Metal Binding by Amino Acids: Preparation and Crystal Structures of Lithium, Sodium, and Potassium Hydrogen Bis-L-pyroglutamate

Lithium, sodium and potassium hydrogen bis‐L‐pyroglutamate [Li(L‐pGlu)(L‐pGluH)], [Na(L‐pGlu)(L‐pGluH)] and [K(L‐pGlu)(L‐pGluH)], respectively, have been prepared by reaction of aqueous solutions of the metal hydroxides with L‐pyroglutamic acid in the molar ratio 1:2, or in methanol. Crystalline samples of the salts could be obtained by slow evaporation of solutions of the salts in methanol. In the solid state, Li(L‐pGlu)(L‐pGluH) adopts a chain structure with the lithium atoms arranged in double strands. Adjacent metal centers are bridged by the carboxylate groups of the L‐pGlu− ligands. The L‐pGluH ligands are attached to the metal centers through the amide oxygen atoms. Na(L‐pGlu)(L‐pGluH) forms a three‐dimensional coordination polymer with two half‐occupied sodium positions, one L‐pGlu− and one L‐pGluH ligand in the asymmetric unit. The carboxyl and carboxylate groups of the L‐pGluH and the L‐pGlu− ligands are bridging the sodium centers, giving rise to eight‐membered rings, which consist of two sodium atoms, a carboxyl and a carboxylate group. The hydrogen atoms of the carboxyl group of the L‐pGluH ligands are involved in strong hydrogen bonds between a carboxyl and an adjacent carboxylate oxygen atom. K(L‐pGlu)(L‐pGluH) adopts a layer structure with strands of potassium atoms. The metal centers are connected through bridging carboxyl and carboxylate groups of the L‐pGluH and L‐pGlu− ligands, respectively. The structure features eight‐membered ring units comparable to those observed for Na(L‐pGlu)(L‐pGluH), again with the hydrogen atom of the carboxyl group engaged in a strong transannular hydrogen bond. The amide oxygen atoms of the L‐pGlu− and the L‐pGluH ligands are coordinated to the metal centers of adjacent coordination chains. The results of 1H‐ and 13CNMR investigations of aqueous solutions of the compounds indicate extensive electrolytic dissociation in dilute solutions.