Bis(DL-aspartic acid) sulfate

In the title compound, 2C₄H₈NO₄⁺·SO₄²⁻, the sulfate anion connects the aspartic acid cations by strong hydrogen bonds. Both independent cations have a gauche II form, and the C^γ is trans to the C′ atom. An intramolecular N—H...O hydrogen bond is observed in one of the cations.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.036
• wR factor = 0.107
• Data-to-parameter ratio = 11.8

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

---

S1. Comment

Aspartic acid is an important compound among naturally occurring α-amino acids. The crystal structures of L-aspartic acid (Derissen et al., 1968), DL-aspartic acid (Rao, 1973), DL-aspartic acid hydrochloride (Dawson, 1977) and DL-aspartic acid nitrate monohydrate (Asath Bahadur & Rajaram, 1994) have been reported earlier. In the present study, DL-aspartic acid with sulfuric acid, (I), was undertaken.

In (I), there are two independent aspartic acid cations, A and B, which have the amino N11 and N21 atoms, respectively (Fig. 1). Their geometries are similar and agree well with those of DL-aspartic acid hydrochloride (Dawson, 1977). The molecule is made up of three four-atom planes consisting of C', C^α, C^β, C^γ (plane 1), α-carboxyl group (plane 2) and the β-carboxyl group (plane 3). The mean deviations of the carbon skeleton from plane 1 are 0.167 and 0.030 Å for A and B, respectively. The C^γ is trans to the C' atom for both the molecules. The conformation angles ψ¹ are 15.4 (3) and -8.0 (3)° for A and B, respectively (Table 1). This tendency of twisting of the C—N bond from the amino carboxyl plane is found in various amino acids (Lakshiminarayanan et al., 1967). The branched chain conformation angle χ¹ is in a gauche II form for both molecules; the N11—C12—C13—C14 and N21—C22—C23—C24 torsion angle are -85.3 (2) and -64.7 (2)°, respectively.

The sulfate anion plays a vital role in forming hydrogen bonds with both the cations and stabilizing the structure (Fig. 2). The sulfate anion, as acceptor, links the O1B, O1D, N11, O2B, O2D and N21 atoms. The O^γ atom of each molecule links the amino N atom of the other molecule (Table 2). A bifurcated hydrogen bond is observed in the case of N21 with O1C^v and O2C, the latter being an intramolecular hydrogen bond. But an intramolecular hydrogen bond is not favoured in molecule A, since the χ²² for A and B are -33.4 (3) and 3.9 (3)°, respectively. A chelated three-centre hydrogen bond is observed in the case of the amino N21 atom of molecule B with the sulfate O1 and O2 atoms (Jeffrey & Saenger, 1991). An infinite chain of hydrogen bonds runs along the b axis, linking the sulfate anion and the N21 atom.

S2. Experimental

The title compound was crystallized from aqueous solution of DL-aspartic acid and sulfuric acid in a stoichiometric ratio of 2:1.

S3. Refinement

All H atoms were fixed by geometric restraints using HFIX and were allowed to ride on the preceding atom.

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 structures of the two independent cations of (I) showing the atomic numbering scheme and 50% probability displacement ellipsoids (Johnson, 1976).
	Fig. 2. Packing diagram of the title compound viewed down the b axis

Bis(DL-aspartic acid) sulfate

Crystal data

2C4H8NO4^+·SO4^2^−	F(000) = 760
Mr = 364.29	Dx = 1.683 Mg m^−3 Dm = 1.674 Mg m^−3 Dm measured by flotation in a mixture of bromoform and xylene
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71069 Å
a = 16.774 (4) Å	Cell parameters from 25 reflections
b = 5.9697 (9) Å	θ = 8.9–14.5°
c = 14.5276 (18) Å	µ = 0.30 mm^−1
β = 98.66 (2)°	T = 293 K
V = 1438.1 (4) Å^3	Needles, colorless
Z = 4	0.5 × 0.4 × 0.2 mm

