Nakajima Y, Sakagishi Y, Shiibashi M, Suzuki Y, Kato H
Department of Biochemistry, Saitama Medical School, Japan.
Biochem Biophys Res Commun. 1993 Aug 31;195(1):449-54. doi: 10.1006/bbrc.1993.2064.
We have obtained the optimized geometrical structure of 2-(1-carboxy-1-hydroxyethyl)-3,4-dimethylthiazolium dipolar ion and investigated its geometric and electric changes during decarboxylation process by the MNDO-PM3 method, a molecular orbital method. The salient features of the optimized structure are that the dihedral angle of C4-C1-C2-S3 is 5.4 degrees and the distance between thiazolium S3 and carboxyl O6 is about 1.8 A (the bond order between S3 and O6 is about 0.4). The lowest energy decarboxylation profile is the following process. First the dihedral angle of C4-C1-C2-S3 becomes about 90 degrees, then the distance between C1-C2 increases while the dihedral angle holds about 90 degrees, and finally the C1-C2 bond disappears. The most remarkable change caused by the 90 degrees rotation is the disappearance of the S3-O6 bond, and this disappearance causes electric changes that prompt the decarboxylation.
我们已获得2-(1-羧基-1-羟乙基)-3,4-二甲基噻唑鎓偶极离子的优化几何结构，并通过分子轨道方法MNDO-PM3研究了其在脱羧过程中的几何和电学变化。优化结构的显著特征是C4-C1-C2-S3的二面角为5.4度，噻唑鎓S3与羧基O6之间的距离约为1.8埃（S3与O6之间的键级约为0.4）。能量最低的脱羧过程如下。首先，C4-C1-C2-S3的二面角变为约90度，然后C1-C2之间的距离增加，而二面角保持约90度，最后C1-C2键消失。90度旋转引起的最显著变化是S3-O6键的消失，这种消失导致电学变化从而促使脱羧反应发生。
