L-Aspartic acid nitrate–L-aspartic acid (1/1)

In the title compound, C₄H₈NO₄⁺·C₄H₇NO₄·NO₃⁻, the cation and neutral mol­ecule are connected by an asymmetric hydrogen bond. The cation, residue 1, exists in a gauche I conformation, whereas the neutral mol­ecule, residue 2, exhibits a gauche II conformation. A syn–syn orientation is also observed in this structure and residue 1 is involved in a straight (S1) head-to-tail sequence. The structure is stabilized by both inter- and intramolecular N—H...O hydrogen bonding.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.007 Å
• R factor = 0.041
• wR factor = 0.118
• Data-to-parameter ratio = 7.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

General Notes



REFLT_03
         From the CIF: _diffrn_reflns_theta_max           24.97
         From the CIF: _reflns_number_total               1578
         Count of symmetry unique reflns         1409
         Completeness (_total/calc)            111.99%
         TEST3: Check Friedels for noncentro structure
         Estimate of Friedel pairs measured       169
         Fraction of Friedel pairs measured     0.120
         Are heavy atom types Z>Si present           no
          ALERT: MoKa measured Friedel data cannot be used to
                 determine absolute structure in a light-atom
                 study EXCEPT under VERY special conditions.
                 It is preferred that Friedel data is merged in such cases.

---

S1. Comment

Aspartic acid is a non-essential amino acid, widely distributed in proteins, which plays a major role in the energy cycle of the human body. The crystal structures of L-aspartic acid (Derissen et al., 1968), DL-aspartic acid (Rao, 1973; Sequeira et al., 1989), DL-aspartic acid nitrate monohydrate (Asath Bahdur & Rajaram, 1995) and bis(DL-aspartic acid) sulfate (Srinivasan et al., 2001) have been reported. In the present paper, the crystal structure of the product, (I), of L-aspartic acid reacted with nitric acid is reported.

The asymmetric unit of (I) contains one aspartic acid cation (residue 1), one neutral aspartic acid molecule (residue 2) and one nitrate anion. Superposition of residue 1 on residue 2 results in an r.m.s. deviation of the constituent atoms of 1.088 Å. Examination of the coordinates suggests that the two residues might be related by a pseudo-inversion centre. The unsymmetrical carboxyl bond distances and angles [1.217 (7)/1.297 (6) Å and 122.3 (5)/109.9 (5)°] of residue 1 clearly indicate protonation of the carboxyl group, whilst, in the case of residue 2, the equality of C—O bond distances [1.240 (7)/1.250 (6) Å] and O—C—C bond angles [117.4 (5)/115.5 (5)°] represent the deprotonated carboxylate group (Table 1).

The backbone conformation angle ψ¹ is the cis form for both residues [O1A—C11—C12—N1 − 7.0 (7)° and O2A—C21—C22—N21 − 1.7 (7)°]. The deviations of the amino atoms N11 and N21 from the planar carboxyl groups at C11 and C22 are 0.198 (8) and 0.044 (1) Å, respectively. This non-planarity of the amino nitrogen and carboxyl group is also found in other amino acids (Lakshminarayanan et al., 1967). The side-chain conformation angle χ¹ is in a gauche I form [66.1 (6)°] for residue 1 and a gauche II form [−64.8 (6)°] for residue 2. The branched chain conformation angles, χ¹¹ and χ²¹, are in cis and trans forms [10.0 (8)/-5.5 (8) and −170.8 (5)/174.3 (5)°] for both residues. In residue 1, the C^γ atom C14 is in the gauche II [−58.1 (6)°] conformation with respect to C11, while, in the case of residue 2, the C^γ atom C24 is trans [170.9 (5)°] with respect to C21.

The average N—O and O—N—O values are 1.248 Å and 120°, respectively, clearly showing the nearly ideal trigonal symmetry of the anion, which plays a vital role in hydrogen bonding and the resulting stabilization of the structure.

