NMR Spectroscopy Study of Enol–Enol Tautomerism of Nitisinone in Solutions

The structure of nitisinone (β, β′-triketone reversible inhibitor of the enzyme 4-hydroxyphenylpyruvate dioxygenase) in various protic and aprotic solvents was studied using PMR and 13C NMR spectroscopy. Nitisinone was shown to exist in solutions at room temperature as the endo-enol tautomer while the exo-enol tautomer was not found. Prototropic ring tautomerism (migration of a proton between the 1- and 3-positions), the rate of which increases with increasing solvent polarity, occurs in the endo-enol tautomer. The mineral composition of the biological fluid does not affect the tautomeric equilibrium. The conformational rigidity of the nitisinone molecule accounts for the nonequivalence of the 1- and 3-positions in the cyclohexene fragment from the standpoint of binding of nitisinone to the Fe(II)*4-hydroxyphenylpyruvate dioxygenase complex.