Metal Ion Sensing by BAPTA: Investigation Using 1H NMR Spectroscopy

Abstract

Many  biological  macromolecules  rely  on  metal  ions  to  maintain  structural  integrity  and  control  their  regulatory  function. In biological fluids, detection and identification of metal ions requires sensitive analytical tools with clear readouts.  In  this  work,  we  sought  to  investigate  the  potential  of  solution  Nuclear  Magnetic  Resonance  (NMR)  spectroscopy to analyze metal ion solutions and mixtures. To enable 1H NMR detection, we prepared the complexes of  eight  metal  ions  with  the  chelating  agent,  1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic  acid  (BAPTA).  The 1H NMR spectra were collected for BAPTA samples as a function of metal ion concentrations. The analysis of NMR data revealed that all metal ions with a notable exception of Mg2+ bind BAPTA with high affinities and form complexes with 1:1 metal-to-chelator stoichiometry. Both methylene and aromatic regions of the BAPTA 1H NMR spectra  experience  significant  changes  upon  the  metal  ion  complex  formation.  We  identified  the  spectroscopic  signatures  of  trivalent  and  paramagnetic  ions  and  demonstrated  that  the  binary  Zn2+/Pb2+  metal  ion  mixture  can  be  successfully analyzed by NMR. We conclude that complexation with BAPTA followed by the 1H NMR analysis is a sensitive method to detect and identify both nutritive and xenobiotic metal ions.