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

•Complexation of XO with Ca2 +, Mn2 +, Zn2 + and Tb3 + was compared with Gd3 +.•XO detects Gd3 +, Zn2 + and Tb3 + at low concentration range (< 1 μM).•Mn2 + and Ca2 + form complexes with XO only at higher i.e. micromolar concentrations.•Gd–XO, Zn–XO and Tb–XO are all 1:1 complexes with Kf 5.9, 7.2 and 6.1 respectively.•Ion charge density accounts for the complexation ability of these metals with XO.

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 (< 1 μM) 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 +.

Graphical abstractThe complexation behavior of XO with Zn2 +, Tb3 +, Mn2 + and Ca2 + was compared with Gd3 + by UV–Vis absorption spectroscopy. Ion charge density accounts for the discrepancy in complexation ability among these electronically similar metal cations.Figure optionsDownload full-size imageDownload as PowerPoint slide