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Crystals of anhydrous L-aspargine, C4H8N2O3, were obtained from a saturated aqueous solution. The mol­ecules are in their zwitterionic form. Although the carboxyl group is deprotonated, the distances of the two C[pdbond]O bonds are significantly different [1.2407 (19) and 1.262 (2) Å], due to different hydrogen-bond environments. The conformation of the side chain is trans, which distinguishes it significantly from that of L-asparagine monohydrate.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807039505/zl2057sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536807039505/zl2057Isup2.hkl
Contains datablock I

CCDC reference: 660286

Key indicators

  • Single-crystal X-ray study
  • T = 90 K
  • Mean [sigma](C-C) = 0.002 Å
  • R factor = 0.032
  • wR factor = 0.085
  • Data-to-parameter ratio = 7.6

checkCIF/PLATON results

No syntax errors found



Alert level C PLAT063_ALERT_3_C Crystal Probably too Large for Beam Size ....... 0.65 mm PLAT089_ALERT_3_C Poor Data / Parameter Ratio (Zmax .LT. 18) ..... 7.59 PLAT353_ALERT_3_C Long N-H Bond (0.87A) N1 - H1NB ... 1.01 Ang. PLAT720_ALERT_4_C Number of Unusual/Non-Standard Label(s) ........ 5
Alert level G REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is correct. If it is not, please give the correct count in the _publ_section_exptl_refinement section of the submitted CIF. From the CIF: _diffrn_reflns_theta_max 30.09 From the CIF: _reflns_number_total 865 Count of symmetry unique reflns 868 Completeness (_total/calc) 99.65% TEST3: Check Friedels for noncentro structure Estimate of Friedel pairs measured 0 Fraction of Friedel pairs measured 0.000 Are heavy atom types Z>Si present no PLAT791_ALERT_1_G Confirm the Absolute Configuration of C1 = . S PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints ....... 1
0 ALERT level A = In general: serious problem 0 ALERT level B = Potentially serious problem 4 ALERT level C = Check and explain 3 ALERT level G = General alerts; check 1 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 0 ALERT type 2 Indicator that the structure model may be wrong or deficient 4 ALERT type 3 Indicator that the structure quality may be low 2 ALERT type 4 Improvement, methodology, query or suggestion 0 ALERT type 5 Informative message, check

Comment top

L-Asparagine is one of the fundamental natural amino acid residues in proteins. It has been believed that it plays an important role in the formation of the secondary structures in proteins due to the fact that the side chain can form efficient hydrogen bonds with the peptide backbone. In general, amino acids very often have polymorphs. The crystal structures of L-asparagine monohydrate (Kartha & de Vries, 1961; Verbist et al., 1972; Ramanadham et al., 1972; Wang et al., 1985; Weisinger-Lewin et al., 1989; Smirnova et al., 1990; Arnold et al., 2000; Flaig et al., 2002; Chandrasekhar et al., 2003) and D-asparagine monohydrate (Chandrasekhar et al., 2003) have been reported so far. A powder X-ray diffraction study (PDF:37–1659) has been also reported for anhydrous L-asparagine. In the present study, a single-crystal structure determination of anhydrous L-asparagine, (I), is reported.

The single-crystal diffraction analysis confirms the space group and the unit-cell dimensions previously proposed by the powder diffraction study, and shows that, as expected, the title molecule exists as a zwitter ion in the crystal (Fig. 1). The distances of the CO bonds in the carboxylate group are significantly different although the group is deprotonated. The corresponding distances are 1.2407 (19) and 1.262 (2) Å for C2—O1 and C2—O2, respectively. The discrepancy is attributed to the number and kind of the intermolecular hydrogen bonds each O atom of the carboxylate participates in. The O2 atom forms two strong hydrogen bonds with neighboring cationic ammonium groups. O1, on the other hand, forms only one relatively weak hydrogen bond with the neutral amide group (Table 2 and Fig. 2). Owing to the formation of two strong hydrogen bonds, the C1—O2 bond is strongly polarized, and the distance of the C1—O2 bond is elongated accordingly. The carbonyl oxygen in the side chain, O3, also forms two hydrogen bonds with each one ammonium and amide group of neighboring molecules.

