Cytidine (Cyd, 99% purity), 5-methylcytidine (5mCyd, > 99%), and extra dry acetonitrile (ACN) were purchased from Sigma Aldrich used as received. Phosphate buffer solutions with total phosphate concentration of 16 mM from monosodium and disodium phosphate salts dissociated in ultrapure water (Millipore) were freshly prepared the day of each laser irradiation experiment. The pH of the solution was adjusted using 0.1 M solutions of NaOH and HCl to the desired pH of 7.4 (± 0.1 pH units).