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 (mGluR_1a), 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^–].