It is of interest to compare the present structure with that of L-asparagine monohydrate (Ramanadham et al., 1972). In the L-asparagine monohydrate crystal, the CO bonds in the ionized carboxyl group are 1.243 and 1.257 Å, which is in good agreement with those in (I), but with a slightly less pronounced difference in C—O bond lengths. Both oxygen atoms in the monohydrate exhibit each one relatively weak N—H···O hydrogen bond to an amide group, but the oxygen atom with the longer C—O distance forms two additional strong H bonds with solvate water molecules. The oxygen atom with the shorter C—O bond, on the other hand, forms only one strong hydrogen bond, in this case to the ammonium group. As the difference in the hydrogen bonding environment is thus less pronouced for the monohydrate than in the anhydrous structure this may also explain the more pronounced difference in the C—O distances found in the structure of the title compound.

The conformation of the backbone of (I) is quite different from that of the monohydrate. In (I), the torsion angle of C2—C1—C3—C4 is 170.64 (14)°, while, in the monohydrate, the corresponding angle is -53.08°. As mentioned, there are significant differences between the crystal structures and the side-chain conformations of anhydrous and monohydrate asparagines, which can be attributed most likely to the different hydrogen bonding environment induced by the presence of the water molecules. Similar differences are also found in the crystal structures of L-aspartic acid (Derissen et al., 1968) and L-aspartic acid monohydrate (Umadevi et al., 2003). The corresponding torsion angles of the side-chains, for example, are 178.2° and 52.8°, for L-aspartic acid and its monohydrate, respectively.

Related literature top

For related literature on single-crystal diffraction studies of L-asparagine monohydrate, see: Arnold et al. (2000); Flaig et al. (2002); Kartha & de Vries (1961); Ramanadham et al. (1972); Smirnova et al. (1990); Verbist et al. (1972); Wang et al. (1985); Weisinger-Lewin et al. (1989). The unit cell and space group of the title compound were previously determined by powder X-ray diffraction (PDF: 37–1659). For the sample preparation of the title compound, see Yamada et al. (2007)·For other related literature, see: Chandrasekhar et al. (2003); Derissen et al. (1968); Umadevi et al. (2003).

Experimental top

The title compound, asparagine oxygen-17 isotope enriched at the carboxyl group, was synthesized with the aim to perform solid-state 17O NMR experiments. L-Asparagine was obtained by deprotection of both the N-terminus and side-chain groups from 17O-enriched N-α-Fmoc-N-β-trityl-L-asparagine. Detailed procedures have been described elsewhere (Yamada et al., 2007).

Colorless crystals of L-asparagine monohydrate can be obtained by slow cooling of an aqueous solution (Verbist et al., 1972; Ramanadham et al., 1972). Colorless platelike crystals of anhydrous L-asparagine used in the present study, on the other hand, were obtained from a saturated aqueous solution after it was left standing at room temperature for a few months.

Refinement top

All H atoms were found in difference density Fourier maps. Their positions and isotropic displacement parameters were freely refined. The refined C—H and N—H bond lengths are in the expected range: 1.00 (3) Å and 107.6 (18)–109.8 (15)° for the methyne C—H distance and the C/N—C—H angle, respectively; 1.01 (3)–1.03 (3) Å, 107.3 (15)–110.9 (18)° and 110 (2)° for the methylene C—H, C—C—H and H—C—H values, respectively; 0.92 (4)–1.01 (3) Å, 109 (2)–114.5 (18)° and 105 (2)–112 (3)° for the ammonium N—H, C—N—H and H—N—H values, respectively; 0.92 (3)–0.94 (3) Å, 118.6 (15)–120.2 (16)° and 121 (2)° for the amide N—H, C—N—H and H—N—H values, respectively. The range of the Uiso values for the H atoms is 0.020 (7)–0.044 (9) Å2.

Structure description top

L-Asparagine is one of the fundamental natural amino acid residues in proteins. It has been believed that it plays an important role in the formation of the secondary structures in proteins due to the fact that the side chain can form efficient hydrogen bonds with the peptide backbone. In general, amino acids very often have polymorphs. The crystal structures of L-asparagine monohydrate (Kartha & de Vries, 1961; Verbist et al., 1972; Ramanadham et al., 1972; Wang et al., 1985; Weisinger-Lewin et al., 1989; Smirnova et al., 1990; Arnold et al., 2000; Flaig et al., 2002; Chandrasekhar et al., 2003) and D-asparagine monohydrate (Chandrasekhar et al., 2003) have been reported so far. A powder X-ray diffraction study (PDF:37–1659) has been also reported for anhydrous L-asparagine. In the present study, a single-crystal structure determination of anhydrous L-asparagine, (I), is reported.