The aspartic acid cation and neutral aspartic acid molecule are linked, by strong O—H···O hydrogen bonding, to form dimers. This hydrogen bond may be termed an asymmetric hydrogen bond, since the H atom is closer to one of the O atoms of the carboxyl group (Olovsson et al., 2001). Atom H1B is in a syn orientation with respect to both donor carboxyl group and acceptor carboxylate group; the torsion angles H1B—O1B—C11—O1A and H1B—O2Bⁱ—C21ⁱ—O2Aⁱ [symmetry code: (i) 1 + x, y, z] are 13 (5) and 29 (3)°, respectively. This type of syn–syn orientation is also found in betaine betainium oxalate (Rodrigues et al., 2001). The β-carboxyl group of residue 1 forms a strong O—H···O hydrogen bond with the α-carboxylate group of residue 2. In the case of residue 2, the β-carboxyl group forms a rather strong O—H···O hydrogen bond with the carbonyl O atom of the β-carboxyl group of residue 1.

The amino N atom of both residues forms N—H···O hydrogen bonds with the nitrate anion, and the α- and β-carboxyl groups. Three-centered hydrogen bonding is observed in both residues. Interestingly, in residue 1, three such three-centered bonds are observed, leading to a class IV hydrogen-bonding pattern (Jeffrey & Saenger, 1991). The frequency of such class IV hydrogen-bonding pattern is very low. A class II hydrogen-bonding pattern is observed in residue 2, having two two-centered and one three-centered hydrogen bonds. In both residues, intramolecular N—H···O hydrogen bonding is present, involving the amino nitrogen and the carbonyl oxygen of a carboxylic acid group. A straight (S1) head-to-tail sequence is observed in residue 1, connecting two amino acids separated by a unit translation (Vijayan, 1988). Each aspartic acid residue is linked by the nitrate anion through N—H···O hydrogen bonding, forming a chain running along the a axis: (a) (x − 1, y, z)O2···H11B—N11—H11A···O3(x − 2, y, z) and (b) (−1/2 − x, 2 − y, z − 1/2)O1···H21A—N21—H21B···O3(1/2 − x, 2 − y, z − 1/2).

S2. Experimental

The title compound was crystallized by slow evaporation of an aqueous solution of L-aspartic acid and nitric acid in a 2:1 stoichiometric ratio.

S3. Refinement

The carboxyl H atoms were located in a difference Fourier synthesis and refined isotropically [O—H = 0.96 (6)–1.06 (6) Å]. All other H atoms were placed in geometrically calculated positions and included in the refinement in a riding-model approximation, with U_iso equal to 1.2U_eq of the carrier atom (1.5 U_eq for methyl and H atoms attached to nitrogen).

Computing details

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, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure, with the atom numbering scheme and 50% probability displacement ellipsoids (Johnson, 1976).
	Fig. 2. Packing diagram of the title molecule, viewed down the a axis.

Bis(L-aspartic acid) nitrate

Crystal data

C4H8NO4^+·NO3^−·C4H7NO4	Dx = 1.620 Mg m^−3 Dm = 1.58 Mg m^−3 Dm measured by flotation in mixture of carbon tetrachloride and xylene
Mr = 329.23	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, P212121	Cell parameters from 25 reflections
a = 5.5840 (7) Å	θ = 11.0–14.8°
b = 11.491 (3) Å	µ = 0.15 mm^−1
c = 21.043 (5) Å	T = 293 K
V = 1350.2 (5) Å^3	Needle, colourless
Z = 4	0.5 × 0.2 × 0.2 mm
F(000) = 688

