DL-Valine-fumaric acid (2/1)

In the title compound, C₅H₁₁NO₂·0.5C₄H₄O₄, the valine mol­ecule exists as a zwitterion and the fumaric acid mol­ecule in the unionized state, forming an adduct, a feature uncommon in similar crystal structures. The fumaric acid mol­ecule has a centre of symmetry and is planar with a trans configuration about the central C=C bond. The fumaric acid mol­ecules have no hydrogen-bonded interactions among themselves and only mediate interactions between DL-valine layers, leading to a three-dimensional network of mol­ecules.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.058
• wR factor = 0.154
• Data-to-parameter ratio = 13.7

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Fumaric acid, a key intermediate in the organic acid biosynthesis, is known to readily form adducts/complexes with other organic molecules. Valine, an essential amino acid, is hydrophobic with a non-polar hydrocarbon chain and plays a vital role in the stabilization of protein molecules. A determination of the present crystal structure, (I), was carried out to examine the stoichiometry and ionization states and it appears to be the first of its kind involving fumaric acid and an amino acid. Moreover, the aggregation and the interaction patterns observed in amino acid–carboxylic acid complexes might possibly contribute to the understanding of the self-assembly processes that might have led to the emergence of the primitive multimolecular systems. Recently, the crystal structures of complexes of DL-valine with maleic acid (Alagar et al., 2001) and trichloroacetic acid (Rajagopal et al., 2002) were reported from our laboratory.

Fig. 1 shows the molecular structure of (I) with the adopted atom-numbering scheme. The valine molecule exists as a zwitterion, and the fumaric acid molecule in the unionized state forming an adduct involving the two distinct species, a feature uncommon in similar crystal structures. Usually in the crystals of amino acid–carboxylic acid complexes, the amino acid molecule is expected to exist in the cationic state (with a neutral carboyxlic acid group and a positively charged amino group) and the dicarboxylic acid in the anionic state (with a neutral carboxylic acid group and a negatively charged carboxylate group) facilitated by a proton transfer. The observed zwitterionic form of DL-valine and the unionized state of fumaric acid in the present structure is due to a `break down' in the otherwise routine proton transfer observed in such complexes. The coformation of the valine molecule determined by χ¹¹ [−58.9 (2)°] and χ¹² [68.5 (2)°] differs significantly from the values observed for the monoclinic form of DL-valine (Mallikarjunan & Rao, 1969) and for the triclinic form of DL-valine (Dalhus & Görbitz, 1996). However, the values agree well with those observed in DL-valinium maleate (Alagar et al., 2001), in spite of the difference in the ionization states of the amino acid molecules. The fumaric acid molecule has a centre of symmetry and is planar with a trans conformation about the central C═C bond.

The adduct formed by DL-valine and fumaric acid are held together by hydrogen bonded interactions (Fig. 2). DL-valine molecules aggregate into layers parallel to the bc plane in which glide and screw related head-to-tail hydrogen bonds between the amino acids are present. The fumaric acid molecules have no hydrogen-bonded interactions among them. They only mediate interactions between DL-valine layers through hydrogen bonds leading to a three-dimensional network of molecules. The aggregation pattern of individual molecules is distinctly different from those observed in the complexes of DL-alanine with maleic acid and trichloroacetic acid.

S2. Experimental

Colorless single crystals of (I) were grown as transparent needles, from a saturated aqueous solution containing DL-valine and fumaric acid, in 1:1 stoichiometric ratio.

S3. Refinement

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

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

	Fig. 1. The molecular structure of (I), with the atom-numbering scheme and ellipsoids at the 50% probability level [symmetry code: (i) −x, −y, −z].
	Fig. 2. Packing diagram of the molecules of (I), viewed down tha b axis.

