Crystal structures of three halide salts of l-asparagine: an isostructural series

The structures of three monohydrated halide salt forms of l-asparagine are presented, viz. l-asparaginium chloride monohydrate, C(4)H(9)N(2)O(3) (+)·Cl(−)·H(2)O, (I), l-asparaginium bromide monohydrate, C(4)H(9)N(2)O(3) (+)·Br(−)·H(2)O, (II), and l-asparaginium iodide monohydrate, C(4)H(9)N(2)O(3) (+)·I(−)·H(2)O, (III). These form an isomorphous and isostructural series. The C—C—C—C backbone of the amino acid adopts a gauche conformation in each case [torsion angles for (I), (II) and (III) = −55.4 (2), −55.6 (5) and −58.3 (7)°, respectively]. Each cation features an intra­molecular N—H⋯O hydrogen bond, which closes an S(6) ring. The extended structures feature chains of cations that propagate parallel to the b-axis direction. These are formed by carb­oxy­lic acid/amide complimentary O—H⋯O + N—H⋯O hydrogen bonds, which generate R (2) (2)(8) loops. These chains are linked by further hydrogen bonds mediated by the halide ions and water mol­ecules to give a layered structure with cation and anion layers parallel to the ab plane. Compound (III) was refined as an inversion twin.