The single-crystal diffraction analysis confirms the space group and the unit-cell dimensions previously proposed by the powder diffraction study, and shows that, as expected, the title molecule exists as a zwitter ion in the crystal (Fig. 1). The distances of the CO bonds in the carboxylate group are significantly different although the group is deprotonated. The corresponding distances are 1.2407 (19) and 1.262 (2) Å for C2—O1 and C2—O2, respectively. The discrepancy is attributed to the number and kind of the intermolecular hydrogen bonds each O atom of the carboxylate participates in. The O2 atom forms two strong hydrogen bonds with neighboring cationic ammonium groups. O1, on the other hand, forms only one relatively weak hydrogen bond with the neutral amide group (Table 2 and Fig. 2). Owing to the formation of two strong hydrogen bonds, the C1—O2 bond is strongly polarized, and the distance of the C1—O2 bond is elongated accordingly. The carbonyl oxygen in the side chain, O3, also forms two hydrogen bonds with each one ammonium and amide group of neighboring molecules.

It is of interest to compare the present structure with that of L-asparagine monohydrate (Ramanadham et al., 1972). In the L-asparagine monohydrate crystal, the CO bonds in the ionized carboxyl group are 1.243 and 1.257 Å, which is in good agreement with those in (I), but with a slightly less pronounced difference in C—O bond lengths. Both oxygen atoms in the monohydrate exhibit each one relatively weak N—H···O hydrogen bond to an amide group, but the oxygen atom with the longer C—O distance forms two additional strong H bonds with solvate water molecules. The oxygen atom with the shorter C—O bond, on the other hand, forms only one strong hydrogen bond, in this case to the ammonium group. As the difference in the hydrogen bonding environment is thus less pronouced for the monohydrate than in the anhydrous structure this may also explain the more pronounced difference in the C—O distances found in the structure of the title compound.

The conformation of the backbone of (I) is quite different from that of the monohydrate. In (I), the torsion angle of C2—C1—C3—C4 is 170.64 (14)°, while, in the monohydrate, the corresponding angle is -53.08°. As mentioned, there are significant differences between the crystal structures and the side-chain conformations of anhydrous and monohydrate asparagines, which can be attributed most likely to the different hydrogen bonding environment induced by the presence of the water molecules. Similar differences are also found in the crystal structures of L-aspartic acid (Derissen et al., 1968) and L-aspartic acid monohydrate (Umadevi et al., 2003). The corresponding torsion angles of the side-chains, for example, are 178.2° and 52.8°, for L-aspartic acid and its monohydrate, respectively.

For related literature on single-crystal diffraction studies of L-asparagine monohydrate, see: Arnold et al. (2000); Flaig et al. (2002); Kartha & de Vries (1961); Ramanadham et al. (1972); Smirnova et al. (1990); Verbist et al. (1972); Wang et al. (1985); Weisinger-Lewin et al. (1989). The unit cell and space group of the title compound were previously determined by powder X-ray diffraction (PDF: 37–1659). For the sample preparation of the title compound, see Yamada et al. (2007)·For other related literature, see: Chandrasekhar et al. (2003); Derissen et al. (1968); Umadevi et al. (2003).

