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

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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.

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