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

• The conformational equilibrium of tryptophan methyl ester was analysed.
• Experimental 3JHH and theoretical calculations were employed.
• Conformer populations were not dependent on the solvent.
• Hyperconjugation and steric effects govern the ester’s conformational isomerism.

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 (σC4-C13⁎, σC1-C4⁎ and σC4-H12⁎) and also to the steric effects from the nonbonding electron pairs of oxygen atoms and the main and side chain of this system.

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