Data collection

Enraf-Nonius CAD-4 diffractometer	929 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.020
Graphite monochromator	θmax = 25.0°, θmin = 1.9°
ω–2θ scans	h = 0→6
Absorption correction: ψ scan (North et al., 1968)	k = −1→13
Tmin = 0.932, Tmax = 0.968	l = −1→24
1640 measured reflections	3 standard reflections every 60 min
1578 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.041	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.118	w = 1/[σ^2(Fo^2) + (0.058P)^2 + 0.0829P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max < 0.001
1578 reflections	Δρmax = 0.24 e Å^−3
212 parameters	Δρmin = −0.30 e Å^−3
0 restraints	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.011 (3)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
O1A	1.2219 (7)	−0.1138 (3)	0.48094 (17)	0.0314 (10)
O1B	1.1335 (8)	0.0165 (4)	0.40417 (18)	0.0396 (11)
H1B	1.289 (15)	0.009 (8)	0.376 (3)	0.11 (3)*
C11	1.0862 (11)	−0.0487 (5)	0.4529 (2)	0.0280 (14)
C12	0.8217 (10)	−0.0363 (4)	0.4712 (2)	0.0213 (13)
H12	0.7266	−0.0545	0.4334	0.026*
N11	0.7613 (8)	−0.1242 (4)	0.5204 (2)	0.0286 (12)
H11A	0.6072	−0.1179	0.5307	0.043*
H11B	0.8509	−0.1123	0.5548	0.043*
H11C	0.7894	−0.1952	0.5052	0.043*
C13	0.7539 (11)	0.0870 (4)	0.4922 (2)	0.0273 (13)
H13A	0.7759	0.1390	0.4563	0.033*
H13B	0.5849	0.0876	0.5029	0.033*
C14	0.8914 (11)	0.1353 (5)	0.5478 (2)	0.0270 (14)
O1C	1.0679 (7)	0.0892 (4)	0.57031 (16)	0.0375 (11)
O1D	0.8000 (8)	0.2330 (3)	0.56814 (19)	0.0414 (12)
H1D	0.849 (11)	0.270 (5)	0.607 (3)	0.05 (2)*
O2A	0.5429 (9)	0.1827 (4)	0.33741 (18)	0.0469 (13)
O2B	0.4999 (8)	−0.0102 (4)	0.33645 (18)	0.0402 (11)
C21	0.5667 (10)	0.0847 (5)	0.3135 (2)	0.0283 (13)
C22	0.6907 (9)	0.0773 (5)	0.2487 (2)	0.0240 (13)
H22	0.5733	0.0462	0.2186	0.029*
N21	0.7516 (8)	0.1980 (4)	0.22727 (19)	0.0268 (11)
H21A	0.6204	0.2420	0.2281	0.040*
H21B	0.8092	0.1955	0.1879	0.040*
H21C	0.8613	0.2283	0.2531	0.040*
C23	0.9042 (11)	−0.0038 (5)	0.2478 (2)	0.0322 (14)
