作者:Tadao KURATA、Yoko NISHIKAWA
DOI:10.1271/bbb.64.1651
日期:2000.1
Dehydro-L-ascorbic acid (DAA) exists mainly in its C2 hydrated bicyclic form (5) in an aqueous solution, and monocyclic DAA (3), which is the expected reaction product immediately after the oxidation of AA, has not been observed by NMR spectroscopy. The formation mechanism for 5 from 3 and the stability of 5 were examined by the semi-empirical molecular orbital method (MOPAC). It was indicated that the protonation reaction was the key step in the formation of 5, therefore, the formation of 5 is thought to be more difficult under physiological conditions which mostly involve in the neutral or slightly alkaline state. However, by NMR, it was confirmed that, even in a neutral or slightly alkaline state very close to physiological conditions, the predominant form of DAA existing in an aqueous solution immediately after the enzymatic oxidation of AA was confirmed to be 5, although the possible existence of other forms of DAA at very low concentrations could not be completely excluded.
脱氢-L-抗坏血酸(DAA)在水溶液中主要以 C2 水合双环形式(5)存在,而单环 DAA(3)是 AA 氧化后的预期反应产物,但核磁共振光谱尚未观察到。通过半经验分子轨道法(MOPAC)研究了 3 形成 5 的机理以及 5 的稳定性。结果表明,质子化反应是形成 5 的关键步骤,因此认为在生理条件下形成 5 比较困难,因为生理条件大多涉及中性或微碱性状态。不过,核磁共振证实,即使在非常接近生理条件的中性或微碱性状态下,在 AA 被酶氧化后立即存在于水溶液中的 DAA 的主要形式也是 5,尽管不能完全排除在极低浓度下可能存在其他形式的 DAA。