Data collection

Enraf-Nonius sealed tube diffractometer	2113 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.014
Graphite monochromator	θmax = 25.0°, θmin = 2.5°
ω–2θ scans	h = −19→19
Absorption correction: ψ scan (North et al., 1968)	k = 0→7
Tmin = 0.868, Tmax = 0.943	l = 0→17
2616 measured reflections	25 standard reflections every 3 reflections
2502 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.107	H-atom parameters constrained
S = 0.81	w = 1/[σ^2(Fo^2) + (0.0827P)^2 + 1.6327P] where P = (Fo^2 + 2Fc^2)/3
2502 reflections	(Δ/σ)max = 0.001
212 parameters	Δρmax = 0.38 e Å^−3
0 restraints	Δρmin = −0.38 e Å^−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
S	0.73852 (3)	0.11327 (8)	0.26890 (3)	0.02349 (17)
O1	0.71791 (9)	0.1535 (3)	0.36215 (10)	0.0353 (4)
O2	0.72809 (11)	0.3215 (3)	0.21513 (11)	0.0459 (4)
O3	0.68580 (9)	−0.0628 (3)	0.22228 (10)	0.0328 (4)
O4	0.82218 (9)	0.0354 (3)	0.27733 (13)	0.0476 (5)
O1A	0.98758 (9)	0.4639 (3)	0.21248 (12)	0.0412 (4)
O1B	1.04825 (9)	0.6711 (3)	0.11540 (12)	0.0475 (5)
H1B	1.0898	0.6229	0.1460	0.071*
C11	0.98595 (12)	0.6028 (4)	0.15248 (15)	0.0302 (5)
C12	0.90784 (11)	0.7087 (4)	0.10616 (14)	0.0288 (4)
H12	0.8922	0.6331	0.0463	0.035*
N11	0.84489 (10)	0.6584 (3)	0.16526 (14)	0.0360 (5)
H11A	0.7982	0.7187	0.1397	0.054*
H11B	0.8596	0.7157	0.2218	0.054*
H11C	0.8392	0.5107	0.1696	0.054*
C13	0.91137 (13)	0.9605 (4)	0.08663 (15)	0.0340 (5)
H13A	0.9011	1.0427	0.1412	0.041*
H13B	0.9651	0.9991	0.0750	0.041*
C14	0.85100 (12)	1.0295 (4)	0.00446 (15)	0.0294 (5)
O1C	0.83116 (10)	0.9103 (3)	−0.06259 (12)	0.0432 (4)
O1D	0.82279 (10)	1.2349 (3)	0.01231 (11)	0.0398 (4)
H1D	0.7922	1.2675	−0.0353	0.060*
O2A	0.52138 (10)	0.5126 (3)	0.25354 (12)	0.0492 (5)
O2B	0.45700 (9)	0.4946 (3)	0.37783 (11)	0.0410 (4)
H2B	0.4164	0.4785	0.3392	0.061*
C21	0.51934 (13)	0.5317 (3)	0.33496 (14)	0.0298 (5)
C22	0.59179 (12)	0.6065 (3)	0.40399 (14)	0.0265 (4)
H22	0.6018	0.4964	0.4545	0.032*
N21	0.66305 (10)	0.6158 (3)	0.35483 (13)	0.0312 (4)
H21A	0.7060	0.6593	0.3944	0.047*
H21B	0.6539	0.7130	0.3080	0.047*
