Bis(glycylglycinium) oxalate at 100 K

The structure of the title compound, 2C₄H₉N₂O₃⁺·C₂O₄²⁻, which has been determined by X-ray diffraction, contains discrete glycyl­glycine (HGly–Gly)⁺ cations in general positions and oxalate anions which lie across centres of inversion. Although the geometry of the (HGly–Gly)⁺ cation is not significantly different compared with other structures containing this residue, a few changes in conformation are observed which indicate the presence of molecular interactions. The molecular network in the crystal consists of one nearly linear O—H...O, five N—H...O and two weak C—H...O hydrogen bonds.

S1. Comment

Compounds derived from amino acids, often formed by weak van der Waals interactions and hydrogen bonds, possess a high degree of delocalization and hence are expected to be more non-linear than their inorganic counterparts. Furthermore, amino acids have peculiar physical and chemical properties which are attributed to the presence of an H-atom donor carboxylic acid group (–COO) and an H-atom acceptor amino group (–NH₂). Due to this dipolar nature, amino acids and related compounds often have physical properties which make them potential candidates for non-linear optical (NLO) activity (Fuchs et al., 1989).

The geometry of glycylglycine (Gly-Gly) has been investigated by X-ray and neutron diffraction at room temperature (Biswas et al., 1968; Hughes, 1968; Freeman et al., 1970; Griffin & Coppens, 1975) and at 82 K (Kvick et al., 1977). The X-ray charge densities have been used in the calculation of intermolecular interactions and lattice energies in the crystal of Gly-Gly (Abramov Volkov & Coppens, 2000; Abramov, Volkov et al., 2000). Analyses of the hydrogen bonds present and their influence on the conformation of the Gly-Gly moiety in the structures of Gly-Gly as the hydrochloride (Parthasarathy, 1969), monohydrochloride monohydrate (Koetzle et al., 1972), nitrate (Narasinga Rao & Parthasarathy, 1973), phosphate monohydrate (Freeman et al., 1972) and phosphite (Averbuch-Pouchot, 1993) have been reported. The interaction between Gly-Gly and polyoxometalates has been examined for an understanding of their antitumour and anti-HIV activity (Crans et al., 1994; Han et al., 2002). We present here the crystal structure of the title salt, (I), obtained from Gly-Gly and oxalic acid. \sch

The crystals of (I) consist of discrete glycylglycine cations, (HGly-Gly)⁺, and oxalic dianions, C₂O₄²⁻ (Fig. 1). The characteristic structural features of peptides are the planarity of the peptide group and the constancy of the dimensions of the peptide unit, which are independent of different amino acid constituents. The geometry of the (HGly-Gly)⁺ cation in (I), as defined by the bond distances and angles, is comparable with the corresponding parameters observed in similar structures containing this residue (Parthasarathy, 1969; Koetzle et al., 1972; Narasinga Rao & Parthasarathy, 1973; Han et al., 2002).

Atoms C2, C3, O4, N5, C6 and H5 define the peptide unit, which is almost planar. The average deviaton from the least-squares plane through atoms C2—C6 is 0.01 Å. Furthermore, the planarity of the peptide unit is defined by the torsion angle ω, which characterizes the rotation around the C—N peptide bond. An ω value of 180° corresponds to a planar peptide unit (trans conformation). In compound (I), the deviation of ω (C2—C3—N5—C6) from 180° is about 2° (Table 3). The maximum twist around the peptide bond in Gly-Gly structures has been observed in (HGly-Gly)₃PMo₁₂O₄₀·4H₂O (167.1°; Han et al., 2002); this deviation was attributed to the interaction between the Gly-Gly unit and the polyanion. The full conformation of a peptide chain is described by two additional torsion angles, ψ and ϕ. These angles characterize the twist around the C—C^α and N—C^α bonds, respectively. These torsion angles for (I) are given in Table 3 and suggest that the (HGly-Gly)⁺ cation is in an almost extended conformation. A similar conformation of the (HGly-Gly)⁺ cation has been reported in Gly-Gly nitrate (Narasinga Rao & Parthasarathy, 1973) and (HGly-Gly)₃PMo₁₂O₄₀·4H₂O (Han et al., 2002).

