L-cysteinium semioxalate: a new monoclinic polymorph or a hydrate?

The title compound, C(3)H(8)NO(2)S(+)·C(2)HO(4)(-), (I), crystallizes in the monoclinic C2 space group and is a new form (possibly a hydrate) of L-cysteinium semioxalate with a stoichiometric cation-anion ratio of 1:1. In contrast to the previously known orthorhombic form of L-cysteinium semioxalate, (I) has a layered structure resembling those of monoclinic L-cysteine, as well as of DL-cysteine and its oxalates. The conformations of the cysteinium cation and the oxalate anion in (I) differ substantially from those in the orthorhombic form. The structure of (I) has voids with a size sufficient to incorporate water molecules. The residual density, however, suggests that if water is in fact present in the voids, it is strongly disordered and its amount does not exceed 0.3 molecules per void. The difference in conformation of the cysteinium cations in (I) and in the orthorhombic form is similar to that in DL-cysteine under ambient conditions and in DL-cysteine under high pressure or at low temperature.