A theoretical and experimental1H NMR spectroscopy study of the stereoelectronic interactions that rule the conformational energies of alanine and valine methyl ester

Amino acids 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 amino acids have been widely attributed to intramolecular 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. The3JHHspin–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.