The carboxylate end of the (HGly-Gly)⁺ cation of (I) (atoms C6, C7, O8 and O9) is almost planar: the dihedral angle describing the inclination of the carboxylate moiety to the peptide plane (atoms C2, C3, O4, N5, C6) is 32.57 (6)°. A similar dihedral angle of 32.22 (7)° is observed when the peptide plane is defined only by atoms C2, C3, O4 and N5, as has been performed by Kvick et al. (1977). Furthermore, the dihedral angle between the peptide plane (atoms C2, C3, O4, N5, C6) and the carboxylate group (atoms C7, O8 and O9) is 31.9 (1)°, which is not significantly different from the values given above. Han et al. (2002) calculated this angle by defining the peptide-group plane using only C, N and O atoms in the structure of (HGly-Gly)₃PMo₁₂O₄₀·4H₂O, where there are three symmetrically independent (HGly-Gly)⁺ cations. The dihedral angles in that structure between the plane of the peptide group so defined and the carboxylate group range from 48.3 to 62.6°, which is unusual (Han et al., 2002). By calculating the dihedral angle in this way for (I), using atoms C3, O4, N5 and C7, O8, O9, we have obtained a value of 31.6 (2)°. In the present structure, the inclination of the peptide plane to that of the carboxylic acid end or group is greater than 30°, no matter how the peptide or carboxylate (carboxylate end or group) planes are defined. This is in contrast with the principle that the carboxylate and peptide groups are approximately coplanar or perpendicular, as is observed, for example, in glycylglycine nitrate (16.5°; Narasinga Rao & Parthasarathy, 1973) and in glycylglycine hydrochloride (77.9°; Parthasarathy, 1969).

The centrosymmetric oxalate dianion in (I) is strictly coplanar. The C10—O11 and C10—O12 bond distances (Table 1) clearly indicate that the carboxylate groups are not protonated. These values and the other geometric parameters correlate well with the corresponding values found in crystal structures containing the C₂O₄²⁻ moiety (Newkome et al., 1985; Van der Brempt et al., 1985; Braga et al., 2002).

The crystal packing in (I) is stabilized by ionic interactions between the (HGly-Gly)⁺ cations and oxalic dianions, and by strong hydrogen bonds. The N—H···O hydrogen bond involving the peptide N atom is significantly longer than the hydrogen bonds donated by the N atom of the ammonium group, similar to what is observed in the structures of glycylglycine nitrate (Narasinga Rao & Parthasarathy, 1973) and α-glycylglycine (Kvick et al., 1977). The three H atoms of the ammonium group participate in four strong hydrogen bonds. One is to the carboxyl O atom of an (HGly-Gly)⁺ cation (atom H1B) and three involve the oxalate anion (atoms H1A and H1C; Table 2). Atom H5 of the amide group takes part in only one intercation N—H···O hydrogen bond, involving the amide O atom of a neighbouring cation. Atom H9 of the carboxyl group participates in a nearly linear strong O—H···O hydrogen bond with the O atom of an oxalate anion. Weak C—H···O hydrogen bonds are formed between the H atom of the α C atom (C2) and the O atoms of neighbouring (HGly-Gly)⁺ cations (Table 2). Overall, the hydrogen bonds link the ions into a three-dimensional framework.

S2. Experimental

Crystals of (I) were grown by slow evaporation at room temperature of an aqueous solution containing glycylglycine and oxalic acid in a 1:1 stoichiometric ratio.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2002); cell refinement: CrysAlis RED (Oxford Diffraction, 2002); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure of the (HGly-Gly)^+ cation and oxalate dianion in (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii [symmetry code: (i) 2 − x, 1 − y, 1 − z].
	Fig. 2. The packing diagram for (I), showing the hydrogen bonding (dashed lines).