H21C	0.6720	0.4807	0.3327	0.047*
C23	0.57612 (12)	0.8343 (4)	0.44503 (15)	0.0303 (5)
H23A	0.5695	0.9450	0.3956	0.036*
H23B	0.5262	0.8277	0.4710	0.036*
C24	0.64290 (12)	0.9077 (4)	0.51963 (14)	0.0276 (4)
O2C	0.70441 (9)	0.8008 (3)	0.54036 (10)	0.0367 (4)
O2D	0.62699 (10)	1.0965 (3)	0.55860 (13)	0.0488 (5)
H2D	0.6583	1.1135	0.6072	0.073*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S	0.0146 (3)	0.0262 (3)	0.0281 (3)	−0.00044 (18)	−0.00200 (18)	0.00032 (19)
O1	0.0320 (8)	0.0412 (9)	0.0320 (8)	−0.0044 (7)	0.0022 (6)	−0.0036 (7)
O2	0.0594 (11)	0.0364 (9)	0.0379 (9)	−0.0061 (8)	−0.0059 (8)	0.0080 (7)
O3	0.0243 (8)	0.0321 (8)	0.0386 (8)	−0.0021 (6)	−0.0065 (6)	−0.0058 (6)
O4	0.0149 (8)	0.0656 (12)	0.0600 (11)	0.0047 (7)	−0.0016 (7)	−0.0144 (9)
O1A	0.0283 (8)	0.0393 (9)	0.0524 (10)	0.0002 (7)	−0.0053 (7)	0.0155 (8)
O1B	0.0202 (8)	0.0658 (12)	0.0564 (10)	0.0056 (8)	0.0049 (7)	0.0179 (9)
C11	0.0205 (10)	0.0316 (11)	0.0357 (11)	0.0004 (8)	−0.0044 (8)	−0.0011 (9)
C12	0.0181 (9)	0.0322 (11)	0.0335 (10)	−0.0024 (8)	−0.0042 (8)	−0.0002 (9)
N11	0.0164 (8)	0.0380 (11)	0.0514 (11)	−0.0016 (7)	−0.0015 (8)	0.0085 (9)
C13	0.0264 (10)	0.0325 (12)	0.0404 (12)	−0.0037 (9)	−0.0043 (9)	0.0032 (9)
C14	0.0229 (10)	0.0306 (11)	0.0339 (11)	−0.0032 (8)	0.0018 (8)	0.0049 (9)
O1C	0.0426 (10)	0.0412 (10)	0.0414 (9)	0.0004 (8)	−0.0079 (7)	−0.0035 (8)
O1D	0.0404 (9)	0.0375 (9)	0.0380 (8)	0.0078 (7)	−0.0050 (7)	0.0008 (7)
O2A	0.0439 (10)	0.0662 (12)	0.0348 (9)	−0.0095 (9)	−0.0029 (8)	−0.0100 (8)
O2B	0.0246 (8)	0.0567 (11)	0.0387 (8)	−0.0073 (8)	−0.0045 (6)	−0.0031 (8)
C21	0.0292 (11)	0.0232 (10)	0.0342 (11)	0.0001 (8)	−0.0040 (8)	−0.0015 (8)
C22	0.0221 (10)	0.0271 (11)	0.0286 (10)	0.0003 (8)	−0.0020 (8)	−0.0004 (8)
N21	0.0251 (9)	0.0310 (10)	0.0371 (10)	0.0024 (7)	0.0033 (7)	−0.0065 (7)
C23	0.0214 (10)	0.0333 (12)	0.0337 (11)	0.0040 (8)	−0.0036 (8)	−0.0061 (9)
C24	0.0207 (10)	0.0335 (11)	0.0279 (10)	−0.0022 (9)	0.0016 (8)	−0.0016 (8)
O2C	0.0256 (8)	0.0434 (9)	0.0373 (8)	0.0078 (7)	−0.0078 (6)	−0.0060 (7)
O2D	0.0349 (9)	0.0530 (11)	0.0529 (11)	0.0136 (8)	−0.0116 (8)	−0.0287 (9)

