supplementary materials


hg5362 scheme

Acta Cryst. (2013). E69, o1856-o1857    [ doi:10.1107/S1600536813032170 ]

2-Methyl­aspartic acid monohydrate

G. Brewer, A. S. Burton, J. P. Dworkin and R. J. Butcher

Abstract top

The title compound, C5H9NO4·H2O, is an isomer of the [alpha]-amino acid glutamic acid that crystallizes from water in its zwitterionic form as a monohydrate. It is not one of the 20 proteinogenic [alpha]-amino acids that are used in living systems and differs from the natural amino acids in that it has an [alpha]-methyl group rather than an [alpha]-H atom. In the crystal, an O-H...O hydrogen bond is present between the acid and water mol­ecules while extensive N-H...O and O-H...O hydrogen bonds link the components into a three-dimensional array.

Comment top

The α-amino acids are essential for life as they are the building blocks of all proteins and enzymes. Nature uses almost exclusively the L form of the nineteen common chiral amino acids. However, there are over eighty amino acids that have been identified in meteorites (Pizzarello et al., 2006; Burton et al., 2012). One of these extraterrestrial non-proteinogenic amino acids is 2-methylaspartic acid. The majority of meteoritic amino acids show little or no enrichment of one enantiomer over the other. However, a number of alpha methyl amino acids have been observed with L-enantiomeric excesses up to 20% that are not believed to be the result of contamination (Pizzarello & Cronin, 2000; Glavin & Dworkin, 2009; Glavin et al., 2011; Glavin et al., 2012; Burton et al., 2013). An intriguing question is the process that leads to the separation and enrichment of the L enantiomer over the D. There are several possible explanations for this including the role that crystallization plays (Blackmond et al., 2007; Glavin et al., 2012). Only two of the twenty amino acids used biologically crystallize in a chiral space group from a racemic solution, which allows for spontaneous separation of enantiomers, at the level of the crystal (Blackmond et al., (2008).

Racemic 2-methylaspartic crystallizes from water in an achiral space group acid, forming a racemic compound, in which there are equal numbers of D and L enantiomers in the unit cell. Thus, crystallization under these conditions would not provide a mechanism for separation of enantiomers at the level of the crystal. Another important aspect in the prebiotic chemistry of the amino acids is the role of racemization. All of the nineteen naturally occurring chiral amino acids have a hydrogen atom on the alpha carbon atom, which enhances the rate of racemization (Yamada et al., 1983). However, little is known about the mechanism of racemization of amino acids lacking an alpha hydrogen atom (Pizzarello et al., 2011). We recently reported the structure of another non-proteinogenic amino acid, isovaline, which crystallized as a racemic conglomerate from water in contrast to the present example which crystallizes in a centro-symmetric space group and is thus a racemate as indicated above (Butcher et al., 2013). Resolved 2-methylaspartic acid and the structure given here can be used as a starting point in mechanistic studies of racemization mechanisms of amino acids lacking an alpha hydrogen atom.

In the structure of the title compound the amino acid is in the usual zwitterionic form involving the α carboxylate group and all the the bond lengths and angles are in the normal range for such compounds (Orpen, 1993). There is extensive N—H···O and O—H···O hydrogen bonding linking the zwitterions into a 3-D array.

Related literature top

For the eighty amino acids that have been detected in meteorites or comets, see: Pizzarello et al. (2006); Glavin & Dworkin, (2009); Burton et al. (2012). For the role that crystallization plays in chiral separation, see: Blackmond & Klussmann (2007); Blackmond et al. (2008). For the role of H atom on the α-C atom in enhancing the rate of racemization, see: Yamada et al. (1983). For the mechanism of racemization of amino acids lacking an α-H atom, see: Pizzarello & Groy (2011). For the role that crystallization can play in the enrichment of L isovaline and its structure, see: Glavin & Dworkin (2009); Butcher et al. (2013). For normal bond lengths and angles, see: Orpen (1993). For the number of α-methyl amino acids that have been observed with L-enantiomeric excesses up to 20% that are not believed to be the result of contamination, see: Pizzarello & Cronin (2000); Glavin & Dworkin (2009); Glavin et al. (2011, 2012); Burton et al. (2013).

Experimental top

2-Methylaspartic acid was purchased from Nagase and Co. Ltd. Crystals of the title compound were grown from slow evaporation of a racemic solution of the amino acid in water.

Refinement top

H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with a C—H distances of 0.98 and 0.99 Å. The protons on the N and O were refined isotropically with the O—H distances for the water H's constrained to be 0.82 Å and the H—O—H angle close to 104.5°.

