Influence of pH on the cis–trans isomerization of Valine-Proline dipeptide: An integrated NMR and theoretical investigation

The pH influence on the cis – trans isomerization of the Valine-Proline dipeptide was investigated by means of NMR spectral techniques and quantum chemical calculations at different computational levels. It was found that the process of isomerization is strongly pH dependent. The trans -isomer of Val-Pro is the more abundant isomer when the amino group is protonated, while an approximately equal distribution between the cis and trans -isomers for the neutral and anion forms of the dipeptide was confirmed.