Crystal structure of glycidamide: the mutagenic and genotoxic metabolite of acrylamide

The title compound, glycidamide, was synthesized via the reaction of acrylonitrile and hydrogen peroxide. Both enantiomers occur as two crystallographically independent molecules in the asymmetric unit. In the crystal, molecules are linked by N—H⋯O hydrogen bonds forming β-sheet structures. The β-sheets are linked by weaker C—H⋯O hydrogen bonds, forming a supramolecular three-dimensional structure.

The title compound, glycidamide (systematic name: oxirane-2-carboxamide), C 3 H 5 NO 2 , is the mutagenic and genotoxic metabolite of acrylamide, a food contaminant and industrial chemical that has been classified as being probably carcinogenic to humans. Synthesized via the reaction of acrylonitrile and hydrogen peroxide, it crystallizes with both enantiomers occurring as two crystallographically independent molecules (A and B) in the asymmetric unit. They have similar conformations with an r.m.s. deviation of 0.0809 Å for molecule B inverted on molecule A. In the crystal, molecules are linked by N-HÁ Á ÁO hydrogen bonds, which lead to the formation of -sheet structures enclosing R 2 2 (8) and R 4 2 (8) loops. The -sheets are linked by weaker C-HÁ Á ÁO hydrogen bonds, forming a supramolecular three-dimensional structure.

Chemical context
The formation of glycidamide (GA) is considered to cause the carcinogenicity of acrylamide (AA; Udovenko & Kolzunova, 2008), which is a widely used chemical in industry (EPA, 1994). Typical applications include the production of copolymers, flocculation agents and carrier material for gel electrophoresis. Moreover, it is formed if certain foods are heated to temperatures above 393 K at low moisture. AA was found at the highest levels in solid coffee substitutes, fried potato products and gingerbread, thus contributing to human exposure (EFSA, 2015). AA forms predominantly from asparagine in the presence of reducing sugars during the Maillard reaction via a Strecker-type degradation (Mottram et al., 2002;Stadler et al., 2002;Tareke et al., 2002;Yaylayan et al., 2003). Besides being a food contaminant, AA is also a component of tobacco smoke (Papoušek et al., 2014). It has also been classified as 'probably carcinogenic to humans (Group 2A)' by the International Agency for Research on Cancer (IARC, 1994). It has not been found to be mutagenic or genotoxic without metabolic activation to GA at biologically relevant concentrations (Watzek et al., 2012).
GA is a genotoxic and mutagenic compound formed in vivo metabolically from AA, mainly in the liver by cytochrome P450 2E1 (Baum et al., 2005). As a reactive epoxide, GA is able to react with nucleophilic centers of proteins and DNA, thus forming DNA adducts and hemoglobin conjugates (Ghanayem et al., 2005). As a consequence, mutations may occur, which represent stages of chemical mutagenesis and carcinogenesis (Gamboa da Costa et al., 2003). We synthesized GA via the reaction of acrylonitrile and hydrogen peroxide.

Structural commentary
Owing to its size, GA shows few structural features. Both enantiomers occur as two crystallographically independent molecules (A and B) in the asymmetric unit ( Fig. 1). They have similar conformations with an r.m.s. deviation of 0.0809 Å for molecule B inverted on molecule A. The amide group is inclined to the epoxide plane by 77.9 (2) in molecule A (N1/ C1/O1 vs O2/C2/C3), and by 72.6 (2) in molecule B (N11/C11/ O11 vs O12/C12/C13).
Of interest are the C-C as well as the C-O bond lengths in the epoxide fragment (Table 1). The values of bond lengths in the epoxide fragments of both enantiomers are compared to the mean values and their standard deviation of a selection of 149 similarly substituted compounds featuring an epoxide fragment (Table 1), which were reported to the Cambridge Structural Database (CSD, Version 5.37, Update 2 Feb 2016; Groom et al., 2016). While the epoxide C-O bonds in GA match the mean values quite well, the C-C bond is more at the upper end for bond lengths. Nevertheless, the C-C bond length is still in the range of one standard deviation to the mean value of the database entries. Omitting oxirane itself, GA is the smallest example of an epoxide crystal structure reported in the CSD. Summing up, the epoxide fragment in GA seems to be representative for this class of epoxides.

Supramolecular features
In the crystal, there are as expected, hydrogen bonds dominating the solid-state structure. The protons of the amino moiety undergo strong N-HÁ Á ÁO hydrogen bonding to the carbonyl groups of adjacent GA molecules (Table 2, Fig. 2). This results in the formation of a -sheet structure, which is parallel to the crystallographic b axis and encloses R 2 2 (8) and R 2 4 (8) loops. The -sheets are also oriented parallel to each other ( Fig. 3). They are further interlinked by additional but weaker C-HÁ Á ÁO hydrogen bonds (Table 2), between the protons ot the -CH 2 -units with the carbonyl group and the epoxy function from the neighbouring -sheets, which leads to the formation of a supramolecular three-dimensional structure (Fig. 4).

Database survey
As noted in Section 2, a search of the Cambridge Structural Database (Groom et al., 2016) revealed the presence of 149 similarly substituted compounds featuring an epoxide fragment. However, up to now there has been no report of the structure of the title compound (GA). The molecular structure of the two independent molecules (A and B) of the title compound, glycidamide (GA), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A partial view of the crystal packing of the title compound, showing the -sheet arrangement, formed through strong N-HÁ Á ÁO hydrogen bonds (dashed lines; see Table 2 for details), propagating along the b-axis direction.

Synthesis and crystallization
The synthesis of the title compound (GA) was performed according to a published method with modifications (Payne & Williams, 1961). The conventional literature procedure by controlled pH and temperature resulted in an unfavorable decomposition of hydrogen peroxide. GA was synthesized by dropwise addition of 1 M NaOH (60 ml) to acrylonitrile (80.1 g, 1.22 mol) in water (500 ml) and 30% H 2 O 2 (102 ml, 1 mol). The pH was kept at 7.3-7.5 and the temperature was maintained at 308-310 K. After the reaction was completed (about 12 h), the mixture was treated with 5% palladium on charcoal, stored overnight in a refrigerator and then filtered.
The solvent was evaporated and the crude product (yield: 55 g; 63%) was recrystallized from dry acetone at low temperature. Colourless crystals formed after 3-5 days at 243 K. GA is very hygroscopic, so purification of the raw product was carried out in an inert atmosphere. The compound was stored in dry argon at 243 K. Identity and purity were checked by NMR spectroscopic methods and elemental analysis.

Figure 3
A view along the b axis of the crystal packing of the title compound, showing the -sheet arrangement formed through strong N-HÁ Á ÁO hydrogen bonds (dashed lines; see Table 2 for details). The C-bound H atoms have been omitted for clarity (A molecules = black; B molecules = red).   SIR2014 (Burla et al., 2015); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008) and PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2009).

Oxirane-2-carboxamide
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.