Computing details top

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Diagram of the title compound showing atom labeling. Atomic displacement parameters are at the 30% probability level. Hydrogen bonds are shown as dashed lines.
[Figure 2] Fig. 2. Packing diagram of the title compound viewed along the c axis showing the extensive N—H···O and O—H···O hydrogen bonds as dashed lines.
2-Methylaspartic acid monohydrate top
Crystal data top
C5H9NO4·H2OF(000) = 352
Mr = 165.15Dx = 1.527 Mg m3
Monoclinic, P21/cCu Kα radiation, λ = 1.54178 Å
Hall symbol: -P 2ybcCell parameters from 3596 reflections
a = 9.9690 (6) Åθ = 3.4–77.1°
b = 12.8677 (6) ŵ = 1.20 mm1
c = 5.8409 (3) ÅT = 123 K
β = 106.491 (6)°Plate, colourless
V = 718.44 (7) Å30.49 × 0.12 × 0.04 mm
Z = 4
Data collection top
Agilent Xcalibur Ruby Gemini
diffractometer
1498 independent reflections
Radiation source: Enhance (Cu) X-ray Source1436 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.038
Detector resolution: 10.5081 pixels mm-1θmax = 77.3°, θmin = 3.4°
ω scansh = 1212
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2012)
k = 1516
Tmin = 0.682, Tmax = 1.000l = 75
5544 measured 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)] = 0.090Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.279H atoms treated by a mixture of independent and constrained refinement
S = 1.20 w = 1/[σ2(Fo2) + (0.171P)2 + 1.5626P]
where P = (Fo2 + 2Fc2)/3
1498 reflections(Δ/σ)max < 0.001
123 parametersΔρmax = 0.50 e Å3
0 restraintsΔρmin = 0.53 e Å3
Crystal data top
C5H9NO4·H2OV = 718.44 (7) Å3
Mr = 165.15Z = 4
Monoclinic, P21/cCu Kα radiation
a = 9.9690 (6) ŵ = 1.20 mm1
b = 12.8677 (6) ÅT = 123 K
c = 5.8409 (3) Å0.49 × 0.12 × 0.04 mm
β = 106.491 (6)°
Data collection top
Agilent Xcalibur Ruby Gemini
diffractometer
1498 independent reflections
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2012)
1436 reflections with I > 2σ(I)
Tmin = 0.682, Tmax = 1.000Rint = 0.038
5544 measured reflectionsθmax = 77.3°
Refinement top
R[F2 > 2σ(F2)] = 0.090H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.279Δρmax = 0.50 e Å3
S = 1.20Δρmin = 0.53 e Å3
1498 reflectionsAbsolute structure: ?
123 parametersAbsolute structure parameter: ?
0 restraintsRogers parameter: ?
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. 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
O10.1948 (3)0.4358 (2)0.7672 (5)0.0230 (7)
O20.3545 (3)0.3270 (2)0.6994 (5)0.0214 (6)
O30.3796 (3)0.5820 (2)0.5612 (5)0.0212 (6)
O40.2105 (3)0.6995 (2)0.4131 (6)0.0236 (7)
H4O0.276 (7)0.743 (6)0.457 (13)0.042 (17)*
O1W0.3772 (3)0.8584 (2)0.5403 (6)0.0246 (7)
H1W10.469 (7)0.851 (5)0.608 (12)0.033 (15)*
H1W20.380 (6)0.879 (4)0.409 (11)0.018 (12)*
N10.3151 (3)0.3888 (3)0.2516 (6)0.0183 (7)
H1A0.294 (6)0.405 (5)0.095 (11)0.027*
H1B0.332 (6)0.320 (5)0.249 (10)0.027*
H1C0.393 (6)0.422 (5)0.325 (11)0.027*
C10.2565 (4)0.3913 (3)0.6364 (7)0.0182 (8)
C20.2010 (4)0.4134 (3)0.3646 (7)0.0174 (8)
C30.0787 (4)0.3399 (3)0.2591 (7)0.0198 (8)
H3A0.04580.34950.08570.030*
H3B0.00230.35520.32890.030*
H3C0.10950.26790.29500.030*
C40.1559 (4)0.5261 (3)0.3097 (7)0.0188 (8)
H4A0.13730.53790.13600.023*
H4B0.06710.53740.35020.023*
