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Highlights: [•] Phenylalanine and tyrosine were investigated by theoretical and experimental methods. [•] The lowest energy conformers are not stabilized by hydrogen bonding. [•] Steric and hyperconjugative effects were analyzed for all conformers. [•] Several theoretical methods were used to explain the conformational preferences. [Copyright &y& Elsevier]

Thiswork reports a detailed study regarding the conformational preferencesof l-proline methyl ester (ProOMe) and its N-acetylated derivative (AcProOMe) to elucidate the effects that ruletheir behaviors, through nuclear magnetic resonance (NMR) and infrared(IR) spectroscopies combined with theoretical calculations. Thesecompounds do not present a zwitterionic form in solution, simulatingproperly amino acid residues in biological media, in a way closerthan amino acids in the gas phase. Experimental 3JHHcoupling constants and infrared data showedexcellent agreement with theoretical calculations, indicating no variationsin conformer populations on changing solvents. Natural bond orbital(NBO) results showed that hyperconjugative interactions are responsiblefor the higher stability of the most populated conformer of ProOMe,whereas for AcProOMe both hyperconjugative and steric effects ruleits conformational equilibrium. [ABSTRACT FROM AUTHOR]

exhibit a bipolar zwitterionic structure (+H3N-CHR-COO−) in solution; hence, conformational studies of these compounds have been limited to the gas phase. The conformational preferences of have been widely attributed to hydrogen bonding, despite steric and hyperconjugative effects. In this work, we propose the conformational study of alanine and valine methyl esters, which do not show zwitterionic structures in solution, by1H NMR and theoretical calculations. The 3 JHH spin-spin coupling constants and theoretical calculations were found to be in agreement, showing that the interplay between steric hindrance and hyperconjugation is the forces that are responsible for determining the conformational preferences of alanine and valine methyl esters. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] The conformational equilibrium of tryptophan methyl ester was analysed. [•] Experimental 3 J HH and theoretical calculations were employed. [•] Conformer populations were not dependent on the solvent. [•] Hyperconjugation and steric effects govern the ester’s conformational isomerism. [ABSTRACT FROM AUTHOR]