Conformational analysis by NMR spectroscopy and molecular simulation in water of methylated glutamic acids, agonists at glutamate receptors

Abstract

We have undertaken a conformational analysis by 1H and 13C NMR spectroscopy and molecular modelling of five glutamic acid analogues substituted in position-3 or -4 by a methyl (3T, 3E, 4T, 4E) or a methylene group (4M). These analogues interact with glutamate receptors of the central nervous system, especially the ligand 4T, at one cloned metabotropic receptor (mGluR1a), and the ligand 4E, at the ionotropic receptor (KA). A combination of one- and two-dimensional NMR techniques was used to completely assign the 1H and 13C NMR chemical shifts of the different isomers according to the geometrical isomerism of the methyl. Hetero- and homo-nuclear coupling constants were measured in order to assign the diastereotopic methylene protons at C(3) or C(4), and used for comparison in molecular dynamics (MD) simulations. The system with two carbon–carbon single bonds allows the possibility of trans and gauche forms ‘A, B, C’ and ‘a, b, c’ corresponding to the χ1 and χ2 backbone torsion angles, respectively. The hydrogen-bonding possibility, steric effect or electrostatic interaction may be a considerable influence in stabilizing the different conformations in D2O solution. The conformations may be grouped by the two characteristic distances between the potentially active functional groups, αN+⋯γCO2–(d1) and αCO2–⋯γCO2–(d2) and by the two backbone torsion angles, χ1[α-CO2––C(2)–C(3)–C(4)] and χ2[+NC(2)–C(3)–C(4)–γCO2–].