title: Stabilization of calcium- and terbium-carboxylate bonds by NH...O hydrogen bonds in a mononuclear complex: a functional model of the active site of calcium-binding proteins.
abstract: Novel benzoic acid ligands with bulky amide groups at the ortho position, 2,6-(MeCONH)(2)C(6)H(3)CO(2)H (1) and 2,6-(t-BuCONH)(2)C(6)H(3)CO(2)H (2), and their tris- and tetrakis(carboxylate) complexes with Ca(II) and Tb(III) ions, (NEt(4))(2)[Ca(II)[O(2)C-2,6-(t-BuCONH)(2)C(6)H(3)](4)] (4), [Tb[O(2)C-2,6-(t-BuNHCO)(2)C(6)H(3)](3)(H(2)O)(3)]] (5), and (NMe)(4)[Tb[O(2)C-2,6-(t-BuNHCO)(2)C(6)H(3)](4)(thf)] (6), were synthesized. The formation of the NH...O hydrogen bonds between the amide NH and carboxylate for 2, (NEt(4))[2,6-(t-BuCONH)(2)C(6)H(3)CO(2)] (3), and 4 was determined by (1)H NMR spectroscopy in solution and in the solid state (CRAMPS, IR). The ligand exchange reactions were attempted between 4 and a large excess of 2,4,6- Me(3)C(6)H(3)CO(2)H in chloroform-d solution; however, exchange reaction did not take place, indicating that the Ca(II) ions bound strongly to the carboxylate in 4. The Ca(II) ion binding properties with the benzoate derivatives were also examined using Tb(III) ion as a fluorescence probe. These results indicate that the NH...O hydrogen bonding between the amide NH and the oxygen atom of the carboxylate contributes to strong Ca(II) binding and prevents the dissociation of the calcium-carboxylate bond. The X-ray structural analyses of these complexes revealed that the NH.O hydrogen-bonded carboxylate ligands prefer the chelate-type coordination and create a mononuclear [Ca(O(2)CR)(4)](2)(-) or [Tb(O(2)CR)(4)](-) core with anionic charge, which is known only in the active site of calcium-binding proteins.
authors.affiliation: Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Onoda
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Yamada
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Nakayama
authors.forename: Kazuyuki
authors.initials: K
authors.lastname: Takahashi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Adachi
authors.forename: Taka-Aki
authors.initials: TA
authors.lastname: Okamura
authors.forename: Akira
authors.initials: A
authors.lastname: Nakamura
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Norikazu
authors.initials: N
authors.lastname: Ueyama
authors.forename: Drahomir
authors.initials: D
authors.lastname: Vyprachticky
authors.forename: Yoshi
authors.initials: Y
authors.lastname: Okamoto
mesh_terms: D002118:Calcium / Q000737:chemistry*; D002135:Calcium-Binding Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D013725:Terbium / Q000737:chemistry*
chemical_list: D002135:Calcium-Binding Proteins; D008024:Ligands; D009942:Organometallic Compounds; D013725:Terbium; D002118:Calcium