 1950). A similar phenomenon has been observed in the case of L-allothreonine (Swaminathan & Srinivasan, 1975). The present study reports the crystal structure of a complex of DL-threonine with oxalic acid.

Fig. 1 shows the molecular structure of the title compound (I) with atom numbering scheme. The amino acid exists in the cationic form with a positively charged amino group and a protonated carboxylic acid group. The torsion angles Ψ1 (N1—C2—C1—O1) and Ψ2 (N1—C2—C1—O2) describing the torsions of the two C—O bonds around C1—C2 are -2.5 (4) and 177.2 (2)°, indicating that the carboxylic acid and the amino group lie in the same plane. Interestingly, L-threoninium cations exhibit a significant deviation from this planarity in the crystal structures of bis(L-threoninium) sulfate monohydrate (Sridhar et al., 2001), O-phospho-L-threonine and O-phospho-DL-threonine (Maniukiewicz et al., 1996). The conformation of the molecule about the Cα–Cβ bond corresponds to the staggered ethane type. The sidechain conformation is described by the torsion angles Ξ11, Ξ12 and Ξ13 of -59.0 (3), 60.1 (3) and -178°, respectively.

The oxalic acid molecule exists as a semi-oxalate anion. Unlike in the crystal structures of other similar complexes, in the present case the semi-oxalate ion deviates far from planarity as the carboxyl groups are rotated by 33.9 (3)° with respect to the C5—C6 bond. The C—O distances in the carboxylate group of the semi-oxalate anion are unequal, presumably due to the participation of one atom (O6) in one hydrogen bond (O–H···O) and the other (O7) in two hydrogen bonds (O–H···O and N–H···O). Usually, the semi-oxalate ion has a tendency to be planar and the observed departure from planarity seems to be necessitated by requirements for optimum packing within the lattice.

Fig. 2 shows the packing of molecules of (I) viewed down the b axis. The semi-oxalate ions form hydrogen-bonded strings generated by the glide plane as in DL-arginine semi-oxalate complex (Chandra et al., 1998). The threoninium and semi-oxalate ions are tied together by an infinite network of hydrogen bonds between them. The O3 (Oγ) atom participates in the hydrogen-bonding network both as an acceptor and as a donor mediating the amino acid–amino acid interactions. No classic head-to-tail hydrogen bonds are observed in the crystal structure. The molecules aggregate into infinite parallel layers which extend along diagonal of the ac plane. These layers have no hydrogen-bonded interactions among them apart from van der Waals interactions.

Experimental top

Crystals of (I) were grown as fine transparent needles from a saturated aqueous solution containing DL-threonine and oxalic acid in stoichiometric ratio. The density was determined by flotation method using a liquid mixture of xylene and bromoform.

Refinement top

The H atoms were placed at calculated positions and were allowed to ride on their respective parent atoms with HFIX instructions using SHELXL97 (Sheldrick, 1997) defaults.

Computing details top

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: CAD-4 Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. The molecular structure of (I) with atom-numbering scheme and 50% probability displacement ellipsoids.
[Figure 2] Fig. 2. Packing diagram of the molecules of (I) viewed down the b axis.
DL-threoninium oxalate top
Crystal data top
C4H10NO3+·C2HO4F(000) = 440
Mr = 209.16Dx = 1.590 Mg m3
Dm = 1.61 (2) Mg m3
Dm measured by flotation
Monoclinic, P21/nCu Kα radiation, λ = 1.54178 Å
a = 8.325 (5) ÅCell parameters from 25 reflections
b = 10.957 (4) Åθ = 9–29°
c = 10.363 (4) ŵ = 1.30 mm1
β = 112.39 (5)°T = 293 K
V = 874.0 (7) Å3Needle, colourless
Z = 40.35 × 0.21 × 0.15 mm
Data collection top
Enraf-Nonius CAD-4
diffractometer
1252 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.000
Graphite monochromatorθmax = 72.1°, θmin = 5.8°
ω–2θ scansh = 109
Absorption correction: ψ scan
(North et al., 1968)
k = 013
Tmin = 0.76, Tmax = 0.82l = 012
1605 measured reflections2 standard reflections every 60 min
1605 independent reflections intensity decay: <2%
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.053Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.175H-atom parameters constrained
S = 1.10 w = 1/[σ2(Fo2) + (0.1106P)2 + 0.3459P]
where P = (Fo2 + 2Fc2)/3
1605 reflections(Δ/σ)max < 0.001
171 parametersΔρmax = 0.36 e Å3
0 restraintsΔρmin = 0.30 e Å3
Crystal data top
C4H10NO3+·C2HO4V = 874.0 (7) Å3
Mr = 209.16Z = 4
Monoclinic, P21/nCu Kα radiation
a = 8.325 (5) ŵ = 1.30 mm1
b = 10.957 (4) ÅT = 293 K
c = 10.363 (4) Å0.35 × 0.21 × 0.15 mm
β = 112.39 (5)°
Data collection top
Enraf-Nonius CAD-4
diffractometer
1252 reflections with I > 2σ(I)
Absorption correction: ψ scan
(North et al., 1968)
Rint = 0.000
Tmin = 0.76, Tmax = 0.822 standard reflections every 60 min
1605 measured reflections intensity decay: <2%
1605 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0530 restraints
wR(F2) = 0.175H-atom parameters constrained
S = 1.10Δρmax = 0.36 e Å3
1605 reflectionsΔρmin = 0.30 e Å3
171 parameters
Special details top

