Ion charge density governs selectivity in the formation of metal–Xylenol Orange (M–XO) complexes

Abstract

Clinically-approved Magnetic Resonance Imaging (MRI) contrast agents (CAs) are Gd-based small-molecule chelates which possess high thermodynamic stability and kinetic inertness in vivo. Free Gd3+ ion in plasma is toxic and has been implicated in Nephrogenic Systemic Fibrosis (NSF) in renally-compromised patients. Any preparation of Gd-based MRI CAs must therefore be carefully examined for the presence of free Gd3+ ion. A simple assay involves the use of Xylenol Orange (XO) as a colorimetric indicator under slightly acidic (pH=5.8) buffered conditions. However, the structure of XO suggests that other ions possessing similar ionic size and/or coordination number could form complexes with XO as well. In this study, we investigated the complexation of XO with other metal ions, namely, Ca2+, Mn2+, Zn2+ and Tb3+, by UV–Vis absorption spectroscopy and compared these results with those of Gd–XO solutions. Ligand-to-metal ratios were determined using Job plots, and the relative stabilities of the complexes were investigated by competition with EDTA. In summary, XO can be used to detect Gd3+, Tb3+ and Zn2+ at the same concentration (<1μM) range, but not Mn2+ and Ca2+ which form M–XO only at higher concentrations. This discrepancy was rationalized by differences in ion charge density. Hence, XO can be used to discriminate between, for example, Gd3+ and Ca2+, but not between Gd3+, Tb3+ or Zn2+.