Determination of the preferred tautomeric form of 4-nitrohistidine

Spectrophotometric pKa determinations of methyl derivatives of N^α-acetyl-4-nitrohistidine methyl ester 1 have been used to determine the position of the tautomeric equilibrium of 1 . The N₁-H tautomer is the predominant form with an equilibrium constant K_T of 48. The conclusion is supported qualitatively by the study of ¹H-nmr and ¹³C-nmr chemical shifts of the imidazole ring atoms and their changes from neutral to acidic media.