H23A	1.0300	0.0291	0.2740	0.039*
H23B	0.8578	−0.0776	0.2665	0.039*
C24	1.0026 (11)	−0.0258 (5)	0.1819 (2)	0.0277 (14)
O2C	0.9084 (8)	0.0106 (3)	0.13420 (17)	0.0367 (11)
O2D	1.2010 (7)	−0.0881 (4)	0.18295 (17)	0.0404 (11)
H2D	1.274 (13)	−0.120 (6)	0.140 (3)	0.07 (2)*
N1	0.2416 (10)	0.7685 (4)	0.6397 (2)	0.0316 (12)
O1	0.2544 (7)	0.7870 (3)	0.69917 (17)	0.0397 (11)
O2	0.0374 (7)	0.7696 (4)	0.61466 (19)	0.0447 (12)
O3	0.4204 (8)	0.7467 (4)	0.60902 (19)	0.0471 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1A	0.024 (2)	0.039 (2)	0.031 (2)	0.004 (2)	−0.004 (2)	−0.0002 (19)
O1B	0.039 (3)	0.042 (2)	0.038 (2)	0.002 (2)	0.014 (2)	0.015 (2)
C11	0.032 (4)	0.030 (3)	0.022 (3)	−0.007 (3)	0.002 (3)	−0.011 (3)
C12	0.027 (3)	0.019 (3)	0.018 (3)	0.003 (3)	−0.001 (2)	0.001 (2)
N11	0.022 (3)	0.031 (3)	0.032 (2)	−0.003 (3)	0.001 (3)	−0.001 (2)
C13	0.025 (3)	0.031 (3)	0.026 (3)	0.001 (4)	−0.005 (3)	−0.002 (3)
C14	0.031 (4)	0.031 (3)	0.020 (3)	−0.001 (3)	0.000 (3)	0.000 (3)
O1C	0.032 (3)	0.045 (2)	0.036 (2)	0.011 (2)	−0.014 (2)	−0.009 (2)
O1D	0.050 (3)	0.028 (2)	0.045 (2)	0.009 (2)	−0.013 (2)	−0.018 (2)
O2A	0.061 (3)	0.043 (3)	0.037 (2)	−0.007 (2)	0.019 (2)	−0.015 (2)
O2B	0.040 (3)	0.043 (2)	0.038 (2)	0.003 (2)	0.017 (2)	0.007 (2)
C21	0.025 (3)	0.037 (3)	0.023 (3)	−0.001 (3)	0.002 (3)	0.000 (3)
C22	0.019 (3)	0.029 (3)	0.024 (3)	−0.001 (3)	0.005 (3)	0.001 (3)
N21	0.019 (3)	0.031 (3)	0.031 (2)	−0.005 (3)	0.007 (2)	−0.004 (2)
C23	0.032 (3)	0.037 (3)	0.028 (3)	0.008 (3)	0.005 (3)	0.005 (3)
C24	0.033 (4)	0.025 (3)	0.026 (3)	−0.003 (3)	0.004 (3)	0.008 (3)
O2C	0.041 (3)	0.044 (2)	0.024 (2)	0.010 (2)	0.003 (2)	0.001 (2)
O2D	0.041 (3)	0.051 (3)	0.029 (2)	0.025 (2)	0.009 (2)	0.004 (2)
N1	0.028 (3)	0.030 (3)	0.037 (3)	−0.006 (3)	0.006 (3)	−0.007 (2)
O1	0.028 (2)	0.055 (3)	0.036 (2)	−0.001 (2)	0.000 (2)	−0.007 (2)
O2	0.028 (3)	0.061 (3)	0.046 (2)	−0.003 (2)	−0.013 (2)	0.012 (2)
O3	0.040 (3)	0.047 (3)	0.054 (3)	−0.002 (2)	0.011 (3)	−0.019 (2)

