Proton and carbon lanthanide-induced shifts are reported for aqueous alanine at pH 3. A comparison of the measured shifts with theoretical values provides evidence for changes in the coordination behavior of alanine between Tb3+ and Dy3+. An apparent stability constant (K1=0.7 ± 0.1 M-l) was measured for eight lanthanide ions and found insensitive to changes in alanine coordination geometry. The measured shifts are separated into their contact and pseudocontact components and the resulting corrected pseudocontact shift ratios are compared with values calculated using the axial symmetry model to deduce the dynamic alanine structures. A monodentate coordination geometry is proposed for the ions Pr3+ → Tb3+ and a bidentate carboxyl coordination structure is evident for the ions Dy3+→Yb3+. The carbon shifts are dominated by polarization of unpaired spin away from the ligating oxygen(s) and mediated by a direct delocalization of spin resulting in a greater unpaired spin density at Cα than C0. Although the proton shifts are primarily pseudocontact in origin, the separation method reveals a change in sign and increased magnitude of spin density at the protons on changing from monodentate to bidentate coordination. This behavior may parallel known hydration sphere alterations along the lanthanide series.
ASJC Scopus subject areas
- Colloid and Surface Chemistry