Revealing the Conformational Preferences of Proteinogenic Glutamic Acid Derivatives in Solution by 1H NMR Spectroscopy and Theoretical Calculations

The conformational preferences of proteinogenic glutamic acid esterified (GluOMe) and N-acetylated (Ac-GluOMe) derivatives have been determined in solution for the first time. Theoretical calculations at the wB97X-D/aug-cc-pVTZ made possible the assignment of six and eight stable conformers for GluOMe and AcGluOMe, respectively. The conformational equilibrium of the studied compounds was evaluated in different organic solvents using a combination of the integral equation formalism polarizable continuum model (IEF-PCM) and H-1 NMR spectroscopy data. The results showed that the conformational equilibrium of both derivatives change in the presence of solvent. According to the quantum theory of atoms in molecules (QTAIM), non-covalent interactions (NCI), and natural bond orbitals (NBO) analyses, the conformational preferences observed for GluOMe and AcGluOMe are not dictated by the presence of a specific interaction but are due to a combination of hyperconjugative and steric effects.