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    Effects of C5-substituent group on the hydrogen peroxide-mediated tautomerisation of protonated cytosine: a theoretical perspective

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    <p>The direct tautomerism (path A) and H<sub>2</sub>O<sub>2</sub> as a catalyst (path B) have been studied in conversion of Cyt2t<sup>+</sup> into CytN3<sup>+</sup> isomer. The protonated 5-carboxycytosine (5-caCyt) is represented and has been further explored in the presence of H<sub>2</sub>O<sub>2</sub> (path C). In going from a four-membered-ring transition state in the case of the direct tautomerism to the six-membered ring for H<sub>2</sub>O<sub>2</sub>, the H<sub>2</sub>O<sub>2</sub> significantly contributes to decreasing the free energy barrier of tautomerisation. Although the carboxylic substituent of 5-carboxycytosine has certain affected on the electron distribution of the pyrimidine ring, the six-membered-ring transition state has not changed. This result illustrates that the C5-carboxylation has no significant effect on the H<sub>2</sub>O<sub>2</sub>-mediated isomerisation of Cyt2t<sup>+</sup> to CytN3<sup>+</sup> isomer. Meanwhile, these paths A–C have been further explored in the presence of two water molecules. Use of implicit solvent models (PCM) does not significantly alter the energetics of water-mediated paths A–C compared to those in gas phase. Furthermore, the rate constant with Wigner tunnelling correction of path A is obviously smaller than those of paths B and C. Finally, the lifetime τ<sub>99.9%</sub> of paths B and C is 10<sup>−5</sup> s, which is implemented by the mechanism of the concerted synchronous double proton transfer.</p
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