Nuclear magnetic resonance study of Ln3+ complexes with aspartate and glutamate residues. Thermodynamic and structural analysis

Abstract

Binding of aspartate and glutamate residues with the lanthanide cation Yb³⁺ has been studied in D₂O, involving two kinds of peptide backbones: N-acetylamino acid methyl esters and cyclic dipeptides. A method is described for the quantitative analysis of the lanthanide-induced shifts (LIS) from which the best model of solution, the binding constants, the induced shifts, and the geometry of the complexes have been determined. Aspartate and glutamate derivatives are found to form both 1 : 1 and 2 : 1 complexes (peptide:cation). Backbone flexibility and, to a lesser extent, side-chain length seem to govern the magnitude of the binding constants. Only one oxygen of the carboxylate group is involved in the complex. The Yb³⁺⋯ O^– distances are found to be different according to the Asp (2.70–2.75 Å) and Glu (2.50–2.55 Å) residues. The predominant conformer has an extended side-chain so no chelation occurs with the peptide backbone.