Microhydration of cytosine and its radical anion: cytosine.(H2O)n (n=1-5).

Microhydration effects on cytosine and its radical anion have been investigated theoretically, by explicitly considering various structures of cytosine complexes with up to five water molecules. Each successive water molecule (through n=5) is bound by 7-10 kcal mol(-1) to the relevant cytosine complex. The hydration energies are uniformly higher for the analogous anion systems. While the predicted vertical detachment energy (VDE) of the isolated cytosine is only 0.48 eV, it is predicted to increase to 1.27 eV for the lowest-lying pentahydrate of cytosine. The adiabatic electron affinity (AEA) of cytosine was also found to increase from 0.03 to 0.61 eV for the pentahydrate, implying that the cytosine anion, while questionable in the gas phase, is bound in aqueous solution. Both the VDE and AEA values for cytosine are smaller than those of uracil and thymine for a given hydration number. These results are in qualitative agreement with available experimental results from photodetachment-photoelectron spectroscopy studies of Schiedt et al. [Chem. Phys. 239, 511 (1998)].