Title Page

Abstract

Contents

1. Introduction 15

1.1. Solar UV-induced DNA damages 15

1.2. Generation of CPDs in humans 16

1.3. Repair mechanisms of UV-induced DNA damages 16

1.4. Release of 8-oxo-dG while repairing oxidative DNA damage 17

1.5. Oxidatively damaged DNA nucleoside 17

1.5.1. Contribution of 8-oxo-dG to neurodegenerative diseases 18

1.5.2. CPDs photorepair by 8-oxo-dG 19

1.5.3. Photophysical studies on 8-oxo-dG 20

1.6. Ionization processes of water by two-photon absorption 22

1.6.1. Two-photon ionization of water at 266 nm 23

1.7. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran 23

1.7.1. Emissive states of DCM 24

1.8. Transient absorption spectroscopy 26

2. Methods 28

2.1. Sample preparation 28

2.1.1. 8-Oxo-2'-deoxyguanosine 28

2.1.2. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran 28

2.2. Transient absorption spectroscopy setup 29

2.2.1. Ultrashort pulse laser 32

2.2.2. Home-built TA spectroscopy setup 32

2.2.3. Instrumental setup 33

2.3. Data fit 35

3. Photochemical pathways of 8-oxo-2'-deoxyguanosine under photodegradation 37

3.1. Steady-state absorption/fluorescence spectra of 8-oxo-2'-deoxyguanosine 37

3.1.1. Absorption spectra in acidic/alkaline conditions 39

3.1.2. Fluorescence spectra in acidic/alkaline conditions 41

3.2. TA kinetics of the 8-oxo-dG photodegradation 42

3.3. Mechanisms of the photochemical pathways under the 8-oxo-dG photodegradation 50

4. A twisted intramolecular charge transfer (TICT) state in a DCM molecule analyzed by isosbestic points 59

4.1. Steady-state absorption/fluorescence spectroscopy 59

4.2. TA kinetics and isosbestic points 60

5. Summary 70

References 72

Table I. Time constants of 8-oxo-dG at ~pH 8 under photodegradation (0-400 min) probed at 650 nm with 266 nm excitation. 45

Fig. 1. Cyclobutane pyrimidine dimer (left) and 8-oxoguanine (right). 15

Fig. 2. Tautomers of the neutral nucleobase (8-oxoguanine). The calculated relative populations⁴⁴ are indicated. The populations less than 0.0001% are not indicated. 21

Fig. 3. Major tautomers of the anionic nucleobase (8-oxoguanine). The calculated relative populations⁴⁴ are indicated. 21

Fig. 4. Major tautomers of the anionic nucleoside (8-oxo-2'-deoxyguanosine). The calculated relative populations⁴⁴ are indicated. R＝Deoxyribose. 22

Fig. 5. The structure of a DCM molecule. 25

Fig. 6. Schematic view of the transient absorption spectroscopy setup used in this study. 30

Fig. 7. Transient absorption spectroscopy setup. 31

Fig. 8. Steady-state absorption (black line)/fluorescence (red dots) spectra of 8-oxo-dG at~pH 8. 39

Fig. 9. Steady-state absorption spectra of 8-oxo-dG with (a) 0-20 ml of HCl added, (b) 0-1.3 ml of NaOH added, and (c) 1.3-81 ml of NaOH added when 1M HCl/NaOH solution is... 40

Fig. 10. (a) Steady-state fluorescence spectra of 8-oxo-dG with 10 and 100 ml of HCl added, and 0 and 20 ml of NaOH added (ex/emi slit＝2.5/10 nm with 266 nm excitation). (b) Steady-... 42

Fig. 11. Transient absorption spectra and kinetics of 8-oxo-dG at ~pH 8 under 40 min and 400 min photodegradation. 44

Fig. 12. TA kinetics of 8-oxo-dG at ~pH 8 under photodegradation probed at 650 nm wavelength with 266 nm excitation. ES: the early stage of the photodegradation (0-160 min).... 46

Fig. 13. Kinetic schemes proposed for the 8-oxo-dG photodegradation (~pH 8) by UV radiation. S0 is the ground state. S₁' is the vibrationally relaxed state of the first excited state...[이미지참조] 47

Fig. 14. (a) Steady-state absorption spectra of 8-oxo-dG at ~pH 8 under photodegradation. A: 0-400 min photodegradation. B: 840-1700 min photodegradation. (b) Isosbestic points... 51

Fig. 15. Normalized steady-state absorption spectra of the photodegradation. The spectra were normalized at the (a) S₂ and (b, c) S₁ states. Blue lines: 8-oxo-dG at ~pH 8 with 0 min... 53

Fig. 16. Mechanisms proposed for the photochemical pathways of 8-oxo-dG¯ under photodegradation with UV radiation. 55

Fig. 17. Steady-state absorption/fluorescence spectra of DCM in methanol. The fluorescences were measured with the excitation of 3.1, 4.7, and 6.2 eV (400, 266, and 200... 60

Fig. 18. Transient absorption spectra of DCM in methanol with 4.7 eV excitation. 61

Fig. 19. Transient absorption kinetics of DCM in methanol with 4.7 eV excitation. (a) The absorption kinetics at 543 nm. (b) The fluorescence kinetics at 634 nm with an inset (〉 ~1... 62

Fig. 20. Spectral changes of DCM in methanol. The wavelengths of isosbestic points are shown. 64

Fig. 21. Schematic energy diagram proposed for the photophysical pathways of DCM in methanol after exciting the Sn state (n ≥ 2). The TICT state before solvation relaxation is... 66

Fig. 22. Analysis with the function C(t) of the emission band maximum of DCM in methanol with 4.7 eV excitation. 67

Fig. 23. Contour plot showing the emission band shift in the TA dynamics of DCM in methanol. The emission band maximum (dotted line) has been red shifted from 625 to 640... 68