Excited-state dynamics of an oxidatively damaged DNA nucleoside under photodegradation and a TICT state in DCM

Abstract

An oxidatively damaged DNA nucleoside under photodegradation and a twisted intramolecular charge transfer (TICT) excited state of the DCM molecule were investigated by UV-pump/visible-probe transient absorption (TA) spectroscopy. 8-Oxo-2′-deoxyguanosine (8-oxo-dG), the common product of DNA in oxidative stress, is known for photorepairing cyclobutane pyrimidine dimers (CPDs). The 8-oxo-dG photodegradation at a weakly basic condition showed different excited-state dynamics, the energy shift, and the pH change. Two main events were observed and differentiated into the early stage that electron ejection occurred and the late stage that showed different photochemical dynamics. Two mechanisms were proposed for the photochemical dynamics of the 8-oxo-dG photodegradation. One of the mechanisms (8-oxo-dG• + •Haq → 8-oxo-dG) represents the pathway increasing the photophysical properties (photostability from UV radiation and the reactivity toward radicals) in the system, and the other mechanism describes the pathway degrading the system (8-oxo-dG• + •OHaq → 5-OH-8-oxo-dG∗ → Sp or Gh/Ia). The results might imply that the photodegradation of the oxidatively damaged DNA nucleoside would provide the electrons required for the CPD photorepair and the advantageous photophysical properties simultaneously. We also presented the photoinduced dynamics of an emissive intermediate TICT state in 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). The excited-state dynamics of its fluorescence have been debated but not fully understood so far. Intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states have been suggested for dual fluorescence in DCM. In this study, DCM was excited by UV light to populate the S2 state in a twisted conformation of the electron donor since it has been suggested that the twisted S2 state is responsible for the TICT dynamics. We observed the formation and completion of the TICT state from the spectral changes of temporary isosbestic points (within 46 ps) in the TA spectra and the dynamics of the TA kinetics (~50 ps). The solvation relaxation lifetime (~3 ps) of the TICT state was estimated, which is shorter than that of the ICT state (~10 ps).