N 2-(4-Methoxysalicylidene)arginine hemihydrate

The title compound, C14H20N4O4·0.5H2O [systematic name: (2S)-5-{[amino(iminiumyl)methyl]amino}-2-{[(1Z)-4-methoxy-2-oxidobenzylidene]azaniumyl}pentanoate hemihydrate], has been synthesized by the reaction of l-arginine and 4-methoxysalicylaldehyde and crystallizes with two independent substituted l-arginine molecules and one water molecule of solvation in the asymmetric unit. Each molecule exists as a zwitterion and adopts a Z configuration about the central C=N. The molecular conformation is stabilized by strong intramolecular N—H⋯O hydrogen bonds that generate S(6) and S(10) ring motifs. Intermolecular N—H⋯O and O—H⋯O hydrogen bonds involving also the water molecule and weak intermolecular C—H⋯Owater interactions link the molecules into an infinite one-dimensional ribbon structure extending along the b axis. The known (2S) absolute configuration for l-arginine was invoked. Weak intermolecular C—H⋯π interactions are also present.

The title compound, C 14 H 20 N 4 O 4 Á0.5H 2 O [systematic name: (2S)-5-{[amino(iminiumyl)methyl]amino}-2-{[(1Z)-4-methoxy-2-oxidobenzylidene]azaniumyl}pentanoate hemihydrate], has been synthesized by the reaction of l-arginine and 4-methoxysalicylaldehyde and crystallizes with two independent substituted l-arginine molecules and one water molecule of solvation in the asymmetric unit. Each molecule exists as a zwitterion and adopts a Z configuration about the central C N. The molecular conformation is stabilized by strong intramolecular N-HÁ Á ÁO hydrogen bonds that generate S(6) and S(10) ring motifs. Intermolecular N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds involving also the water molecule and weak intermolecular C-HÁ Á ÁO water interactions link the molecules into an infinite one-dimensional ribbon structure extending along the b axis. The known (2S) absolute configuration for l-arginine was invoked. Weak intermolecular C-HÁ Á Á interactions are also present.
The H atoms attached to the phenolic groups (O2 and O6) are transferred to the basic centres N1 and N5 respectively, generating the iminium groups. Also, the carboxylic H-atoms on O4 and O7 have been transferred to N4 and N8, respectively, to generate the common amino acid zwitterions.
In both molecules A and B, all nitrogen H-atoms are involved in hydrogen bonding (Table 1). In each, intramolecular N -H···O hydrogen bonds lead to the formation of a six-and a ten-membered ring motif [S(6) and S(10), respectively (Bernstein et al., 1995)] (Fig.1). Intermolecular N-H···O and O-H···O hydrogen bonds involving also the water molecule and weak intermolecular C-H···O water interactions link the molecules into an infinite one-dimensional ribbon structure extending along the b axis (Fig. 2). Present also are weak intermolecular C-H..π interactions.

Experimental
L-Arginine and 4-methoxy salicylaldehyde (E-Merck-analar grades) were mixed in 1:1 stoichiometric proportions and dissolved in a triply distilled water-ethanol mixture using a mechanical stirrer for about four hours. The raw reaction product was removed by filtration, then re-dissolved in a water-ethanol solvent mixture and kept aside to allow crystal growth at ambient temperature. Bright yellowish crystals formed in 3 days and on removal were recrystallized several times to obtain the crystal specimen used in the X-ray analysis.

Refinement
The H atoms were positioned geometrically, with methyl C-H distances of 0.96 Å (methylene), 0.93 Å (aromatic) and the N2-H and N6-H distances of 0.86 Å, and were refined as riding on their parent atoms, with U iso (H) = 1.2-1.5 U eq of the parent atom. The remaining N-H atoms and water molecule H atoms were located from a difference Fourier map and refined with distance restraints [N-H = 0.90 (2) and O-H = 0.91 (2) Å] with U iso (H) = 1.2U eq (N) and U iso (H) = 1.5U eq (O). The known (2S) absolute configuration for L-arginine was invoked at the trivially numbered chiral centres of the A and B molecules (C9 and C23, respectively) (Flack parameter: 0.01 (14) for 3448 Friedel pairs).

Computing details
Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 and SAINT (Bruker, 2004); data reduction: SAINT and XPREP (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009    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 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 > σ(F 2 ) is used only for calculating R-factors(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.