On the substituent effect of tert-butylaminoxyl group for the acid-dissociation equilibria of alpha-substituted acetic acids.

alpha-Substituted acetic acids with tert-butylaminoxyl groups, t-BuN(O.)-CHX-COOH, were produced as spin adducts in gamma-irradiated aqueous solutions of amino acids and peptides with a spin trap, 2-methyl-2-nitrosopropane. The spin adducts were isolated and characterized by means of high-performance liquid chromatography and EPR spectroscopy. Their EPR spectra in acidic region changed reversibly with pH through the acid-dissociation of the carboxyl groups. The spectra at pH around the pK value were weighted averages of acid forms and conjugate base forms. The pKCOOH values for the dissociation were determined to be 3.0, 3.2, 2.0, 1.8, and 1.6 for t-BuN(O.)-CH2COOH, t-BuN(O.)-CH(CH3)COOH, NH3+CH2CONHCH(COOH)N(O.)-t-Bu, and NH3(+)-*CH(CH3)-CONH-*CH(COOH)N(O.)-t-Bu [a pair of diastereomers], respectively. The electron-withdrawing character of the aminoxyl group is responsible for the observation that the pKCOOH values are lower than those of X-CH2-COOH. The substituent effect of the tert-butylaminoxyl group on the acid-dissociation equilibria of alpha-substituted acetic acids was investigated in relation to the Taft equation. The aliphatic polar substituent constant (sigma*) has been evaluated to be approximately +0.9 for the t-BuN(O.)CH2 group [or the inductive substituent constant sigma 1 approximately +0.4 for the t-BuN(O.) group].