DL-valine hemifumaric acid

Crystal data

C5H11NO2·0.5C4H4O4	F(000) = 752
Mr = 175.18	Dx = 1.332 Mg m^−3 Dm = 1.34 (2) Mg m^−3 Dm measured by flotation in xylene–bromoform
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 25 reflections
a = 24.417 (4) Å	θ = 7–13°
b = 7.5713 (10) Å	µ = 0.11 mm^−1
c = 10.013 (2) Å	T = 293 K
β = 109.268 (10)°	Needle, colourless
V = 1747.4 (5) Å^3	0.28 × 0.22 × 0.14 mm
Z = 8

Data collection

Enraf-Nonius CAD-4 diffractometer	1356 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.10
Graphite monochromator	θmax = 25.0°, θmin = 2.8°
ω–2θ scans	h = 0→28
Absorption correction: ψ scan (North et al., 1968)	k = −8→8
Tmin = 0.88, Tmax = 0.98	l = −11→11
2600 measured reflections	2 standard reflections every 100 reflections
1524 independent reflections	intensity decay: <1%

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)] = 0.058	H-atom parameters constrained
wR(F^2) = 0.154	w = 1/[σ^2(Fo^2) + (0.0869P)^2 + 1.3337P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
1524 reflections	Δρmax = 0.32 e Å^−3
111 parameters	Δρmin = −0.28 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.007 (2)

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
O1	0.27658 (5)	0.5467 (2)	0.38149 (14)	0.0333 (4)
O2	0.24030 (5)	0.4490 (2)	0.15951 (14)	0.0351 (4)
O3	0.12567 (5)	−0.0296 (2)	0.12750 (15)	0.0371 (4)
H3A	0.1555	0.0033	0.1147	0.056*
O4	0.08326 (5)	0.1308 (2)	−0.06608 (15)	0.0431 (5)
N1	0.17990 (5)	0.7020 (2)	0.38135 (14)	0.0240 (4)
H1A	0.1447	0.7406	0.3754	0.036*
H1B	0.2029	0.7937	0.3831	0.036*
H1C	0.1946	0.6390	0.4602	0.036*
C1	0.23603 (7)	0.5223 (2)	0.26606 (18)	0.0230 (4)
C2	0.17563 (7)	0.5892 (2)	0.25650 (17)	0.0230 (4)
H2	0.1617	0.6641	0.1722	0.028*
C3	0.13226 (8)	0.4356 (3)	0.2373 (2)	0.0332 (5)
H3	0.1350	0.3647	0.1578	0.040*
C4	0.14757 (14)	0.3151 (4)	0.3640 (3)	0.0630 (8)
H4A	0.1870	0.2760	0.3863	0.094*
H4B	0.1221	0.2147	0.3432	0.094*
H4C	0.1434	0.3778	0.4434	0.094*
C5	0.06993 (10)	0.5041 (4)	0.1948 (4)	0.0689 (9)
H5A	0.0622	0.5788	0.1133	0.103*
H5B	0.0649	0.5703	0.2717	0.103*
H5C	0.0435	0.4060	0.1729	0.103*
C6	0.08116 (7)	0.0364 (2)	0.0300 (2)	0.0277 (5)
C7	0.02520 (7)	−0.0165 (3)	0.0492 (2)	0.0317 (5)
H7	0.0259	−0.0738	0.1319	0.038*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0259 (7)	0.0428 (9)	0.0285 (8)	0.0043 (5)	0.0053 (5)	−0.0091 (6)
O2	0.0365 (7)	0.0389 (9)	0.0297 (8)	0.0075 (6)	0.0107 (6)	−0.0098 (6)
O3	0.0263 (7)	0.0401 (9)	0.0437 (8)	−0.0014 (6)	0.0101 (6)	0.0121 (7)
O4	0.0314 (7)	0.0500 (10)	0.0471 (9)	−0.0044 (6)	0.0119 (6)	0.0192 (8)
N1	0.0253 (7)	0.0219 (8)	0.0249 (8)	0.0007 (5)	0.0084 (6)	−0.0034 (6)
C1	0.0287 (9)	0.0179 (8)	0.0237 (9)	0.0011 (6)	0.0102 (7)	−0.0007 (7)
C2	0.0268 (8)	0.0212 (9)	0.0197 (8)	0.0016 (7)	0.0059 (6)	−0.0022 (7)
C3	0.0351 (10)	0.0323 (11)	0.0331 (10)	−0.0100 (8)	0.0128 (8)	−0.0127 (8)
C4	0.097 (2)	0.0484 (16)	0.0454 (13)	−0.0372 (14)	0.0264 (13)	−0.0037 (12)
C5	0.0340 (12)	0.0668 (18)	0.104 (2)	−0.0138 (12)	0.0200 (13)	−0.0308 (18)
C6	0.0279 (9)	0.0236 (10)	0.0325 (10)	−0.0022 (7)	0.0113 (7)	0.0004 (8)
C7	0.0300 (9)	0.0295 (10)	0.0376 (11)	−0.0019 (7)	0.0138 (7)	0.0075 (8)

