1H NMR and theoretical studies on the conformational equilibrium of tryptophan methyl ester

Selected 3JHH coupling constants and theoretical calculations were used to explain the conformational equilibrium of L-tryptophan methyl ester (Trp-OMe) in several solvents. The obtained 3JHαHβ values did not exhibit any significant variability and thus indicate that there are no conformational population variations for the side chain of the Trp-O-Me depending on the solvent. Moreover, the potential energy surfaces obtained at the B3LYP/cc-pVDZ theoretical level produced eight energy minima that were analysed by QTAIM and NBO methods. It was possible to conclude that the Trp-OMe conformational preferences were due to hyperconjugative effects involving the nonbonding electron pairs of the main chain nitrogen atom and certain antibonding orbitals ( σ C 4 -C 13 ⁎ , σ C 1 -C 4 ⁎ and σ C 4 -H 12 ⁎ ) and also to the steric effects from the nonbonding electron pairs of oxygen atoms and the main and side chain of this system.