Photophysics of Deoxycytidine and 5-Methyldeoxycytidine in Solution: A Comprehensive Picture by Quantum Mechanical Calculations and Femtosecond Fluorescence Spectroscopy

The study concerns the relaxation of electronic excited states of the DNA nucleoside deoxycytidine (dCyd) and its methylated analogue 5-methyldeoxycridine (5mdCyd), known to be involved in the formation of UV-induced lesions of the genetic code: Due to the existence of four closely lying and potentially coupled excited states, the deactivation pathways in these systems are particularly complex and have hot been assessed so far. Here, we provide a complete mechanistic picture of the excited state relaxation of dCyd/5mdCyd in three solvents water, acetonitrile; and tetrahydrofuran by combining femtosecond fluorescence experiments, addressing the effect of solvent proticity on the relaxation dynamics of dCyd and 5mdCyd for the first time, and two Complementaryquantum mechanical approaches (CASPT2/MM and PCM/TD-CAM-B3LYP). The lowest energy pi pi* State is responsible for the sub-picosecond lifetime observed for dCyd in all the solvents. In addition, computed excited state absorption and transient IR spectra allow one, for the first time; to assign the tens of picoseconds time constant,. reported previously,to a dark state (n(O)pi*) involving the carbonyl lone pair. A second low-lying dark state, involving the nitrogen lone pair (n(N)pi*), does significantly participate. in the excited state dynamics. The 267 nm excitation of dCyd leads to a non negligible population of the second bright pi pi* state, which affects the dynamics, acting mainly as a doorway state for the n(O)pi* State. The solvent plays a key role governing the interplay between the different excited states; unexpectedly, water favors papulation of the dark,states. In the case of 5mdCyd, an energy barrier present on the main nonradiative decay route explains the 6-fold lengthening of the excited state lifetime compared to that of dCyd, observed for all the examined solvents. Moreover) C5-methylation destabilizes both n(O)pi* and n(N)pi* dark states, thus preventing them from being populated.