Computing details top

Data collection: CrystalClear SM (Rigaku/MSC Inc., 2005); cell refinement: CrystalClear SM; data reduction: HKL-2000 (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR2004 (Burla et al., 2005); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1] Fig. 1. A view of the molecular structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.
[Figure 2] Fig. 2. A packing diagram of (I). Broken lines indicate the hydrogen bonds.
L-Asparagine top
Crystal data top
C4H8N2O3F(000) = 140
Mr = 132.12Dx = 1.607 Mg m3
Monoclinic, P21Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ybCell parameters from 1254 reflections
a = 5.0622 (4) Åθ = 2.5–32.2°
b = 6.7001 (5) ŵ = 0.14 mm1
c = 8.0543 (5) ÅT = 90 K
β = 91.706 (5)°Plate, colourless
V = 273.06 (3) Å30.65 × 0.36 × 0.08 mm
Z = 2
Data collection top
AFC-8 with Saturn70 CCD
diffractometer
815 reflections with I > 2σ(I)
Radiation source: fine-focus rotating anodeRint = 0.045
Confocal monochromatorθmax = 30.1°, θmin = 2.5°
Detector resolution: 28.5714 pixels mm-1h = 77
ω scansk = 99
3379 measured reflectionsl = 1111
865 independent reflections
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)] = 0.032Hydrogen site location: difference Fourier map
wR(F2) = 0.085All H-atom parameters refined
S = 1.08 w = 1/[σ2(Fo2) + (0.0469P)2 + 0.0383P]
where P = (Fo2 + 2Fc2)/3
865 reflections(Δ/σ)max < 0.001
114 parametersΔρmax = 0.22 e Å3
1 restraintΔρmin = 0.29 e Å3
Crystal data top
C4H8N2O3V = 273.06 (3) Å3
Mr = 132.12Z = 2
Monoclinic, P21Mo Kα radiation
a = 5.0622 (4) ŵ = 0.14 mm1
b = 6.7001 (5) ÅT = 90 K
c = 8.0543 (5) Å0.65 × 0.36 × 0.08 mm
β = 91.706 (5)°
Data collection top
AFC-8 with Saturn70 CCD
diffractometer
815 reflections with I > 2σ(I)
3379 measured reflectionsRint = 0.045
865 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0321 restraint
wR(F2) = 0.085All H-atom parameters refined
S = 1.08Δρmax = 0.22 e Å3
865 reflectionsΔρmin = 0.29 e Å3
114 parameters
Special details top

Experimental. All Friedel pairs were merged, and all f"s of containing atoms were set to zero.

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 > 2σ(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
O10.2240 (2)0.1487 (2)0.59976 (15)0.0152 (3)
O20.1823 (2)0.1359 (2)0.48234 (15)0.0146 (3)
O30.5526 (2)0.0384 (3)0.00341 (16)0.0159 (3)
N10.4009 (3)0.3459 (2)0.32483 (18)0.0113 (3)
H1NA0.353 (6)0.451 (6)0.391 (4)0.044 (9)*
H1NB0.458 (5)0.393 (5)0.212 (4)0.031 (8)*
H1NC0.563 (5)0.287 (5)0.380 (3)0.024 (7)*
N20.1172 (3)0.0801 (2)0.06008 (19)0.0137 (3)
H2NA0.151 (5)0.112 (5)0.171 (3)0.028 (7)*
H2NB0.051 (5)0.079 (5)0.022 (3)0.020 (7)*
C10.1734 (3)0.2065 (3)0.3083 (2)0.0097 (3)
H10.034 (5)0.273 (5)0.237 (3)0.019 (6)*
C20.0645 (3)0.1617 (3)0.48026 (19)0.0097 (3)
C30.2553 (4)0.0114 (3)0.2244 (2)0.0126 (3)
H3A0.420 (6)0.049 (5)0.283 (3)0.025 (7)*
H3B0.102 (5)0.084 (4)0.231 (3)0.021 (7)*
C40.3218 (3)0.0440 (3)0.04381 (19)0.0108 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0139 (6)0.0228 (7)0.0088 (5)0.0022 (6)0.0002 (4)0.0018 (5)
O20.0096 (6)0.0183 (6)0.0160 (6)0.0017 (6)0.0020 (4)0.0045 (5)
O30.0106 (7)0.0229 (7)0.0143 (5)0.0015 (5)0.0016 (4)0.0015 (5)
N10.0128 (7)0.0115 (7)0.0097 (6)0.0022 (6)0.0017 (5)0.0008 (5)
N20.0127 (7)0.0190 (8)0.0096 (6)0.0011 (6)0.0013 (5)0.0007 (5)
C10.0090 (7)0.0120 (7)0.0081 (6)0.0011 (6)0.0013 (5)0.0016 (5)
C20.0116 (8)0.0076 (8)0.0099 (7)0.0007 (6)0.0024 (5)0.0007 (5)
C30.0166 (8)0.0123 (8)0.0091 (6)0.0016 (7)0.0028 (5)0.0004 (6)
C40.0129 (8)0.0094 (8)0.0103 (7)0.0010 (6)0.0018 (5)0.0011 (6)
Geometric parameters (Å, º) top
O1—C21.2407 (19)N2—H2NA0.94 (3)
O2—C21.262 (2)N2—H2NB0.92 (3)
O3—C41.240 (2)C1—C31.535 (3)
N1—C11.485 (2)C1—C21.536 (2)
N1—H1NA0.92 (4)C1—H11.00 (3)
N1—H1NB1.01 (3)C3—C41.518 (2)
N1—H1NC1.00 (3)C3—H3A1.03 (3)
N2—C41.334 (2)C3—H3B1.01 (3)
C1—N1—H1NA109 (2)C2—C1—H1109.8 (15)
C1—N1—H1NB111.0 (18)O1—C2—O2126.99 (15)
H1NA—N1—H1NB112 (3)O1—C2—C1118.07 (14)
C1—N1—H1NC114.5 (18)O2—C2—C1114.90 (13)
H1NA—N1—H1NC106 (3)C4—C3—C1111.70 (14)
H1NB—N1—H1NC105 (2)C4—C3—H3A107.3 (15)
C4—N2—H2NA118.6 (15)C1—C3—H3A110.9 (18)
C4—N2—H2NB120.2 (16)C4—C3—H3B109.5 (15)
H2NA—N2—H2NB121 (2)C1—C3—H3B107.4 (15)
N1—C1—C3110.86 (14)H3A—C3—H3B110 (2)
N1—C1—C2109.90 (14)O3—C4—N2122.28 (15)
C3—C1—C2109.84 (14)O3—C4—C3121.80 (14)
N1—C1—H1107.6 (18)N2—C4—C3115.92 (15)
C3—C1—H1108.8 (17)
N1—C1—C2—O136.1 (2)N1—C1—C3—C467.71 (17)
C3—C1—C2—O186.1 (2)C2—C1—C3—C4170.64 (14)
N1—C1—C2—O2145.97 (16)C1—C3—C4—O3107.35 (19)
C3—C1—C2—O291.81 (18)C1—C3—C4—N272.2 (2)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1NA···O2i0.92 (4)1.83 (4)2.741 (2)167 (3)
N1—H1NB···O3ii1.01 (3)1.94 (3)2.908 (2)159 (3)
N1—H1NC···O2iii1.00 (3)1.82 (3)2.807 (2)169 (3)
N2—H2NA···O1iv0.94 (3)1.91 (3)2.8456 (19)174 (3)
N2—H2NB···O3v0.92 (3)2.03 (3)2.921 (2)163 (2)
Symmetry codes: (i) x, y+1/2, z+1; (ii) x+1, y+1/2, z; (iii) x+1, y, z; (iv) x, y, z1; (v) x1, y, z.

