Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
J Phys Chem B. 2011 Feb 10;115(5):1294-305. doi: 10.1021/jp110388v. Epub 2011 Jan 19.
We have simulated ionization of purine nucleic acid components in the gas phase and in a water environment. The vertical and adiabatic ionization processes were calculated at the PMP2/aug-cc-pVDZ level with the TDDFT method applied to obtain ionization from the deeper lying orbitals. The water environment was modeled via microsolvation approach and using a nonequilibrium polarizable continuum model. We have characterized a set of guanine tautomers and investigated nucleosides and nucleotides in different conformations. The results for guanine, i.e., the nucleic acid base with the lowest vertical ionization potential, were also compared to those for the other purine base, adenine. The main findings of our study are the following: (i) Guanine remains clearly the base with the lowest ionization energy even upon aqueous solvation. (ii) Water solvent has a strong effect on the ionization energetics of guanine and adenine and their derivatives; the vertical ionization potential (VIP) is lowered by about 1 eV for guanine while it is ∼1.5 eV higher in the nucleotides, overall resulting in similar VIPs for GMP(-), guanosine and guanine in water. (iii) Water efficiently screens the electrostatic interactions between nucleic acid components. Consequently, ionization in water always originates from the base unit of the nucleic acid and all the information about conformational state is lost in the ionization energetics. (iv) The energy splitting between ionization of the two least bound electrons increases upon solvation. (v) Tautomerism does not contribute to the width of the photoelectron spectra in water. (vi) The effect of specific short-range interactions with individual solvent molecules is negligible for purine bases, compared to the long-range dielectric effects of the aqueous medium.
我们已经模拟了嘌呤核酸成分在气相和水环境中的离化。使用 TDDFT 方法计算了垂直和绝热离化过程，以获得来自更深层轨道的离化。通过微溶剂化方法和非平衡极化连续体模型模拟了水环境。我们已经对一组鸟嘌呤互变异构体进行了特征描述，并研究了不同构象的核苷和核苷酸。对于鸟嘌呤，即具有最低垂直离化势能的核酸碱基，还将其结果与其他嘌呤碱基腺嘌呤进行了比较。我们研究的主要发现如下：（i）即使在水溶剂化的情况下，鸟嘌呤仍然明显是具有最低离化能的碱基。（ii）水溶剂对鸟嘌呤和腺嘌呤及其衍生物的离化能学有很强的影响；对于鸟嘌呤，垂直离化势能（VIP）降低了约 1 eV，而在核苷酸中则升高了约 1.5 eV，总体上导致 GMP(-)、鸟苷和水中的鸟嘌呤具有相似的 VIP。（iii）水有效地屏蔽了核酸成分之间的静电相互作用。因此，水中的离化总是源自核酸的碱基单元，并且在离化能学中失去了关于构象状态的所有信息。（iv）在溶剂化过程中，两个最束缚电子的离化之间的能量分裂增加。（v）互变异构体在水中不会导致光电子能谱变宽。（vi）与水介质的长程介电效应相比，与单个溶剂分子的特定短程相互作用的影响对于嘌呤碱基可以忽略不计。
