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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