Kim Sunghwan, Schaefer Henry F
Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, USA.
J Phys Chem A. 2007 Oct 18;111(41):10381-9. doi: 10.1021/jp072727g. Epub 2007 Aug 18.
Microhydration effects upon the adenine-uracil (AU) base pair and its radical anion have been investigated by explicitly considering various structures of their mono- and dihydrates at the B3LYP/DZP++ level of theory. For the neutral AU base pair, 5 structures were found for the monohydrate and 14 structures for the dihydrate. In the lowest-energy structures of the neutral mono- and dihydrates, one and two water molecules bind to the AU base pair through a cyclic hydrogen bond via the N(9)-H and N(3) atoms of the adenine moiety, while the lowest-lying anionic mono- and dihydrates have a water molecule which is involved in noncyclic hydrogen bonding via the O4 atom of the uracil unit. Both the vertical detachment energy (VDE) and adiabatic electron affinity (AEA) of the AU base pair are predicted to increase upon hydration. While the VDE and AEA of the unhydrated AU pair are 0.96 and 0.40 eV, respectively, the corresponding predictions for the lowest-lying anionic dihydrates are 1.36 and 0.75 eV, respectively. Because uracil has a greater electron affinity than adenine, an excess electron attached to the AU base pair occupies the pi* orbital of the uracil moiety. When the uracil moiety participates in hydrogen bonding as a hydrogen bond acceptor (e.g., the N(6)-H(6a)...O(4) hydrogen bond between the adenine and uracil bases and the O(w)-H(w)...N and O(w)-H(w)...O hydrogen bonds between the AU pair and the water molecules), the transfer of the negative charge density from the uracil moiety to either the adenine or water molecules efficiently stabilizes the system. In addition, anionic structures which have C-H...O(w) contacts are energetically more favorable than those with N-H...O(w) hydrogen bonds, because the C-H...O(w) contacts do not allow the unfavorable electron density donation from the water to the uracil moiety. This delocalization effect makes the energetic ordering for the anionic hydrates very different from that for the corresponding neutrals.
通过在B3LYP/DZP++理论水平上明确考虑腺嘌呤 - 尿嘧啶（AU）碱基对及其自由基阴离子的单水合物和二水合物的各种结构，研究了微水合作用对它们的影响。对于中性AU碱基对，单水合物发现了5种结构，二水合物发现了14种结构。在中性单水合物和二水合物的最低能量结构中，一个和两个水分子通过环状氢键分别经由腺嘌呤部分的N(9)-H和N(3)原子与AU碱基对结合，而最低能量的阴离子单水合物和二水合物有一个水分子通过尿嘧啶单元的O4原子参与非环状氢键。预测AU碱基对的垂直脱附能（VDE）和绝热电子亲和能（AEA）在水合作用时都会增加。未水合的AU对的VDE和AEA分别为0.96和0.40 eV，而最低能量的阴离子二水合物的相应预测值分别为1.36和0.75 eV。由于尿嘧啶比腺嘌呤具有更大的电子亲和能，附着在AU碱基对上的多余电子占据尿嘧啶部分的π*轨道。当尿嘧啶部分作为氢键受体参与氢键作用时（例如，腺嘌呤和尿嘧啶碱基之间的N(6)-H(6a)...O(4)氢键以及AU对与水分子之间的O(w)-H(w)...N和O(w)-H(w)...O氢键），负电荷密度从尿嘧啶部分向腺嘌呤或水分子的转移有效地稳定了体系。此外，具有C-H...O(w)接触点的阴离子结构在能量上比具有N-H...O(w)氢键的结构更有利，因为C-H...O(w)接触点不允许水向尿嘧啶部分进行不利的电子密度捐赠。这种离域效应使得阴离子水合物的能量排序与相应中性水合物的能量排序有很大不同。