C50.2607 (4)0.6045 (3)0.4406 (7)0.0183 (8)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0262 (14)0.0207 (14)0.0234 (14)0.0023 (11)0.0091 (11)0.0003 (11)
O20.0223 (13)0.0144 (13)0.0254 (13)0.0015 (10)0.0033 (11)0.0024 (10)
O30.0178 (12)0.0163 (12)0.0275 (14)0.0010 (10)0.0032 (11)0.0008 (11)
O40.0228 (13)0.0124 (12)0.0333 (15)0.0024 (11)0.0041 (12)0.0012 (11)
O1W0.0222 (14)0.0216 (14)0.0288 (16)0.0019 (11)0.0050 (12)0.0042 (11)
N10.0189 (16)0.0142 (15)0.0225 (16)0.0011 (12)0.0070 (12)0.0011 (12)
C10.0180 (17)0.0103 (16)0.0262 (19)0.0053 (12)0.0063 (15)0.0004 (13)
C20.0167 (17)0.0118 (15)0.0242 (18)0.0009 (13)0.0065 (14)0.0002 (13)
C30.0200 (17)0.0149 (16)0.0232 (18)0.0032 (14)0.0038 (14)0.0029 (14)
C40.0193 (17)0.0141 (17)0.0215 (16)0.0034 (13)0.0034 (14)0.0005 (14)
C50.0212 (17)0.0106 (16)0.0244 (18)0.0014 (13)0.0087 (14)0.0005 (13)
Geometric parameters (Å, º) top
O1—C11.247 (5)N1—H1C0.88 (6)
O2—C11.253 (5)C1—C21.552 (5)
O3—C51.229 (5)C2—C41.525 (5)
O4—C51.313 (4)C2—C31.528 (5)
O4—H4O0.84 (8)C3—H3A0.9800
O1W—H1W10.89 (6)C3—H3B0.9800
O1W—H1W20.82 (6)C3—H3C0.9800
N1—C21.502 (5)C4—C51.497 (5)
N1—H1A0.91 (6)C4—H4A0.9900
N1—H1B0.90 (7)C4—H4B0.9900
C5—O4—H4O110 (5)C3—C2—C1108.1 (3)
H1W1—O1W—H1W299 (6)C2—C3—H3A109.5
C2—N1—H1A114 (4)C2—C3—H3B109.5
C2—N1—H1B112 (4)H3A—C3—H3B109.5
H1A—N1—H1B102 (5)C2—C3—H3C109.5
C2—N1—H1C111 (4)H3A—C3—H3C109.5
H1A—N1—H1C108 (5)H3B—C3—H3C109.5
H1B—N1—H1C110 (5)C5—C4—C2114.3 (3)
O1—C1—O2127.0 (4)C5—C4—H4A108.7
O1—C1—C2116.6 (3)C2—C4—H4A108.7
O2—C1—C2116.3 (3)C5—C4—H4B108.7
N1—C2—C4108.8 (3)C2—C4—H4B108.7
N1—C2—C3108.0 (3)H4A—C4—H4B107.6
C4—C2—C3110.5 (3)O3—C5—O4124.2 (3)
N1—C2—C1108.5 (3)O3—C5—C4123.6 (3)
C4—C2—C1112.8 (3)O4—C5—C4112.2 (3)
O1—C1—C2—N1158.8 (3)N1—C2—C4—C571.6 (4)
O2—C1—C2—N124.4 (4)C3—C2—C4—C5170.0 (3)
O1—C1—C2—C438.2 (4)C1—C2—C4—C548.8 (4)
O2—C1—C2—C4145.0 (3)C2—C4—C5—O38.0 (6)
O1—C1—C2—C384.3 (4)C2—C4—C5—O4171.5 (3)
O2—C1—C2—C392.6 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O4—H4O···O1W0.84 (8)1.78 (8)2.607 (4)165 (7)
O1W—H1W1···O2i0.89 (6)1.83 (6)2.705 (4)168 (6)
O1W—H1W2···O3ii0.82 (6)2.09 (6)2.909 (4)174 (5)
N1—H1A···O1iii0.91 (6)1.93 (6)2.807 (5)164 (5)
N1—H1B···O2iv0.90 (7)1.94 (7)2.832 (4)172 (5)
N1—H1C···O3v0.88 (6)2.18 (6)2.951 (4)146 (5)
N1—H1C···O30.88 (6)2.50 (6)3.033 (4)119 (5)
Symmetry codes: (i) x+1, y+1/2, z+3/2; (ii) x, y+3/2, z1/2; (iii) x, y, z1; (iv) x, y+1/2, z1/2; (v) x+1, y+1, z+1.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O4—H4O···O1W0.84 (8)1.78 (8)2.607 (4)165 (7)
O1W—H1W1···O2i0.89 (6)1.83 (6)2.705 (4)168 (6)
O1W—H1W2···O3ii0.82 (6)2.09 (6)2.909 (4)174 (5)
N1—H1A···O1iii0.91 (6)1.93 (6)2.807 (5)164 (5)
N1—H1B···O2iv0.90 (7)1.94 (7)2.832 (4)172 (5)
N1—H1C···O3v0.88 (6)2.18 (6)2.951 (4)146 (5)
N1—H1C···O30.88 (6)2.50 (6)3.033 (4)119 (5)
Symmetry codes: (i) x+1, y+1/2, z+3/2; (ii) x, y+3/2, z1/2; (iii) x, y, z1; (iv) x, y+1/2, z1/2; (v) x+1, y+1, z+1.
Acknowledgements top

RJB wishes to acknowledge the NSF–MRI program (grant CHE-0619278) for funds to purchase the diffractometer. GB wishes to acknowledge support of this work from NASA (NNX10AK71A).

references
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