National Center for Biomolecular Research, Masaryk University, Kamenice 5/A4, CZ-62500 Brno, Czech Republic.
J Phys Chem A. 2010 Jun 24;114(24):6689-700. doi: 10.1021/jp102186r.
Adenine, an essential building block of nucleic acids present in all living systems, can occur in several tautomeric forms. The phenomenon of tautomerism can be investigated by several experimental methods, including nuclear magnetic resonance. In this study, long-range (1)H-(13)C and (1)H-(15)N coupling constants for N-alkyl derivatives related to four tautomers of adenine are investigated in DMSO and DMF solutions. To investigate the structural dependence of the coupling constants and to understand how polarization propagates in the system, Fermi contact (FC) terms were calculated for the individual isomers and analyzed by using density functional theory (DFT), and the coupling pathways were visualized using real-space functions. The coupling electron deformation densities (CDD) of several (1)H-X (X = (13)C, (15)N) pairs are evaluated and compared. In order to analyze the CDD in more detail, a new approach to break down the CDD into contributions from Boys or Pipek-Mezey localized molecular orbitals (LMOs) has been developed. A similar approach has been applied to split the value of the FC contribution to the J coupling into the LMO contributions. On the basis of chemical concepts, the contributions of sigma-bonds, pi-electrons, and lone pairs of electrons are discussed. The lone pair of electrons at the nitrogen atom contributes significantly to the (1)H-C horizontal line(15)N coupling, whereas the (1)H-C=N-(13)C coupling is affected in a somewhat different way. Surprisingly, the contribution of the intervening C horizontal lineN bond to the FC term for (1)H-C=(15)N coupling originates exclusively in sigma-electrons, with a vanishingly small contribution calculated for the pi-electrons of this fragment. This behavior is rationalized by introducing the concept of "hard and soft J elements" derived from the polarizability of the individual components.
腺嘌呤是所有生命系统中存在的核酸的基本组成部分，它可以以几种互变异构形式存在。互变异构现象可以通过几种实验方法来研究，包括核磁共振。在这项研究中，在 DMSO 和 DMF 溶液中研究了与腺嘌呤的四种互变异构体相关的 N-烷基衍生物的长程（1）H-（13）C 和（1）H-（15）N 偶合常数。为了研究偶合常数的结构依赖性并了解极化如何在系统中传播，为各个异构体计算了费米接触（FC）项，并使用密度泛函理论（DFT）进行了分析，并用实空间函数可视化了偶合途径。评估并比较了几个（1）H-X（X =（13）C，（15）N）对的偶合电子变形密度（CDD）。为了更详细地分析 CDD，开发了一种将 CDD 分解为 Boys 或 Pipek-Mezey 局部分子轨道（LMO）贡献的新方法。还应用了类似的方法将 FC 对 J 偶合的贡献分解为 LMO 的贡献。基于化学概念，讨论了 sigma 键、pi 电子和孤对电子的贡献。氮原子上的孤对电子对（1）H-C（15）N 偶合有很大贡献，而（1）H-C=N-（13）C 偶合的影响方式略有不同。令人惊讶的是，对于（1）H-C =（15）N 偶合，FC 项中 C 水平（13）N 键的贡献完全来自 sigma 电子，而该片段的 pi 电子的贡献可以忽略不计。通过引入源自各个组件的极化率的“硬和软 J 元素”概念，可以合理地解释这种行为。
