METAL-BINDING BY AMINO-ACIDS - PREPARATION AND CRYSTAL-STRUCTURE OF LITHIUM L-PYROGLUTAMATE MONOHYDRATE AND LITHIUM L-PYROGLUTAMATE DIHYDRATE

From aqueous solutions several crystalline lithiUM L-pyroglutamate compounds with different hydrate water content can be obtained. Discrete phases of the stoichiometry Li (L-pGlu) nH2O with n = 0, 1, and 2 are prepared by reaction of one equivalent of lithium hydroxide with one or even up to two equivalents Of L-pyroglutamic acid. Macro- and single-crystalline samples appear either upon layering of concentrated aqueous solutions of the compound with acetone or by slow evaporation of an aqueous solution of the salt. A correlation between the crystallization technique applied and the stoichiometry of the isolated phase is not evident as yet. Li (L-pGlu), which crystallizes without hydrate water, adopts a layer structure with all three oxygen donors of the ligand coordinated to metal centers. The additional hydrate water molecule in Li (L-pGlu).H2O leads to a partial degradation of the sheets in the Li (L-pGlu) structure. The monohydrate thus features a chain structure, with the lithium atoms arranged in double strands. As in Li (L-pGlu), all three oxygen donors are coordinated to metal centers. Li (L-pGlu).2H2O, however, adopts a chain structure, with the lithium ions arranged in a single metal strand. In this phase, only the carboxylate oxygen atoms of the L-pyroglutamate ligand are coordinated to metal ions, while the amide oxygen atom has no metal contacts.