The time evolution of the population of the different states, issued from the SH dynamics, is reported in Figure together with the proposed kinetic model and the fitting time constants. The population of the first singlet state, S1, follows a smooth exponential decay which leads to a global lifetime which may be estimated at about 8.4 ps. It is noteworthy, that the ISC in presence of microhydration is slower than those of the isolated chromophore, which has been determined at about 3.8 ps. Interestingly, however, the kinetic mechanism appears more complex, and while the slower S1àT2 decay is dominating at longer time scale a first fast ISC component can be observed at the very beginning of the SH dynamics. This short component is due to a very fast (t = 461 fs) S1àT3 transfer which is operative around the Franck-Condon region, due to the almost perfect degeneracy between the two states as identified by the Jablonski diagram, but is lost upon the geometrical relaxation along the S1 potential. Thus, a rapid rise of T3 can be observed, followed by its fast decay due to the rapid internal conversion (IC) in the triplet manifold and by the quenching of the population transfer from S1. IC in the triplet manifold is also operative in the main S1àT2 channel leading to the final population of T1, albeit with a much longer persistence of the T2 state. Interestingly, and differently from the case of the isolated chromophore after 2.0 ps the singlet manifold still presents a majority population amounting to about 60%. To rationalize the complex proton transfer and the role of the solvation network we report in Figure ) the time evolution of the most important O-H distance convoluted on the ensemble of the 50 trajectories. Note that the labeling of the involved atoms are also given in Figure ). In Figure we have reported the distance between O26 and H16, i.e. between the hydroxyl group of M and the closest water molecule. At Franck-Condon the average distance over the different trajectories amounts at about 1.75 Å, which is consistent with the formation of a hydrogen bond interaction. However, after 50 fs we observe the appearance of a second branch, involving a shorter O-H distance (~1 Å). The short-distance branch becomes dominant after 800 fs, indicating a transfer of H16 from M to the water molecule. In Figure ) we report the time evolution of the distance between O29 and H28, i.e between the two bridging water molecules. Once again, at short time-scales the average O-H distance lies at about 1.75 Å, coherently with the presence of a hydrogen bond. Yet, after 200 fs a second branch emerge showing a shorter equilibrium distance of 1.0 Å, pointing to the transfer of the hydrogen atom from the first to the second water molecule. Interestingly, this process appears slightly slower than the one observed for O26-H16, and indeed the short-distance branch becomes dominant only after 1200 fs. On Fig. ) we report the evolution of the distance between O13 and H31, i.e. the carboxyl group of M and closest water molecule. While again at Franck-Condon and at very short time of the dynamics the distance peaks at 1.75 Å, a short distance branch presenting an average value of 1.0 Å emerges at 300 fs and becomes dominant at 1300 fs. The emergence of this second branch indicates the completion of the transfer of the hydrogen atom from the water molecule to the carboxyl group of the DNA probe. Therefore, the global analysis of the evolution of the distances reported in Figure ) evidence and describe the emergence of the tautomerization leading to the conversion from M to M T which takes place through the participation of the microhydration network. Interestingly, we may observe the sequential nature of the transfer starting from the abstraction of the hydroxyl hydrogen of M by the closest water molecule and terminating with the donation of the hydrogen to the carboxyl group to form M T , as indicated by the different time-scale of the appearance of the short-distance branches. However, we may also observe, especially in Figure ) and 4b) the appearance of a further secondary branch characterized by large O-H distance (~4.0 Å) which are indicative of the loss of all interactions. As will be detailed in the following, these branches are representing the departure of one water molecule from the microhydration sphere. Finally, the reorganization of the water network can also be observed in Figure ) describing the evolution of the distance between O13 and H27, which from the initial average value of about 4 Å shows rather large fluctuation and the emergence after 1500 fs of a second branch with a distance of 2 Å. Interestingly, some occurrences of extremely large O-H distance of about 10.0 Å are also evidenced, once again indicative of the expulsion of one water molecule from the coordination sphere. Therefore, combining the results presented in Figure with the analysis of the different SH trajectories, we can sketch two possible mechanisms leading to the excited state tautomerization. The most important one, in term of frequency of occurrence, is reported in Figure ) and is characterized by the formation of a network involving both water molecules. As can be appreciated by the snapshots shown in Figure , the hydroxyl H of M is first transferred to the nearest water molecule leading to the formation of a H3O + moiety. Interestingly, this event is then followed by the formation of a bridge between the two solvent molecules which share the proton, leading to a strong reorganization of the solvent shell culminating to the transfer to the carboxyl group of the organic molecule to obtain M T and complete the tautomerization. In the case of the second mechanism (Figure ), which is less important in terms of the frequency of occurrence, after the initial transfer of the hydroxyl hydrogen of M to the nearest water molecule, the obtained H3O + moiety is stabilized by forming a bidentate bridge with the two oxygen atoms of the organic FNA. This occurrence leads, on the one side to the expulsion of the second water molecule from the solvation shell, and on the other side to the possible transfer of the proton to the organic moiety to obtain M T . Figure ) Representative snapshots describing the one-water-molecule driven tautomerization process.