Moriya F, Tanimoto S, Makino K
Department of Chemistry, Kobe University of Commerce, Japan.
Free Radic Res Commun. 1993;19 Suppl 1:S55-61. doi: 10.3109/10715769309056s55.
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].
带有叔丁基氨氧基的α-取代乙酸，即t-BuN(O.)-CHX-COOH，是在氨基酸和肽的γ辐照水溶液中与自旋捕获剂2-甲基-2-亚硝基丙烷反应生成的自旋加合物。这些自旋加合物通过高效液相色谱和电子顺磁共振光谱进行分离和表征。它们在酸性区域的电子顺磁共振光谱通过羧基的酸解离随pH值可逆变化。在pK值附近的pH下的光谱是酸形式和共轭碱形式的加权平均值。对于t-BuN(O.)-CH2COOH、t-BuN(O.)-CH(CH3)COOH、NH3+CH2CONHCH(COOH)N(O.)-t-Bu和NH3(+)-*CH(CH3)-CONH-CH(COOH)N(O.)-t-Bu [一对非对映异构体]，其羧基解离的pKCOOH值分别测定为3.0、3.2、2.0、1.8和1.6。氨氧基的吸电子特性导致观察到pKCOOH值低于X-CH2-COOH的pKCOOH值。结合Taft方程研究了叔丁基氨氧基对α-取代乙酸酸解离平衡的取代基效应。对于t-BuN(O.)CH2基团，脂肪族极性取代基常数（σ）估计约为+0.9 [对于t-BuN(O.)基团，诱导取代基常数σ1约为+0.4]。
