A 1H nuclear magnetic resonance study of the deprotonation of L-dopa and adrenaline

Abstract

The acid–base chemistries of L-3,4-dihydroxyphenylalanine (dopa), L-adrenaline (ad), 3,4-dimethoxyphenylethylamine (dmpe), and 3,4-dihydroxyphenylpropionic acid (dhpa) have been studied by ¹H n.m.r. spectroscopy in D₂O solution. Macroscopic dissociation constants for dmpe and dhpa and macroscopic and microscopic dissociation constants for the phenolic OD and the side-chain ammonium groups of dopa and ad have been calculated from the ¹H chemical shift titration curves for the aryl and allyl protons by a non-linear least-squares curve-fitting program. The isotope effect on the acidity has been determined for several amines and phenols in order to estimate the microscopic dissociation constants of dopa and ad in H₂O from the values obtained in D₂O. The results indicate that the phenolic hydroxy-groups of dopa and of ad are more acidic than their respective side-chain ammonium groups.