6-巯基嘌呤质子转移异构化的量子化学计算研究

采用密度泛函B3LYP方法,在6-311G(d,p)基组水平上对6-巯基嘌呤质子转移引起的硫酮式与硫醇式互变异构反应机理进行了计算研究,获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数。计算结果表明,6-巯基嘌呤无论是孤立分子还是一水合物,其硫酮式TP2是最稳定的异构体。计算结果同已有实验结果相符。由硫酮式通过分子内质子转移向硫醇式异构化找到4条反应通道(P1,P2,P3,P4),各通道的活化能分别为114.0,133.9,128.0,95.1 kJ·mol~(-1)。当水分子参与反应以双质子转移机理异构化时,活化能垒显著降低,各通道的活化能依次降为51.2,63.0,70.5,42.8 kJ·mol~(-1),可见,水助催化有利于硫酮式向硫醇式转变。计算结果还表明,氢键的强弱对TP2一水合物的稳定性会有一定的影响。

A DFT study on the proton transfer isomerization of 6-thiopurine tautomers

The reaction mechanism of thione-thiol tautomerism for 6-thiopurine obtained from proton transfer has been investigated by DFT B3LYP method with 6-311G(d,P)basis set.The reaction enthalpies,activation energies,activation free energies and the rate constants of tautomerism were obtained.The results showed that the thione TP2 is the most stable tautomer of all the tautomers in the form of isolated or hydrated isomers.The 4 reaction pathways(P1,P2,P3,P4)from thione to thiol through intramolecular proton proton transfers mechanism were found and the activation energies are 114.0,133.9,128.0,95.1 kJ·mol~(-1),respectively.In water catalysis,double proton transfers mechanism were found and the activation energies were dramatically decreased to 51.2,63.0,70.5,42.8 kJ·mol~(-1),which was preferable to transform from thione to thiol tautomers. It was also found that the strength of hydrogen bonds has some impact on the stabilities of the TP2 monohydrated complexes.