Semi-maleate salts of L- and DL-serin­ium: the first example of chiral and racemic serinium salts with the same composition and stoichiometry

L-Serinium semi-maleate, (I), and DL-serinium semi-maleate, (II), both C₃H₈NO₃⁺·C₄H₃O₄^-, provide the first example of chiral and racemic anhydrous serine salts with the same organic anion. A comparison of their crystal structures with each other, with the structures of the pure components (L-serine polymorphs, DL-serine and maleic acid) and with other amino acid maleates is important for understanding the formation of the crystal structures, their response to variations in temperature and pressure, and structure-property relationships. As in other known crystal structures of amino acid maleates, there are no direct links between the semi-maleate anions in the two new structures. The serinium cations have different conformations in (I) and (II). In (I), they are linked into infinite chains via hydrogen bonds between carb­oxylic acid and hy­droxy groups. In (II), there are no such chains formed by the serinium cations. In both (I) and (II), there are C₂²(12) chains consisting of alternating semi-maleate anions and serinium cations. Two types of such chains are present in (I) and (II), termed C₂²(12) and C₂²(12)'. In (I), these chains, lying in the same plane, are further linked to each other via hydrogen bonds, whereas in (II) they are not.