Bis(glycylglycinium) oxalate

Crystal data

2C4H9N2O3^+·C2O4^2^−	F(000) = 372
Mr = 354.28	Dx = 1.594 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 4592 reflections
a = 8.2384 (7) Å	θ = 3.2–26.3°
b = 10.0582 (9) Å	µ = 0.14 mm^−1
c = 8.9323 (8) Å	T = 100 K
β = 94.477 (7)°	Cube, colourless
V = 737.90 (11) Å^3	0.20 × 0.18 × 0.16 mm
Z = 2

Data collection

Oxford Diffraction Xcalibur area-detector diffractometer	1301 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.031
Graphite monochromator	θmax = 26.3°, θmin = 3.2°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −10→10
ω scans	k = −12→11
4592 measured reflections	l = −11→10
1495 independent reflections

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)] = 0.029	Hydrogen site location: difference Fourier map
wR(F^2) = 0.078	All H-atom parameters refined
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0487P)^2 + 0.1784P] where P = (Fo^2 + 2Fc^2)/3
1495 reflections	(Δ/σ)max = 0.001
145 parameters	Δρmax = 0.24 e Å^−3
0 restraints	Δρmin = −0.24 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
N1	0.75294 (12)	0.74963 (10)	0.64941 (12)	0.0136 (2)
C2	0.61305 (14)	0.66018 (12)	0.66419 (14)	0.0139 (3)
C3	0.48826 (13)	0.72708 (12)	0.75448 (12)	0.0134 (3)
O4	0.48690 (10)	0.84842 (8)	0.77242 (10)	0.0186 (2)
N5	0.38033 (13)	0.64385 (11)	0.80833 (12)	0.0173 (2)
C6	0.24813 (14)	0.68962 (13)	0.89344 (14)	0.0169 (3)
C7	0.21067 (14)	0.59029 (11)	1.01242 (13)	0.0136 (3)
O8	0.27424 (10)	0.48109 (8)	1.02358 (9)	0.0164 (2)
O9	0.10133 (10)	0.63483 (8)	1.09898 (9)	0.0164 (2)
C10	0.98388 (14)	0.53460 (11)	0.42170 (13)	0.0128 (3)
O11	1.04948 (11)	0.48094 (8)	0.31431 (9)	0.0180 (2)
O12	0.89739 (10)	0.63627 (8)	0.41527 (9)	0.0170 (2)
H1A	0.8036 (19)	0.7761 (15)	0.7424 (18)	0.030 (4)*
H1B	0.7221 (18)	0.8267 (16)	0.6012 (16)	0.020 (4)*
H1C	0.827 (2)	0.7051 (17)	0.5901 (19)	0.036 (4)*
H2A	0.6489 (17)	0.5794 (15)	0.7096 (15)	0.017 (3)*
H2B	0.5646 (17)	0.6400 (13)	0.5620 (16)	0.016 (3)*
H5	0.3958 (18)	0.5616 (17)	0.8049 (16)	0.024 (4)*
H6A	0.1502 (18)	0.7031 (15)	0.8286 (16)	0.022 (4)*
H6B	0.2782 (18)	0.7759 (15)	0.9405 (16)	0.023 (4)*
H9	0.079 (2)	0.568 (2)	1.181 (2)	0.054 (6)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0138 (5)	0.0137 (5)	0.0139 (5)	0.0010 (4)	0.0047 (4)	−0.0003 (4)
C2	0.0138 (6)	0.0137 (6)	0.0147 (6)	−0.0004 (4)	0.0032 (4)	0.0006 (4)
C3	0.0124 (6)	0.0168 (6)	0.0107 (5)	0.0017 (4)	0.0000 (4)	0.0022 (4)
O4	0.0170 (4)	0.0153 (5)	0.0244 (5)	0.0007 (3)	0.0072 (4)	−0.0007 (3)
N5	0.0177 (5)	0.0137 (6)	0.0218 (6)	0.0022 (4)	0.0089 (4)	0.0020 (4)
C6	0.0139 (6)	0.0187 (6)	0.0187 (6)	0.0029 (5)	0.0059 (5)	0.0020 (5)
C7	0.0111 (5)	0.0166 (6)	0.0129 (6)	−0.0016 (4)	−0.0003 (4)	−0.0025 (4)
O8	0.0186 (4)	0.0147 (4)	0.0162 (4)	0.0016 (3)	0.0034 (3)	−0.0007 (3)
O9	0.0168 (4)	0.0180 (4)	0.0153 (4)	0.0034 (3)	0.0061 (3)	0.0020 (3)
C10	0.0109 (5)	0.0137 (6)	0.0140 (6)	−0.0024 (4)	0.0024 (4)	0.0002 (4)
O11	0.0228 (5)	0.0181 (4)	0.0138 (4)	0.0054 (4)	0.0070 (3)	0.0015 (3)
O12	0.0191 (4)	0.0178 (4)	0.0145 (4)	0.0057 (3)	0.0040 (3)	0.0016 (3)