Experimental details

Crystal data
Chemical formulaC4H8N2O3
Mr132.12
Crystal system, space groupMonoclinic, P21
Temperature (K)90
a, b, c (Å)5.0622 (4), 6.7001 (5), 8.0543 (5)
β (°) 91.706 (5)
V3)273.06 (3)
Z2
Radiation typeMo Kα
µ (mm1)0.14
Crystal size (mm)0.65 × 0.36 × 0.08
Data collection
DiffractometerAFC-8 with Saturn70 CCD
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
3379, 865, 815
Rint0.045
(sin θ/λ)max1)0.705
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.032, 0.085, 1.08
No. of reflections865
No. of parameters114
No. of restraints1
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)0.22, 0.29

Computer programs: CrystalClear SM (Rigaku/MSC Inc., 2005), CrystalClear SM, HKL-2000 (Otwinowski & Minor, 1997), SIR2004 (Burla et al., 2005), SHELXL97 (Sheldrick, 1997), ORTEP-3 for Windows (Farrugia, 1997), SHELXL97.

Selected geometric parameters (Å, º) top
O1—C21.2407 (19)O3—C41.240 (2)
O2—C21.262 (2)
C2—C1—C3—C4170.64 (14)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1NA···O2i0.92 (4)1.83 (4)2.741 (2)167 (3)
N1—H1NB···O3ii1.01 (3)1.94 (3)2.908 (2)159 (3)
N1—H1NC···O2iii1.00 (3)1.82 (3)2.807 (2)169 (3)
N2—H2NA···O1iv0.94 (3)1.91 (3)2.8456 (19)174 (3)
N2—H2NB···O3v0.92 (3)2.03 (3)2.921 (2)163 (2)
Symmetry codes: (i) x, y+1/2, z+1; (ii) x+1, y+1/2, z; (iii) x+1, y, z; (iv) x, y, z1; (v) x1, y, z.
 

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