β-Alaninium maleate

In the title compound, C₃H₈NO₂⁺·C₄H₃O₄⁻, the β-alanine mol­ecule exists in the cationic form with a positively charged amino group and an uncharged carboxyl­ic acid group. The maleic acid mol­ecule exists in the mono-ionized state. In the semi-maleate ion, the intramolecular hydrogen bond between atoms O3 and O5 is found to be asymmetric. There are no direct hydrogen-bonded interactions between the semimaleate anions. The nature of interactions between individual mol­ecules is different from those observed in L-alaninium maleate.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.038
• wR factor = 0.190
• Data-to-parameter ratio = 12.3

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Structural data for complexes of organic acids with amino acids seem to be very limited. Among organic acids, simple carboxylic acids, which are believed to have existed in the prebiotic earth (Miller & Orgel, 1974; Kvenvolden et al., 1971), form crystalline complexes with amino acids. X-ray studies on many such compounds are being carried out in our laboratory with the aim of studying the nature of intermolecular interactions and characteristic aggregation patterns, at atomic resolution. Recently, the crystal structures of glycinium maleate (Rajagopal et al., 2001) and L-alaninium maleate (Alagar et al., 2001) have been reported. The present study reports the crystal structure of β-alaninium maleate, (I), a complex of β-alanine with maleic acid. β-Alanine is the only naturally occurring β-amino acid and is a constituent of the dipeptides carnosine and anserine.

Fig. 1 shows the molecular structure of (I) with the atom-numbering scheme. The β-alanine molecule exists in the cationic form with a positively charged amino group and an uncharged carboxylic acid group. The maleic acid molecule exists in the mono-ionized state. The semi-maleate ion is essentially planar as observed in the crystal structures of similar complexes. In the semi-meleate ion, the intramolecular hydrogen bond between atoms O3 and O5 is found to be asymmetric, as in the crystal structures of maleic acid (James & Williams, 1974), glycinium maleate and L-alaninium maleate. However, in the crystal structures of complexes of maleic acid with DL– and L-arginine (Ravishankar et al., 1998) and L-histidine and L-lysine (Pratap et al., 2000), this intramolecular hydrogen bond between the carboxylic acid and carboxylate groups is symmetric with an H atom shared between the respective O atoms. A common feature observed among the crystal structures of amino acid–maleic acid complexes is that the shortest cell dimension in all of them is close to 5.4 Å, with the exception of lysine–maleic acid complex. It is interesting to note that the shortest cell dimension in almost all the crystal structures of amino acids themselves lie in the neighborhood of 5.4 Å (Suresh & Vijayan, 1983).

Fig. 2 shows the packing of the molecules of (I) viewed down the a axis. The β-alaninium and semi-maleate ions form alternate columns parallel to the axis of intermediate length, as in L-alaninium maleate. The semi-maleate ions do not have direct hydrogen-bonded interactions among themselves. They link the β-alaninium ions into a linear chain running parallel to the longer axis. A head-to-tail hydrogen bond is observed between the n-glide related β-alaninium ions. Strikingly, the cell lengths of (I) are almost similar to those in L-alaninium maleate. However, in (I), β-alaninium interacts with the semi-maleate anion through the ionized carboxylate group, as in glycinium maleate, where as in L-alaninium maleate, the interaction is through the carboxylic acid group. Such differences in the nature of interaction between individual molecules is reflected in the packing mode. The zigzag packing of molecules observed in (I) is distinctly different from those observed in the crystal structures of other amino acid–maleic acid complexes. The crystal packing is also characterized by the presence of two C—H···O hydrogen bonds.

S2. Experimental

Colorless needle-shaped single crystals of (I) were grown from a saturated aqueous solution containing β-alanine and maleic acid in stoichiometric ratio.

S3. Refinement

All the H atoms were generated geometrically and were allowed to ride on their respective parent atoms with SHELXL97 (Sheldrick, 1997) defaults for bond lengths and displacement parameters. The torsion angles about the C—O bonds of the two hydroxyl groups were refined.

Computing details

Data collection: Enraf-Nonius CAD-4; cell refinement: CAD-4 Software (Enraf-Nonius, 1989); 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 50% probability displacement ellipsoids.
	Fig. 2. Packing of the molecules of (I) viewed down the a axis.

