title: Gd3TCAS2: An Aquated Gd(3+)-Thiacalix[4]arene Sandwich Cluster with Extremely Slow Ligand Substitution Kinetics.
abstract: In aqueous solution, Gd(3+) and thiacalix[4]arene-p-tetrasulfonate (TCAS) form the complex [Gd3TCAS2](7-), in which a trinuclear Gd(3+) core is sandwiched by two TCAS ligands. Acid-catalyzed dissociation reactions, as well as transmetalation and ligand exchange with physiological concentrations of Zn(2+) and phosphate, showed [Gd3TCAS2](7-) to be extremely inert compared to other Gd complexes. Luminescence lifetime measurements of the Tb analogue Tb3TCAS2 allowed estimation of the mean hydration number q to be 2.4 per Tb ion. The longitudinal relaxivity of [Gd3TCAS2](7-) (per Gd(3+)) was r1 = 5.83 mM(-1) s(-1) at 20 Hz (37 °C, pH 7.4); however, this relaxivity was limited by an extremely slow water exchange rate that was 5 orders of magnitude slower than the Gd(3+) aqua ion. Binding to serum albumin resulted in no relaxivity increase owing to the extremely slow water exchange kinetics. The slow dissociation and water exchange kinetics of [Gd3TCAS2](7-) can be attributed to the very rigid coordination geometry.
authors.affiliation: Graduate School of Environmental Studies, Tohoku University , 6-6-07 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan.
authors.forename: Nobuhiko
authors.initials: N
authors.lastname: Iki
authors.affiliation: A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School , 149 13th Street, Suite 2301, Charlestown, Massachusetts 02129, United States.
authors.forename: Eszter
authors.initials: E
authors.lastname: Boros
authors.affiliation: Graduate School of Environmental Studies, Tohoku University , 6-6-07 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan.
authors.forename: Mami
authors.initials: M
authors.lastname: Nakamura
authors.affiliation: Graduate School of Environmental Studies, Tohoku University , 6-6-07 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan.
authors.forename: Ryo
authors.initials: R
authors.lastname: Baba
authors.affiliation: A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School , 149 13th Street, Suite 2301, Charlestown, Massachusetts 02129, United States.
authors.forename: Peter
authors.initials: P
authors.lastname: Caravan
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D005682:Gadolinium / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D008024:Ligands; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D012709:Serum Albumin / Q000737:chemistry; D013440:Sulfides / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D056831:Coordination Complexes; D008024:Ligands; D010636:Phenols; D012709:Serum Albumin; D013440:Sulfides; C500450:thiacalix(4)arene; C446239:thiacalix(4)arenetetrasulfonate; D014867:Water; D005682:Gadolinium