Hud Nicholas V, Morton Thomas Hellman
School of Chemistry and Biochemistry, Parker H. Petit Institute of Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Phys Chem A. 2007 May 3;111(17):3369-77. doi: 10.1021/jp065786n. Epub 2007 Apr 6.
Dimers of free nucleobases with their conjugate acid ions can be assigned to either of two categories: protonated dimers or proton-bound dimers. In the former, the extra proton attaches to a lone pair of a neutral dimer. In the latter, the extra proton is situated between two lone pairs and participates in a proton bridge. In general, proton-bound dimers are found to be more tightly held together than protonated dimers. While neutral adenine and its isomer 8-aminopurine (C(5)H(5)N(5)) are substantially more stable than their 7H tautomers, their conjugate acid ions and those of their respective 7H tautomers have nearly the same heats of formation. Correspondingly, the most stable (C(5)H(5)N(5))2H+ structures contain 7H tautomers as the neutral partner. Proton transit from one partner to the other within the most stable protonated dimer of 8-aminopurine has a low barrier (6 kJ mol(-1)). The potential energy curve for the NH stretch in that case is better fitted as a double minimum rather than as a harmonic potential. Purine-purine mismatches have been observed in nucleic acids, to which calculated (C(5)H(5)N(5))2H+ dimer geometries appear nearly isosteric.
质子化二聚体或质子键合二聚体。在前者中，额外的质子附着在中性二聚体的孤对上。在后者中，额外的质子位于两个孤对之间并参与质子桥的形成。一般来说，发现质子键合二聚体比质子化二聚体结合得更紧密。虽然中性腺嘌呤及其异构体8-氨基嘌呤（C(5)H(5)N(5)）比它们的7H互变异构体稳定得多，但它们的共轭酸离子以及各自7H互变异构体的共轭酸离子具有几乎相同的生成热。相应地，最稳定的（C(5)H(5)N(5)）2H+结构包含7H互变异构体作为中性伙伴。在8-氨基嘌呤最稳定的质子化二聚体内，质子从一个伙伴转移到另一个伙伴的势垒较低（6 kJ mol(-1)）。在这种情况下，NH伸缩的势能曲线更好地拟合为双势阱而不是谐振势。在核酸中观察到嘌呤-嘌呤错配，计算得到的（C(5)H(5)N(5)）2H+二聚体几何结构与之几乎等电子。
