Gd3TCAS2: An Aquated Gd(3+)-Thiacalix[4]arene Sandwich Cluster with Extremely Slow Ligand Substitution Kinetics

Inorg Chem. 2016 Apr 18;55(8):4000-5. doi: 10.1021/acs.inorgchem.6b00241. Epub 2016 Mar 28.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Coordination Complexes / chemical synthesis
  • Coordination Complexes / chemistry*
  • Gadolinium / chemistry*
  • Humans
  • Kinetics
  • Ligands
  • Molecular Structure
  • Phenols / chemistry*
  • Serum Albumin / chemistry
  • Sulfides / chemistry*
  • Water / chemistry

Substances

  • Coordination Complexes
  • Ligands
  • Phenols
  • Serum Albumin
  • Sulfides
  • thiacalix(4)arene
  • thiacalix(4)arenetetrasulfonate
  • Water
  • Gadolinium