Geometric parameters (Å, º)

N1—C2	1.4761 (15)	N5—H5	0.838 (16)
N1—H1A	0.939 (17)	C6—C7	1.5081 (17)
N1—H1B	0.913 (16)	C6—H6A	0.965 (15)
N1—H1C	0.950 (18)	C6—H6B	0.988 (15)
C2—C3	1.5132 (16)	C7—O8	1.2176 (14)
C2—H2A	0.945 (15)	C7—O9	1.3110 (14)
C2—H2B	0.989 (14)	O9—H9	1.02 (2)
C3—O4	1.2311 (15)	C10—O12	1.2451 (14)
C3—N5	1.3377 (16)	C10—O11	1.2588 (14)
N5—C6	1.4510 (15)	C10—C10^i	1.566 (2)
C2—N1—H1A	113.0 (10)	C3—N5—H5	119.8 (10)
C2—N1—H1B	111.7 (9)	C6—N5—H5	117.0 (10)
H1A—N1—H1B	105.4 (13)	N5—C6—C7	111.24 (10)
C2—N1—H1C	107.5 (10)	N5—C6—H6A	110.9 (9)
H1A—N1—H1C	111.5 (14)	C7—C6—H6A	108.0 (9)
H1B—N1—H1C	107.7 (13)	N5—C6—H6B	109.2 (9)
N1—C2—C3	110.14 (10)	C7—C6—H6B	109.9 (8)
N1—C2—H2A	110.0 (8)	H6A—C6—H6B	107.6 (12)
C3—C2—H2A	110.9 (8)	O8—C7—O9	124.82 (11)
N1—C2—H2B	107.7 (8)	O8—C7—C6	123.02 (11)
C3—C2—H2B	110.0 (8)	O9—C7—C6	112.15 (10)
H2A—C2—H2B	108.1 (11)	C7—O9—H9	111.7 (11)
O4—C3—N5	124.09 (11)	O12—C10—O11	126.53 (11)
O4—C3—C2	121.58 (10)	O12—C10—C10^i	117.56 (12)
N5—C3—C2	114.31 (11)	O11—C10—C10^i	115.91 (12)
C3—N5—C6	122.48 (11)
N1—C2—C3—O4	18.65 (15)	N5—C6—C7—O8	−6.52 (17)
N1—C2—C3—N5	−162.80 (10)	N5—C6—C7—O9	174.46 (10)
O4—C3—N5—C6	0.56 (19)	O11—C10—C10^i—O11^i	180.0
C2—C3—N5—C6	−177.94 (10)	O11—C10—C10^i—O12^i	−0.2 (2)
C3—N5—C6—C7	−145.99 (11)

Symmetry code: (i) −x+2, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O12^ii	0.939 (17)	1.890 (17)	2.8163 (13)	168.7 (14)
N1—H1B···O8^iii	0.913 (16)	1.913 (16)	2.7934 (13)	161.3 (13)
N1—H1C···O12	0.950 (18)	1.843 (18)	2.7348 (13)	155.2 (15)
N1—H1C···O11^i	0.950 (18)	2.265 (17)	2.8373 (13)	117.9 (13)
N5—H5···O4^iv	0.838 (16)	2.472 (16)	3.2665 (14)	158.5 (14)
O9—H9···O11^v	1.02 (2)	1.51 (2)	2.5310 (12)	174.3 (18)
C2—H2A···O4^iv	0.945 (15)	2.589 (14)	3.3022 (15)	132.5 (11)
C2—H2A···O8^vi	0.945 (15)	2.493 (14)	3.2024 (15)	131.9 (11)

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