(I)

Crystal data

C3H8NO2^+·C4H3O4^−	F(000) = 432
Mr = 205.17	Dx = 1.492 Mg m^−3 Dm = 1.51 Mg m^−3 Dm measured by flotation in mixture of xylene and carbon tetrachloride
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 5.4186 (11) Å	Cell parameters from 25 reflections
b = 22.951 (5) Å	θ = 6–17°
c = 7.4518 (15) Å	µ = 0.13 mm^−1
β = 99.730 (16)°	T = 293 K
V = 913.4 (3) Å^3	Needle, colorless
Z = 4	0.35 × 0.30 × 0.15 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	1344 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.019
Graphite monochromator	θmax = 25.0°, θmin = 2.9°
ω–2θ scans	h = 0→6
Absorption correction: ψ scan (North et al., 1968)	k = 0→27
Tmin = 0.97, Tmax = 0.99	l = −8→8
1775 measured reflections	2 standard reflections every 200 reflections
1600 independent reflections	intensity decay: <2%

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)] = 0.038	H-atom parameters constrained
wR(F^2) = 0.190	w = 1/[σ^2(Fo^2) + (0.1299P)^2 + 0.1847P] where P = (Fo^2 + 2Fc^2)/3
S = 1.16	(Δ/σ)max < 0.001
1600 reflections	Δρmax = 0.30 e Å^−3
130 parameters	Δρmin = −0.30 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: 0.047 (14)

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	0.9600 (3)	0.65798 (8)	0.9827 (3)	0.0496 (6)
H1	0.8833	0.6312	1.0209	0.074*
O2	0.5884 (4)	0.70108 (8)	0.9471 (3)	0.0590 (6)
N1	0.9147 (3)	0.85468 (8)	0.7698 (3)	0.0371 (5)
H1A	0.8328	0.8878	0.7803	0.056*
H1B	0.9073	0.8462	0.6525	0.056*
H1C	1.0739	0.8587	0.8223	0.056*
O3	0.7870 (3)	0.04863 (7)	0.9235 (3)	0.0475 (6)
H3	0.7334	0.0178	0.8765	0.071*
O4	0.6457 (4)	0.13567 (8)	0.9679 (3)	0.0504 (6)
O5	0.6211 (3)	−0.04031 (7)	0.7561 (2)	0.0450 (5)
O6	0.2587 (3)	−0.06948 (7)	0.5973 (2)	0.0482 (6)
C1	0.8066 (4)	0.70073 (10)	0.9310 (3)	0.0378 (6)
C2	0.9311 (5)	0.75014 (10)	0.8468 (3)	0.0408 (6)
H2A	1.1033	0.7537	0.9078	0.049*
H2B	0.9329	0.7414	0.7196	0.049*
C3	0.7980 (5)	0.80692 (10)	0.8605 (4)	0.0421 (6)
H3A	0.6238	0.8029	0.8043	0.051*
H3B	0.8030	0.8166	0.9878	0.051*
C4	0.6116 (4)	0.08756 (10)	0.8980 (3)	0.0350 (6)
C5	0.3642 (4)	0.07430 (10)	0.7857 (3)	0.0374 (6)
H5	0.2522	0.1053	0.7732	0.045*
C6	0.2744 (4)	0.02598 (10)	0.7000 (3)	0.0373 (6)
H6	0.1098	0.0286	0.6399	0.045*
C7	0.3957 (4)	−0.03178 (9)	0.6848 (3)	0.0339 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0440 (11)	0.0380 (10)	0.0668 (12)	0.0048 (8)	0.0094 (8)	0.0142 (8)
O2	0.0438 (11)	0.0502 (12)	0.0876 (15)	0.0067 (8)	0.0241 (10)	0.0223 (10)
N1	0.0342 (11)	0.0277 (10)	0.0491 (12)	0.0015 (7)	0.0056 (8)	−0.0011 (8)
O3	0.0340 (10)	0.0399 (10)	0.0657 (12)	0.0034 (7)	0.0006 (8)	−0.0109 (8)
O4	0.0490 (11)	0.0379 (10)	0.0603 (11)	−0.0004 (7)	−0.0024 (9)	−0.0136 (8)
O5	0.0398 (10)	0.0312 (9)	0.0624 (12)	0.0066 (7)	0.0043 (8)	−0.0059 (7)
O6	0.0483 (11)	0.0353 (10)	0.0600 (11)	−0.0048 (8)	0.0061 (8)	−0.0129 (8)
C1	0.0392 (13)	0.0355 (13)	0.0392 (12)	0.0027 (9)	0.0080 (9)	0.0009 (9)
C2	0.0389 (13)	0.0332 (13)	0.0518 (14)	0.0030 (9)	0.0117 (11)	0.0041 (10)
C3	0.0413 (13)	0.0346 (13)	0.0536 (14)	0.0033 (9)	0.0169 (11)	0.0032 (10)
C4	0.0370 (12)	0.0311 (12)	0.0375 (11)	−0.0008 (9)	0.0076 (9)	0.0004 (9)
C5	0.0346 (12)	0.0317 (12)	0.0457 (12)	0.0059 (9)	0.0064 (10)	−0.0034 (9)
C6	0.0304 (11)	0.0376 (13)	0.0430 (12)	0.0017 (9)	0.0034 (9)	−0.0032 (10)
C7	0.0375 (12)	0.0292 (11)	0.0366 (11)	−0.0012 (9)	0.0106 (9)	0.0008 (9)

