Effect of base stacking on the acid-base properties of the adenine cation radical [A*+] in solution: ESR and DFT studies.
Effect of base stacking on the acid-base properties of the adenine cation radical [A*+] in solution: ESR and DFT studies.
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DOI：
10.1021/ja802122s
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发表时间：
2008-08-06
影响因子：
15
通讯作者：
Sevilla MD
中科院分区：
文献类型：
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作者：
Adhikary A;Kumar A;Khanduri D;Sevilla MD
本研究从实验和理论两个方面研究了腺嘌呤阳离子自由基的酸碱性质。在水玻璃(7.5MLiClH2O和D2O中)中，Cl2·−单电子氧化DAO和堆积的dna齐聚物(Da)6，生成腺嘌呤阳离子自由基(A·+)，并用电子自旋共振波谱对其进行了研究。理论计算和DADO分子中C_8-H和N_6-H上的重氢取代有助于我们的结构归属。我们发现该体系中A·+的pKa值在150K时约为8，似乎与常温下≤-1的可接受值相矛盾。然而，在≥160K热退火后，在这些玻璃系统中，即使在pH约3的情况下，DAO中的A·+也发生了完全的去质子化。在150K时，(DA)6中的A·+在热退火时也发生了去质子化。在这些体系中，A·+在150K时的稳定性归因于堆积碱基之间的电荷离域。在不同水平上(DFT B3LYP/6-31G*、MPWB95和HF-MP2)的理论计算预测了腺嘌呤堆积的二聚体阳离子自由基的结合能为12-16kcal/mol。进一步的密度泛函B3LYP/6-31G*计算预测，在水溶液中，单体A·+应该自发去质子化(预测A·+的pKa约为−0.3)。然而，电荷共振稳定的二聚体AA·+被预测会导致显著的去质子化障碍，并且AA·+二聚体的pKa约为7，比单体高7个pH单位。这些理论和实验结果表明，在溶液中分离的A·+和腺嘌呤堆栈中的A·+具有非常不同的酸碱性质，这是由于腺嘌呤堆栈中的空穴离域引起的稳定所致。
In this study, the acid–base properties of the adenine cation radical are investigated by means of experiment and theory. Adenine cation radical (A•+) is produced by one-electron oxidation of dAdo and of the stacked DNA-oligomer (dA)6 by Cl2•− in aqueous glass (7.5 M LiCl in H2O and in D2O) and investigated by ESR spectroscopy. Theoretical calculations and deuterium substitution at C8–H and N6–H in dAdo aid in our assignments of structure. We find the pKa value of A•+ in this system to be ca. 8 at 150 K in seeming contradiction to the accepted value of ≤ 1 at ambient temperature. However, upon thermal annealing to ≥160 K, complete deprotonation of A•+ occurs in dAdo in these glassy systems even at pH ca. 3. A•+ found in (dA)6 at 150 K also deprotonates on thermal annealing. The stability of A•+ at 150 K in these systems is attributed to charge delocalization between stacked bases. Theoretical calculations at various levels (DFT B3LYP/6-31G*, MPWB95, and HF-MP2) predict binding energies for the adenine stacked dimer cation radical of 12 to 16 kcal/mol. Further DFT B3LYP/6-31G* calculations predict that, in aqueous solution, monomeric A•+ should deprotonate spontaneously (a predicted pKa of ca. −0.3 for A•+). However, the charge resonance stabilized dimer AA•+ is predicted to result in a significant barrier to deprotonation and a calculated pKa of ca. 7 for the AA•+ dimer which is 7 pH units higher than the monomer. These theoretical and experimental results suggest that A•+ isolated in solution and A•+ in adenine stacks have highly differing acid–base properties resulting from the stabilization induced by hole delocalization within adenine stacks.