Depopulation of Highly Excited Singlet States of DNA Model Compounds: Quantum Yields of 193 and 245 nm Photoproducts of Pyrimidine Monomers and Dinucleoside Monophosphates

Abstract— Formation of uracil and orotic acid photodimers, uridine and 5'-UMP photohydrates, TpT photodimers and (6-4)photoproducts, dCpT photohydrates and (6-4)photo-products and UpU, CpC and CpU photohydrates were studied in neutral deoxygenated aqueous solution at room temperature upon irradiation at either 193 or 254 nm. The photoproducts were identified and quantified and the contribution from photoionization to substrate decomposition, using λ_irr= 193 nm, was separated. The ratio of the quantum yields of respective stable products,η=φ¹⁹³/φ²⁵⁴ is indicative of the yield of internal conversion from the second to the first excited singlet state, S₂→ S₁. For the observed photodimers η decreases from 0.94 for uracil to 0.7 for TpT and further to 0.55 for orotic acid. For the (6-4)photoproducts of TpT and dCpT T| = 0.5-0.8 and for the photohydrates in the cases of UpU, CpC, CpU and dCpT TJ ranges from 0.55 to 1.