Nicole Lumbroso-Bader

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

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 A) and Glu (2.50–2.55 A) residues. The predominant conformer has an extended side-chain so no chelation occurs with the peptide backbone.

1H and 13C dynamic nuclear magnetic resonance study of hindered internal rotation in (N,N-dimethylamino)pyrimidines

The free energies of activation ΔG‡ for hindered rotation around the C–N exocyclic bond in a series of 2- and 4-(N,N-dimethylamino)pyrimidines have been determined by 1H and 13C n.m.r. line-shape analysis. Good linear correlations have been obtained between the coupling constant 1J(C,H) for the dimethylamino group and the ΔG‡ parameter. Such correlations have been used to estimate ΔG‡ values of symmetrical or unsymmetrical pyrimidines for which an experimental determination of this thermodynamic parameter is either impossible or extremely difficult. In addition, substituent effects on the barrier heights indicate that: (i) the difference between the ΔG‡4 and ΔG‡2 values decreases with increasing electron-withdrawing power of the substituent, and (ii), substituent effects are larger through a ring nitrogen than a ring carbon atom. Correlation of the ΔG‡ values with Hammett constants is discussed.

Proton and carbon-13 nuclear magnetic resonance studies of substituted pyrimidines. Part 3. Hindered internal rotation in some 4-(NN-dimethylamino)pyrimidines

The free energy of activation for internal rotation about the C–N exocyclic bond of some substituted 4-(NN-di-methylamino)pyrimidines has been determined using 1H and 13C n.m.r. line-shape analysis. Substituent effects on the rotational barrier of the dimethylamino group are evaluated. The influence of the position of the dimethyl-amino group with respect to the nitrogen atoms is discussed, the rotational barrier being higher in the 4- than in the 2-position. An interesting linear correlation between free energies of activation and 1J(C,H) coupling constants for the 4-dimethylamino group has been found.