Greer Edyta M, Uritsky Florence, Herrera Benjamin, Benavides Frankie, Greer Alexander, Doubleday Charles
Department of Natural Sciences, Baruch College of the City University of New York, New York, New York 10010, United States.
Department of Chemistry, Brooklyn College, Brooklyn, New York 11210, United States.
J Org Chem. 2025 Aug 1;90(30):10599-10606. doi: 10.1021/acs.joc.5c00827. Epub 2025 Jul 17.
To assess tunneling, we studied the guanine-cytosine (GC) base pair tautomerization in the gas phase. We applied multidimensional semiclassical reaction path methodology with microcanonically optimized multidimensional tunneling (μOMT) using POLYRATE. The minimum energy path (MEP) has a single saddle point for the double proton transfer. Addition of vibrational zero-point energy (ZPE) to the MEP gives the vibrationally adiabatic ground state curve, , which is the barrier through which tunneling occurs. Unexpectedly, has not one but two well-separated barriers in the transition state region. The first is near the saddle point. The second barrier is entirely due to a large amount of ZPE associated with local reaction path curvature. We refer to it as a quantum barrier. Its height and width reduce the tunneling transmission coefficient. In other words, GC tautomerization has two competing quantum effects, tunneling and ZPE, that have opposite effects on the reaction rate. The transmission coefficient κ is 1.57, and tunneling constitutes 36% of the rate constant at 298 K. Our computed kinetic isotope effects (KIE) are lower than expected, e.g., KIE = 5.05 at 298 K. In the discussion, we show that the quantum barrier is a consequence of reaction path curvature as the tautomer begins to form.
为了评估隧穿效应，我们研究了气相中鸟嘌呤 - 胞嘧啶（GC）碱基对的互变异构。我们使用POLYRATE应用了具有微正则优化多维隧穿（μOMT）的多维半经典反应路径方法。对于双质子转移，最小能量路径（MEP）有一个单一的鞍点。将振动零点能（ZPE）加到MEP上得到振动绝热基态曲线，即发生隧穿的势垒。出乎意料的是，在过渡态区域不是有一个而是有两个明显分开的势垒。第一个靠近鞍点。第二个势垒完全是由于与局部反应路径曲率相关的大量ZPE。我们将其称为量子势垒。它的高度和宽度降低了隧穿透射系数。换句话说，GC互变异构有两种相互竞争的量子效应，隧穿和ZPE，它们对反应速率有相反的影响。透射系数κ为1.57，在298 K时隧穿占速率常数的36％。我们计算的动力学同位素效应（KIE）低于预期，例如在298 K时KIE = 5.05。在讨论中，我们表明量子势垒是互变异构开始形成时反应路径曲率的结果。
