New crystals of L-histidine maleates

Two new crystals were obtained in the system L-histidine (L-His) + maleic acid + H2O: L-His+·maleate−·H2O (I) and L-His2+·2maleate− (II) in addition to two previously known species: L-His+·maleate− (III) and 2L-His+·2maleate−·3H2O (IV). Crystal structures and vibrational spectra of (I) and (II) are determined and compared with structures of (III) and (IV). L-His+·maleate−·H2O is monoclinic (space group P21, Z = 2), L-His2+·2maleate− is triclinic (space group P1, Z = 2). The conformation of the histidine molecules in (I) is closed – as it was found in (III) – whereas in (II) all molecules are in open conformation. Moreover, it is shown that in the structure published for (IV), extremely short CH⋯O type hydrogen bonds are included, which along with several other data indicate that in this structure N and C atoms in imidazole group of one L-His+ cation have been confused.