Polyimidazole Transition Metal Carboxylates: Models for Hydrogen Bonding of Metal Bound Histidines in Proteins. Mark A. Whitener, Cyrene Bolt, Gupreet Kaur, Doan Lieu, Nicole Marchesano, Linton Ordaya, Janine Rosko, Evros Vassiliou, Sandra Victorin Department of Chemistry & Biochemistry, Montclair State University, Upper Montclair, NJ 07043
To model protein hydrogen bonds between metal bound histidines and carboxylate side chains, a series of compounds containing polyimidazole transition metal cations and carboxylate anions have been prepared. Unlike most other side chain ligands in proteins, histidine often has a protonated nitrogen that may act as a hydrogen bond donor to an acceptor elsewhere in the protein. Examples of such acceptors are carboxylate groups of the amino acids aspartate or glutamate and the carbonyl oxygen of a peptide bond. This hydrogen bonding interaction determines the configuration of the histidine about the metal center and provides an electronic influence on the metal site.1
Reaction of polyimidazole-metal-sulfates with barium carboxylates yields barium sulfate and polyimidazole-metal-carboxylates:
[M(Im)n](SO4) + Ba(RCOO)2 --------------> BaSO4 + [M(Im)n](RCOO)2
(Im = Imidazole; M = Cu, n = 4; M = Mn, Co, Ni, or Zn, n = 6; R = methyl, phenyl, 3,5-dinitrophenyl, hippuryl, 4-chlorophenyl, etc.)
The desired products are isolated by filtration and slow evaporation of solvent. The crystal structures of several of the target compounds will be discussed. The nature of the hydrogen bonds between coordinated imidazole ligands and the oxygen atoms of carboxylate anions, amide groups and hydrate water molecules will be emphasized.
1) Lesburg, C. A.; Christianson, D. W. J. Am. Chem. Soc. 1995, 117, 6838-6844.