Proton and carbon lanthanide-induced shifts in aqueous alanine. Evidence for structural changes along the lanthanide series

A. Dean Sherry, E. Pascual

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Abstract

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-1) 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 Co. 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.

Original languageEnglish (US)
Pages (from-to)5871-5876
Number of pages6
JournalJournal of the American Chemical Society
Volume99
Issue number18
StatePublished - 1977

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Lanthanoid Series Elements
Rare earth elements
Alanine
Protons
Carbon
Ions
Geometry
Hydration
Polarization
Oxygen

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

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abstract = "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-1) 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 Co. 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.",
author = "{Dean Sherry}, A. and E. Pascual",
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TY - JOUR

T1 - Proton and carbon lanthanide-induced shifts in aqueous alanine. Evidence for structural changes along the lanthanide series

AU - Dean Sherry, A.

AU - Pascual, E.

PY - 1977

Y1 - 1977

N2 - 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-1) 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 Co. 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.

AB - 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-1) 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 Co. 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.

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