Experimental. The low completeness of data is due to very poor diffraction by the crystal at high angles.

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. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O10.1063 (3)0.88436 (19)0.5225 (2)0.0431 (6)
O20.2280 (3)1.0062 (2)0.4105 (2)0.0458 (6)
H20.14160.98810.34190.069*
O30.2232 (3)1.18732 (18)0.6792 (2)0.0425 (6)
H30.13791.16630.61130.064*
O40.3107 (3)0.72656 (19)0.4016 (2)0.0432 (6)
H40.21650.73240.33620.065*
O50.1712 (3)0.5840 (2)0.4736 (2)0.0449 (6)
O60.5377 (3)0.73866 (19)0.6711 (2)0.0452 (6)
O70.4884 (3)0.53842 (18)0.6839 (2)0.0452 (6)
N10.3322 (3)0.9414 (2)0.7730 (3)0.0371 (6)
H1A0.41640.96530.85200.056*
H1B0.33420.86050.76620.056*
H1C0.22990.96480.77300.056*
C10.2180 (4)0.9568 (2)0.5210 (3)0.0364 (7)
C20.3595 (4)0.9977 (2)0.6520 (3)0.0343 (7)
H2A0.46930.96620.65120.041*
C30.3769 (4)1.1372 (2)0.6686 (3)0.0346 (7)
H3A0.39121.17190.58650.041*
C40.5268 (4)1.1768 (3)0.7975 (3)0.0434 (8)
H4A0.53101.26430.80200.065*
H4B0.63331.14630.79420.065*
H4C0.51201.14510.87860.065*
C50.2987 (4)0.6477 (2)0.4915 (3)0.0355 (7)
C60.4580 (4)0.6422 (2)0.6273 (3)0.0361 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0492 (13)0.0320 (11)0.0506 (13)0.0079 (9)0.0219 (11)0.0050 (9)
O20.0612 (15)0.0328 (11)0.0433 (13)0.0107 (9)0.0197 (11)0.0012 (9)
O30.0505 (13)0.0236 (10)0.0548 (14)0.0018 (8)0.0215 (11)0.0051 (9)
O40.0505 (13)0.0381 (11)0.0424 (12)0.0014 (9)0.0191 (11)0.0100 (9)
O50.0485 (13)0.0394 (12)0.0429 (13)0.0120 (9)0.0132 (10)0.0015 (9)
O60.0524 (13)0.0329 (11)0.0472 (13)0.0083 (9)0.0156 (11)0.0039 (9)
O70.0599 (14)0.0294 (10)0.0370 (13)0.0035 (9)0.0081 (11)0.0040 (8)
N10.0504 (15)0.0194 (10)0.0425 (14)0.0005 (9)0.0189 (12)0.0003 (9)
C10.0449 (16)0.0213 (12)0.0464 (17)0.0026 (11)0.0211 (14)0.0039 (11)
C20.0424 (15)0.0210 (12)0.0432 (17)0.0013 (10)0.0205 (14)0.0031 (10)
C30.0413 (15)0.0234 (13)0.0410 (16)0.0049 (10)0.0179 (14)0.0027 (10)
C40.0541 (19)0.0305 (14)0.0495 (19)0.0095 (13)0.0242 (16)0.0053 (13)
C50.0495 (16)0.0229 (12)0.0386 (16)0.0021 (11)0.0219 (14)0.0004 (10)
C60.0460 (16)0.0275 (13)0.0403 (17)0.0007 (11)0.0228 (14)0.0015 (11)
Geometric parameters (Å, º) top
O1—C11.227 (4)N1—H1B0.8900
O2—C11.298 (4)N1—H1C0.8900
O2—H20.8200C1—C21.488 (4)
O3—C31.434 (3)C2—C31.539 (4)
O3—H30.8200C2—H2A0.9800
O4—C51.302 (3)C3—C41.504 (5)
O4—H40.8200C3—H3A0.9800
O5—C51.224 (4)C4—H4A0.9600
O6—C61.239 (4)C4—H4B0.9600
O7—C61.260 (3)C4—H4C0.9600
N1—C21.490 (4)C5—C61.523 (5)
N1—H1A0.8900
C1—O2—H2109.5O3—C3—C4106.5 (2)
C3—O3—H3109.5O3—C3—C2109.9 (2)
C5—O4—H4109.5C4—C3—C2113.4 (3)
C2—N1—H1A109.5O3—C3—H3A109.0
C2—N1—H1B109.5C4—C3—H3A109.0
H1A—N1—H1B109.5C2—C3—H3A109.0
C2—N1—H1C109.5C3—C4—H4A109.5
H1A—N1—H1C109.5C3—C4—H4B109.5
H1B—N1—H1C109.5H4A—C4—H4B109.5
O1—C1—O2125.8 (3)C3—C4—H4C109.5
O1—C1—C2121.6 (3)H4A—C4—H4C109.5
O2—C1—C2112.6 (2)H4B—C4—H4C109.5
C1—C2—N1108.8 (2)O5—C5—O4124.6 (3)
C1—C2—C3114.2 (2)O5—C5—C6121.1 (3)
N1—C2—C3110.8 (2)O4—C5—C6114.2 (2)
C1—C2—H2A107.6O6—C6—O7127.9 (3)
N1—C2—H2A107.6O6—C6—C5117.4 (3)
C3—C2—H2A107.6O7—C6—C5114.6 (3)
O1—C1—C2—N12.5 (4)C1—C2—C3—C4176.7 (2)
O2—C1—C2—N1177.2 (2)N1—C2—C3—C460.1 (3)
O1—C1—C2—C3126.8 (3)O5—C5—C6—O6144.2 (3)
O2—C1—C2—C352.9 (3)O4—C5—C6—O634.3 (4)
C1—C2—C3—O364.2 (3)O5—C5—C6—O733.4 (4)
N1—C2—C3—O359.0 (3)O4—C5—C6—O7148.2 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O7i0.821.672.483 (4)168
O3—H3···O1ii0.822.052.854 (4)165
O4—H4···O6i0.821.822.623 (4)167
N1—H1A···O5iii0.892.082.810 (4)138
N1—H1A···O5iv0.892.553.065 (3)118
N1—H1B···O60.892.633.217 (3)125
N1—H1B···O3v0.892.092.895 (3)151
N1—H1C···O7iv0.892.183.060 (4)168
C2—H2A···O60.982.553.174 (4)122
Symmetry codes: (i) x1/2, y+3/2, z1/2; (ii) x, y+2, z+1; (iii) x+1/2, y+3/2, z+1/2; (iv) x+1/2, y+1/2, z+3/2; (v) x+1/2, y1/2, z+3/2.

