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

Otto Kumberger, Jürgen Riede, Hubert Schmidbaur

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)1829-1834
Number of pages6
JournalChemische Berichte
Volume125
Issue number8
DOIs
StatePublished - Aug 1992
Externally publishedYes

Keywords

  • Hydrogen bis‐L‐pyroglutamates, lithium, sodium, potassium
  • Pyroglutamate coordination and conformation

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