Conformational analysis of cyclohexane-derived analogues of glutamic acid by X-ray crystallography, NMR spectroscopy in solution, and molecular dynamics

Abstract

The conformational analysis of two glutamic acid analogues containing a cyclohexane ring, substituted in position 1 by an amino and a carboxy group, and in position 3 by a second carboxy group, has been investigated by X-ray crystallography, 1H and 13C NMR spectroscopy in an aqueous environment, and molecular dynamics (MD). These analogues exclusively exhibit chair conformations in aqueous solution. When the two carboxy groups are cisthey are shown to be equatorial and the amino group axial, whatever the pH and the charge are. However, in the trans-isomer the 1-amino and 3-carboxy group are equatorial except around neutral pH, where there is an equilibrium with another conformer, in which the 1-ammonium and the 3-carboxylate groups are axial and stabilized by a large electrostatic interaction or hydrogen bond.