Effects of sequential microhydration on thymine tautomerization.

CONTEXT: The tautomerization of thymine between its keto and enol forms is a key event for spontaneous mutagenesis. While the intrinsic enol-to-keto conversion is kinetically hindered in the gas phase, the presence of a discrete microhydration environment can drastically alter this landscape. Our M06-2X free energy calculations indicate that the cluster with two water molecules provides the lowest Gibbs free energy barrier among the investigated hydrated systems. However, benchmark electronic calculations show that the energetic differences between the mono-, bi-, and trihydrated clusters are small. HOMA index and QTAIM analyses confirm that the third water molecule induces an offset between aromaticity and intermolecular stabilization, providing a structural basis for the observed kinetic stabilization of the rare tautomer in a hydrated environment. METHOD: In this work, we employ Hybrid-Meta DFT calculations to investigate the impact of sequential microhydration (0 to 3 water molecules) on the proton-transfer dynamics. Stable geometries for all stationary points (minima and transition states) were obtained through full optimization using the hybrid-meta M06-2X functional along with the def2-TZVP basis set. The nature of the stationary points was confirmed by harmonic frequency analysis. Furthermore, the normal modes associated with the imaginary frequencies were inspected to confirm that they correspond to the proton-transfer coordinate. Kinetic rate constants were calculated based on Transition State Theory (TST) at 298.15 K. Topological properties of the electronic density were obtained through QTAIM and NCI analyses.