Influence of metal complexation on acidity of cytosine nucleosides: Part I, LiC, NaC and KC cation

Gas-phase acidities of nucleosides, combined with the knowledge of deprotonation sites, could improve our understanding of chemical reactions to biological systems. In this paper, we mainly focus our attention on the influence of cation coordination on acidities of multiple sites in cytosine nucleosides. The acidities of multiple sites inMC-L (where L represents cytosine nucleosides andMC is an alkali metal ion, including LiC, NaC and KC) complexes have been investigated theoretically, employing B3LYP=6 ?? 311CCG .d; p/ basis sets. The geometrical characters, gas-phase acidities, sugar puckering and electronic properties of non-deprotonated and/or deprotonated complexes have been investigated. The shifted 1Hacidity values are a consequence of a combination of metal ion coordination to OH and NH groups and efficient stabilization of the deprotonated species. For instance, after complexation with LiC, NaC, and KC, the 1Hacidity of O20H of the cytidine molecule shifts from 346.2 to 251.6, 258.0, and 275:7 kcal mol??1, respectively. Moreover, for a given coordination site, the metal ion changes the gap between the most and least acidic groups, with respect to that in neutral nucleosides. Such dependence of acidities on the coordination region of the metal ion suggests that acidities of active groups could be controlled by modulating the metal ion coordination site and the type of metal ion.