Geometric parameters (Å, º)

O1—C1	1.263 (2)	C3—C4	1.507 (3)
O2—C1	1.237 (2)	C3—C5	1.529 (3)
O3—C6	1.297 (2)	C3—H3	0.9800
O3—H3A	0.8200	C4—H4A	0.9600
O4—C6	1.214 (2)	C4—H4B	0.9600
N1—C2	1.489 (2)	C4—H4C	0.9600
N1—H1A	0.8900	C5—H5A	0.9600
N1—H1B	0.8900	C5—H5B	0.9600
N1—H1C	0.8900	C5—H5C	0.9600
C1—C2	1.532 (2)	C6—C7	1.496 (2)
C2—C3	1.541 (3)	C7—C7^i	1.322 (4)
C2—H2	0.9800	C7—H7	0.9300
C6—O3—H3A	109.5	C5—C3—H3	106.7
C2—N1—H1A	109.5	C2—C3—H3	106.7
C2—N1—H1B	109.5	C3—C4—H4A	109.5
H1A—N1—H1B	109.5	C3—C4—H4B	109.5
C2—N1—H1C	109.5	H4A—C4—H4B	109.5
H1A—N1—H1C	109.5	C3—C4—H4C	109.5
H1B—N1—H1C	109.5	H4A—C4—H4C	109.5
O2—C1—O1	126.24 (16)	H4B—C4—H4C	109.5
O2—C1—C2	116.59 (14)	C3—C5—H5A	109.5
O1—C1—C2	117.17 (15)	C3—C5—H5B	109.5
N1—C2—C1	109.74 (12)	H5A—C5—H5B	109.5
N1—C2—C3	113.11 (14)	C3—C5—H5C	109.5
C1—C2—C3	111.48 (15)	H5A—C5—H5C	109.5
N1—C2—H2	107.4	H5B—C5—H5C	109.5
C1—C2—H2	107.4	O4—C6—O3	125.38 (16)
C3—C2—H2	107.4	O4—C6—C7	122.61 (16)
C4—C3—C5	112.7 (2)	O3—C6—C7	112.01 (16)
C4—C3—C2	112.48 (16)	C7^i—C7—C6	121.3 (2)
C5—C3—C2	111.04 (19)	C7^i—C7—H7	119.4
C4—C3—H3	106.7	C6—C7—H7	119.4
O2—C1—C2—N1	−169.55 (15)	C1—C2—C3—C4	65.3 (2)
O1—C1—C2—N1	10.6 (2)	N1—C2—C3—C5	68.5 (2)
O2—C1—C2—C3	64.3 (2)	C1—C2—C3—C5	−167.25 (18)
O1—C1—C2—C3	−115.49 (17)	O4—C6—C7—C7^i	9.7 (4)
N1—C2—C3—C4	−58.9 (2)	O3—C6—C7—C7^i	−170.6 (2)

Symmetry code: (i) −x, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3A···O1^ii	0.82	1.68	2.4860 (18)	168
N1—H1A···O4^iii	0.89	2.03	2.8755 (19)	158
N1—H1B···O2^iv	0.89	1.97	2.826 (2)	161
N1—H1C···O2^iii	0.89	2.05	2.9201 (19)	166
C2—H2···O3^v	0.98	2.47	3.233 (2)	135

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