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Protonated primary amines induced thymine quintets studied by electrospray ionization mass spectrometry and density functional theory calculations.

As the novel magic number clusters of nucleobases, the thymine quintets induced by ammonium ion (NH4(+)), and particularly by its derivatives such as protonated alkyl amines and protonated aryl amines, have been studied by electrospray ionization mass spectrometry (ESI-MS) and density functional theory (DFT) calculations. The DFT-optimized geometry of NH4(+) induced thymine quintet ([T5 + NH4](+)) reveals some new features including three additional hydrogen bonds between NH4(+) and its surrounding thymine molecules when compared with that of the alkali metal ions induced thymine quintets. In addition, the fourth hydrogen atom of NH4(+) is sticking out the assembly, and, thus, it might be replaced by an organic group R to form the protonated primary amine induced thymine quintet ([T5 + R - NH3](+)), a hypothesis that has been confirmed by both DFT calculations and ESI-MS experiments. Furthermore, the relative abilities of the different protonated primary amines for inducing the thymine quintets are investigated by ESI-MS competition experiments, and the results have shown a clear trend of stronger ability as the alkyl chain gets longer or as the aryl ring gets larger for the alkyl amines or the aryl amines. Two basic influence factors are consequently identified: one is the ability of the alkyl amine to accept proton, another is the π-π stacking interaction between the aryl ring and the π-surface of the thymine molecule(s), whose explanations are strongly supported by multiple types of thermochemical data, various control experiments and DFT calculations.