(S)-2-[(2-Hydroxybenzyl)azaniumyl]-4-(methylsulfanyl)butanoate

The zwitterionic title compound, C12H17NO3S, is a reduced Schiff base derived from (S)-N-(2-hydroxybenzylidene)methionine. An intramolecular interaction between the N—H and carboxylate groups forms a roughly planar (r.m.s. deviation = 0.1405 Å) five-membered ring containing the H(N), N, Cα, C(carboxylate) and O atoms in a pentagonal conformation. In the crystal, a supramolecular triangle-shaped motif is generated by molecules held together by O—H⋯O and N—H⋯O hydrogen bonds.

The zwitterionic title compound, C 12 H 17 NO 3 S, is a reduced Schiff base derived from (S)-N-(2-hydroxybenzylidene)methionine. An intramolecular interaction between the N-H and carboxylate groups forms a roughly planar (r.m.s. deviation = 0.1405 Å ) five-membered ring containing the H(N), N, C, C(carboxylate) and O atoms in a pentagonal conformation. In the crystal, a supramolecular triangle-shaped motif is generated by molecules held together by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds.

Comment
Considerable attention has been devoted to both Schiff bases and reduced Schiff bases derived from salicylaldehyde and αamino acids, since their transition metal complexes are closely analogous to the metal-free systems formed as intermediates in many reactions involving the vitamin B6, such as transamination, decarboxylation, α-and β-elimination and racemization (Martell, 1989). In this regard, copper(II) complexes of N-(2-hydroxybenzyl)-α-amino acids have been studied as models for the intermediate species that occur in the biological reactions mentioned above (Koh et al., 1996) and such reduced Schiff bases have been used as chelating agents for organoboron (Nefkens et al., 1985;Beltrán et al., 2002) and transition metals such as copper (Koh et al., 1996), zinc (Ritsma, 1975), cobalt (Bandyopadhyay et al., 2006), nickel (Sreenivasulu et al., 2004), manganese (Shongwe et al., 1999), technetium (Wilson, 1990) and vanadium (Maurya, 2003). These ligands are more stable than the Schiff bases from which they originate and are suitable to provide conformationally flexible rings in complexation. This results in different solid state architectures, and the presence of hydrogen bond donors and acceptors enables the design and construction of supramolecular, three-dimensional networks (Ganguly et al., 2008). This paper describes the structural characterization of the reduced Schiff base (S)-N-(2-hydroxybenzyl)methionine (I). Figure 1 shows the molecular structure of (I). The crystal structure analysis clearly indicates that the amino acid is a pure enantiomer, since the compound crystallizes in the chiral P1 space group. In Figure 1 the L-enantiomer is shown with S absolute configuration at C8, in agreement with the synthetic precursor, L-(S)-methionine. The carboxylic acid has been found in the deprotonated -(COO)form, with the C9-O2 and C9-O3 bond distances being very similar. Meanwhile, N1 is protonated in the NH 2 + form. Therefore compound (I) crystallizes as a zwitterion. The angle N1-C8-C9 is lower in comparison to the angles N1-C8-C10 and C10-C8-C9. This asymmetry in the angles at the C8 is due to the presence of an intramolecular interaction between the N-H and carboxylate groups forming a roughly planar five-membered ring containing H, N, Cα, C and O atoms in a pentagonal (C 5 ) conformation.
A very interesting feature encountered in the crystal lattice of (I) is related to the formation of supramolecular triangleshaped motifs [R 2 3 (8)] involving molecules held together by O-H···O and N-H···O hydrogen bonds ( Figure 2, Table  1). Other hydrogen bonding interactions govern the molecular arrangement in parallel strings extending along the (001) crystallographic plane (Figure 3).

Experimental
The synthesis of the title compound (I) was performed according to the method previously employed for similar Schiff bases (Koh, et al., 1996), starting from L-(S)-metionine. Recrystallization of (I) from a methanol solution produced single crystals suitable for X-ray diffraction.
supplementary materials sup-2 Refinement H atoms were located in a difference Fourier map and placed in idealized positions using the riding-model technique, with distances C-H = 0.93-0.97 Å, N-H = 0.90 Å and with U iso (H) = 1.2U eq (C) or U iso (H) = 1.2U eq (N). The best refinement was obtained using the multi-scan SADABS (Sheldrick, 1996) correction. The restraints were generated automatically by SHELXL97 to fix the origin. Fig. 1. : ORTEP drawing with numbering of the atoms. Non H-atoms are represented as displacement ellipsoids plotted at the 50% probability level, while H-atoms bound to heteroatoms are shown as small spheres of arbitrary radius. The intramolecular H-bond is represented by a dashed line.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.