Geometric parameters (Å, º)

O1A—C11	1.217 (7)	O2B—C21	1.250 (6)
O1B—C11	1.297 (6)	C21—C22	1.531 (7)
O1B—H1B	1.05 (8)	C22—N21	1.498 (7)
C11—C12	1.533 (8)	C22—C23	1.513 (7)
C12—N11	1.485 (6)	C22—H22	0.9800
C12—C13	1.532 (7)	N21—H21A	0.8900
C12—H12	0.98	N21—H21B	0.8900
N11—H11A	0.89	N21—H21C	0.8900
N11—H11B	0.8900	C23—C24	1.512 (7)
N11—H11C	0.8900	C23—H23A	0.9700
C13—C14	1.505 (7)	C23—H23B	0.9700
C13—H13A	0.9700	C24—O2C	1.208 (6)
C13—H13B	0.9700	C24—O2D	1.319 (7)
C14—O1C	1.215 (6)	O2D—H2D	1.06 (6)
C14—O1D	1.306 (6)	N1—O3	1.215 (6)
O1D—H1D	0.96 (6)	N1—O2	1.256 (6)
O2A—C21	1.240 (7)	N1—O1	1.272 (5)
C11—O1B—H1B	124 (5)	O2A—C21—C22	117.4 (5)
O1A—C11—O1B	127.7 (6)	O2B—C21—C22	115.5 (5)
O1A—C11—C12	122.3 (5)	N21—C22—C23	112.8 (4)
O1B—C11—C12	109.9 (5)	N21—C22—C21	108.6 (4)
N11—C12—C13	111.9 (4)	C23—C22—C21	113.7 (4)
N11—C12—C11	109.3 (4)	N21—C22—H22	107.1
C13—C12—C11	113.4 (5)	C23—C22—H22	107.1
N11—C12—H12	107.3	C21—C22—H22	107.1
C13—C12—H12	107.3	C22—N21—H21A	109.5
C11—C12—H12	107.3	C22—N21—H21B	109.5
C12—N11—H11A	109.5	H21A—N21—H21B	109.5
C12—N11—H11B	109.5	C22—N21—H21C	109.5
H11A—N11—H11B	109.5	H21A—N21—H21C	109.5
C12—N11—H11C	109.5	H21B—N21—H21C	109.5
H11A—N11—H11C	109.5	C24—C23—C22	113.7 (5)
H11B—N11—H11C	109.5	C24—C23—H23A	108.8
C14—C13—C12	116.0 (5)	C22—C23—H23A	108.8
C14—C13—H13A	108.3	C24—C23—H23B	108.8
C12—C13—H13A	108.3	C22—C23—H23B	108.8
C14—C13—H13B	108.3	H23A—C23—H23B	107.7
C12—C13—H13B	108.3	O2C—C24—O2D	124.6 (5)
H13A—C13—H13B	107.4	O2C—C24—C23	123.1 (5)
O1C—C14—O1D	124.3 (5)	O2D—C24—C23	112.4 (5)
O1C—C14—C13	123.8 (5)	C24—O2D—H2D	120 (4)
O1D—C14—C13	111.8 (5)	O3—N1—O2	121.7 (5)
C14—O1D—H1D	123 (4)	O3—N1—O1	120.8 (6)
O2A—C21—O2B	127.1 (5)	O2—N1—O1	117.5 (5)
O1A—C11—C12—N11	−7.0 (7)	O2A—C21—C22—N21	−1.7 (7)
O1B—C11—C12—N11	171.4 (4)	O2B—C21—C22—N21	178.2 (5)
O1A—C11—C12—C13	118.6 (6)	O2A—C21—C22—C23	124.8 (6)
O1B—C11—C12—C13	−63.0 (5)	O2B—C21—C22—C23	−55.3 (7)
N11—C12—C13—C14	66.1 (6)	N21—C22—C23—C24	−64.8 (6)
C11—C12—C13—C14	−58.1 (6)	C21—C22—C23—C24	170.9 (5)
C12—C13—C14—O1C	10.0 (8)	C22—C23—C24—O2C	−5.5 (8)
C12—C13—C14—O1D	−170.8 (5)	C22—C23—C24—O2D	174.3 (5)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1B—H1B···O2B^i	1.05 (8)	1.47 (8)	2.512 (5)	175 (8)
N11—H11A···O1A^ii	0.89	2.39	3.126 (6)	140
N11—H11A···O3^iii	0.89	2.50	3.050 (6)	121
N11—H11B···O2^iv	0.89	2.12	2.793 (6)	131
N11—H11B···O1C	0.89	2.63	3.170 (6)	120
N11—H11C···O1A^v	0.89	2.25	3.018 (5)	145
N11—H11C···O3^vi	0.89	2.58	3.192 (6)	127
O1D—H1D···O2A^vi	0.96 (6)	1.68 (6)	2.594 (5)	156 (6)
N21—H21A···O1^vii	0.89	2.20	2.892 (6)	134
N21—H21B···O3^viii	0.89	2.34	3.155 (6)	152
N21—H21B···O2C	0.89	2.47	3.040 (5)	122
N21—H21C···O2D^ix	0.89	2.53	3.111 (6)	124
O2D—H2D···O1C^x	1.06 (6)	1.74 (6)	2.699 (5)	148 (5)

Symmetry codes: (i) x+1, y, z; (ii) x−1, y, z; (iii) x, y−1, z; (iv) x+1, y−1, z; (v) x−1/2, −y−1/2, −z+1; (vi) x+1/2, −y+1/2, −z+1; (vii) −x+1/2, −y+1, z−1/2; (viii) −x+3/2, −y+1, z−1/2; (ix) −x+2, y+1/2, −z+1/2; (x) −x+5/2, −y, z−1/2.