| Abstract
| - Dimers of free nucleobases with their conjugate acid ions can be assigned to either of two categories: protonateddimers or proton-bound dimers. In the former, the extra proton attaches to a lone pair of a neutral dimer. Inthe 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 adenineand its isomer 8-aminopurine (C5H5N5) are substantially more stable than their 7H tautomers, their conjugateacid ions and those of their respective 7H tautomers have nearly the same heats of formation. Correspondingly,the most stable (C5H5N5)2H+ structures contain 7H tautomers as the neutral partner. Proton transit from onepartner 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 thanas a harmonic potential. Purine−purine mismatches have been observed in nucleic acids, to which calculated(C5H5N5)2H+ dimer geometries appear nearly isosteric.
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