Geometric parameters (Å, º)

S—O2	1.4646 (17)	C14—O1D	1.325 (3)
S—O4	1.4658 (16)	O1D—H1D	0.8200
S—O1	1.4673 (16)	O2A—C21	1.194 (3)
S—O3	1.4713 (15)	O2B—C21	1.314 (3)
O1A—C11	1.200 (3)	O2B—H2B	0.8200
O1B—C11	1.311 (3)	C21—C22	1.522 (3)
O1B—H1B	0.8200	C22—N21	1.483 (3)
C11—C12	1.518 (3)	C22—C23	1.524 (3)
C12—N11	1.488 (3)	C22—H22	0.9800
C12—C13	1.532 (3)	N21—H21A	0.8900
C12—H12	0.9800	N21—H21B	0.8900
N11—H11A	0.8900	N21—H21C	0.8900
N11—H11B	0.8900	C23—C24	1.502 (3)
N11—H11C	0.8900	C23—H23A	0.9700
C13—C14	1.502 (3)	C23—H23B	0.9700
C13—H13A	0.9700	C24—O2C	1.212 (3)
C13—H13B	0.9700	C24—O2D	1.307 (3)
C14—O1C	1.212 (3)	O2D—H2D	0.8200
O2—S—O4	110.46 (12)	O1C—C14—C13	123.2 (2)
O2—S—O1	109.13 (10)	O1D—C14—C13	113.09 (19)
O4—S—O1	109.29 (10)	C14—O1D—H1D	109.5
O2—S—O3	110.21 (9)	C21—O2B—H2B	109.5
O4—S—O3	108.27 (10)	O2A—C21—O2B	126.7 (2)
O1—S—O3	109.45 (9)	O2A—C21—C22	122.8 (2)
C11—O1B—H1B	109.5	O2B—C21—C22	110.53 (18)
O1A—C11—O1B	125.5 (2)	N21—C22—C21	108.38 (17)
O1A—C11—C12	122.25 (19)	N21—C22—C23	110.74 (17)
O1B—C11—C12	112.11 (18)	C21—C22—C23	110.62 (17)
N11—C12—C11	107.46 (17)	N21—C22—H22	109.0
N11—C12—C13	110.81 (18)	C21—C22—H22	109.0
C11—C12—C13	115.85 (17)	C23—C22—H22	109.0
N11—C12—H12	107.5	C22—N21—H21A	109.5
C11—C12—H12	107.5	C22—N21—H21B	109.5
C13—C12—H12	107.5	H21A—N21—H21B	109.5
C12—N11—H11A	109.5	C22—N21—H21C	109.5
C12—N11—H11B	109.5	H21A—N21—H21C	109.5
H11A—N11—H11B	109.5	H21B—N21—H21C	109.5
C12—N11—H11C	109.5	C24—C23—C22	112.88 (17)
H11A—N11—H11C	109.5	C24—C23—H23A	109.0
H11B—N11—H11C	109.5	C22—C23—H23A	109.0
C14—C13—C12	112.10 (18)	C24—C23—H23B	109.0
C14—C13—H13A	109.2	C22—C23—H23B	109.0
C12—C13—H13A	109.2	H23A—C23—H23B	107.8
C14—C13—H13B	109.2	O2C—C24—O2D	124.65 (19)
C12—C13—H13B	109.2	O2C—C24—C23	122.68 (19)
H13A—C13—H13B	107.9	O2D—C24—C23	112.67 (18)
O1C—C14—O1D	123.66 (19)	C24—O2D—H2D	109.5
O1A—C11—C12—N11	15.4 (3)	O2A—C21—C22—N21	−8.0 (3)
O1B—C11—C12—N11	−168.38 (19)	O2B—C21—C22—N21	172.34 (17)
O1A—C11—C12—C13	139.9 (2)	O2A—C21—C22—C23	113.6 (2)
O1B—C11—C12—C13	−43.9 (3)	O2B—C21—C22—C23	−66.1 (2)
N11—C12—C13—C14	−85.3 (2)	N21—C22—C23—C24	−64.7 (2)
C11—C12—C13—C14	152.00 (19)	C21—C22—C23—C24	175.16 (18)
C12—C13—C14—O1C	−33.4 (3)	C22—C23—C24—O2C	3.9 (3)
C12—C13—C14—O1D	147.60 (19)	C22—C23—C24—O2D	−176.05 (19)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1B—H1B···O4^i	0.82	1.79	2.605 (2)	174
N11—H11A···O2C^ii	0.89	1.97	2.760 (2)	147
N11—H11B···O4^iii	0.89	2.20	2.837 (3)	128
N11—H11C···O2	0.89	2.36	2.972 (3)	126
O1D—H1D···O1^ii	0.82	1.85	2.673 (2)	176
O2B—H2B···O3^iv	0.82	1.83	2.631 (2)	166
N21—H21A···O1C^v	0.89	2.14	2.898 (3)	143
N21—H21A···O2C	0.89	2.29	2.899 (2)	126
N21—H21B···O3^iii	0.89	1.96	2.785 (3)	154
N21—H21C···O1	0.89	2.12	2.906 (2)	147
N21—H21C···O2	0.89	2.28	3.010 (3)	140
O2D—H2D···O2^v	0.82	1.85	2.669 (2)	175

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