Experimental details

Crystal data
Chemical formulaC4H10NO3+·C2HO4
Mr209.16
Crystal system, space groupMonoclinic, P21/n
Temperature (K)293
a, b, c (Å)8.325 (5), 10.957 (4), 10.363 (4)
β (°) 112.39 (5)
V3)874.0 (7)
Z4
Radiation typeCu Kα
µ (mm1)1.30
Crystal size (mm)0.35 × 0.21 × 0.15
Data collection
DiffractometerEnraf-Nonius CAD-4
diffractometer
Absorption correctionψ scan
(North et al., 1968)
Tmin, Tmax0.76, 0.82
No. of measured, independent and
observed [I > 2σ(I)] reflections
1605, 1605, 1252
Rint0.000
(sin θ/λ)max1)0.617
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.053, 0.175, 1.10
No. of reflections1605
No. of parameters171
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.36, 0.30

Computer programs: CAD-4 Software (Enraf-Nonius, 1989), CAD-4 Software, SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), PLATON (Spek, 1999), SHELXL97.

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2···O7i0.821.672.483 (4)168
O3—H3···O1ii0.822.052.854 (4)165
O4—H4···O6i0.821.822.623 (4)167
N1—H1A···O5iii0.892.082.810 (4)138
N1—H1A···O5iv0.892.553.065 (3)118
N1—H1B···O60.892.633.217 (3)125
N1—H1B···O3v0.892.092.895 (3)151
N1—H1C···O7iv0.892.183.060 (4)168
C2—H2A···O60.982.553.174 (4)122
Symmetry codes: (i) x1/2, y+3/2, z1/2; (ii) x, y+2, z+1; (iii) x+1/2, y+3/2, z+1/2; (iv) x+1/2, y+1/2, z+3/2; (v) x+1/2, y1/2, z+3/2.
 

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