Geometric parameters (Å, º)

O1—C1	1.301 (3)	C1—C2	1.509 (3)
O1—H1	0.8200	C2—C3	1.501 (3)
O2—C1	1.209 (3)	C2—H2A	0.9700
N1—C3	1.484 (3)	C2—H2B	0.9700
N1—H1A	0.8900	C3—H3A	0.9700
N1—H1B	0.8900	C3—H3B	0.9700
N1—H1C	0.8900	C4—C5	1.487 (3)
O3—C4	1.295 (3)	C5—C6	1.330 (3)
O3—H3	0.8200	C5—H5	0.9300
O4—C4	1.222 (3)	C6—C7	1.492 (3)
O5—C7	1.262 (3)	C6—H6	0.9300
O6—C7	1.250 (3)
C1—O1—H1	109.5	N1—C3—H3A	109.4
C3—N1—H1A	109.5	C2—C3—H3A	109.4
C3—N1—H1B	109.5	N1—C3—H3B	109.4
H1A—N1—H1B	109.5	C2—C3—H3B	109.4
C3—N1—H1C	109.5	H3A—C3—H3B	108.0
H1A—N1—H1C	109.5	O4—C4—O3	120.4 (2)
H1B—N1—H1C	109.5	O4—C4—C5	118.8 (2)
C4—O3—H3	109.5	O3—C4—C5	120.7 (2)
O2—C1—O1	124.2 (2)	C6—C5—C4	131.5 (2)
O2—C1—C2	123.4 (2)	C6—C5—H5	114.2
O1—C1—C2	112.5 (2)	C4—C5—H5	114.2
C3—C2—C1	111.7 (2)	C5—C6—C7	130.3 (2)
C3—C2—H2A	109.3	C5—C6—H6	114.8
C1—C2—H2A	109.3	C7—C6—H6	114.8
C3—C2—H2B	109.3	O6—C7—O5	124.0 (2)
C1—C2—H2B	109.3	O6—C7—C6	115.1 (2)
H2A—C2—H2B	107.9	O5—C7—C6	120.9 (2)
N1—C3—C2	111.3 (2)
O2—C1—C2—C3	24.6 (4)	O3—C4—C5—C6	1.4 (4)
O1—C1—C2—C3	−155.8 (2)	C4—C5—C6—C7	−1.3 (5)
C1—C2—C3—N1	−177.4 (2)	C5—C6—C7—O6	178.3 (2)
O4—C4—C5—C6	−177.7 (3)	C5—C6—C7—O5	−2.0 (4)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O6^i	0.82	1.71	2.522 (3)	172
N1—H1A···O5^ii	0.89	2.00	2.880 (3)	170
N1—H1B···O2^iii	0.89	2.23	3.012 (3)	146
N1—H1C···O4^iv	0.89	1.99	2.823 (3)	155
O3—H3···O5	0.82	1.66	2.481 (2)	173
C2—H2B···O2^iii	0.97	2.67	3.421 (3)	134
C6—H6···O6^v	0.93	2.61	3.477 (3)	156

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