Study Of The Photoinduced Dynamics Of Cytosine And Its Derivatives

There has been interest for many years in excited electronic states of DNA and RNA bases because of their role in the UV photodamage of nucleic acids. These excited states sometimes decay to deleterious photoproducts, which can cause mutations and interfere with the normal cellular processing of DNA.Over the past two decades, efforts have been made in the study of the relaxation dynamics of excited-state nucleic acid bases. The lifetimes of the first electric dipole-allowed state (S1) have been observed to be generally short (around 1 ps), and fast decay to the ground state via internal conversion (IC) has been proposed. However, this decay mechanism is far from certain, and other possibilities have also been suggested. Some model systems have been widely studied for its extreme ease of experimental manipulation and the simplification as compared to real nucleobase pairs. The research is a front subject in the field of current photochemistry.In this paper, we have studied the short-time photodissociation dynamics of photo-induced Proton-Transfer/Charge-Transfer of 2-mercaptopyridine, 2-hydroxypyridine, cytosine and cttidine in water, methanol, acetonitrile and THF solution by the Resonance Raman spectra combined with DFT and CASSCF calculation and discussed the influential factors to tune the photochemical reactions. Main conclusions of the present work are summarized as follows,(1) The absorption spectrum of 2-thiopyridone has been obtained in water solution. There have two charge-transfer bands, A-band near 341 nm while B-band near 271 nm. The resonance Raman spectra were obtained for 2-thiopyridone (2-TP) and its proton-transfer reaction photoproduct 2-mercaptopyridine (2-MP) in water solution. Density functional calculations were carried out to help elucidate their ultraviolet electronic transitions and vibrational assignments of the resonance Raman spectra associated with their B-band absorptions. The different short-time structural dynamics were examined for both 2TP and 2MP in terms of their resonance Raman intensity patterns. The transition barriers between 2TP and 2MP for S0, T1 and S1 states are determined by using (U)B3LYP-TD and CASSCF level of theories respectively. The excited state proton transfer reaction mechanism is briefly discussed. (2) We obtained the absorption spectra of 2-pyridone (2-PY) in water, acetonitrile and THF solution. The charge-transfer (CT) band of B-band are near 244 nm, 214 nm and 215 nm respectively. Resonance Raman spectra were obtained for 2-pyridone in water, acetonitrile and THF solution with 223.1 nm and 228.7 nm excitation wavelength in resonance with the CT-band absorption spectrum. The results from the present investigation suggest that 2-pyridone is with strongly solvent effect. The structure of 2PY-(H₂O)₂ complex has been determined from FT-Raman spectrum in the water solution, and the structure of 2-PY has been determined from FT-Raman spectrum in acetonitrile and THF solution. We have compareed the vibrational modes in water and THF solution, and it shows that the hydrogen bond of solute and proton polarity solvent can influence the Franck-Condon region structural dynamics. Two water molecules act as a bridge, which makes proton transfer easy in the complexes.(3) The absorption spectra of cytosine and cytidine in water and methanol solution are obtained , and also the Resonance Raman spectra. The absorption spectra of cytosine in water and methanol solution which has a charge-transfer (CT) band near 263 nm have been acquired. Resonance Raman spectrums were obtained for cytosine in water and methanol solution with 252.7 nm , 266 nm, 273.9 nm and 282.4 nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short time photodissociation dynamics of cytosine have multidimensional characters with motion predominantly along the nominal C₅=C₆/C2=O7 stretch+N3=C₄-N8 asymmetry stretch mode(ν₇,1643 cm^-1), the nominal N3=C₄-C₅ asymmetry stretch mode(ν9,1526 cm^-1), the nominal C₄=C8 stretch +C₅-H/C₆-H/N_-H bend mode (ν₁₂, 1283 cm^-1), the nominal C₄-C₅ stretch +ring deformation mode (ν₂₀,780 cm^-1).We obtained the absorption spectra of cytidine in water and methanol solution which has a charge-transfer(CT) band near 268 nm. Resonance Raman spectra were obtained for cytidine in water and methanol solution with 252.7 nm , 266 nm, 273.9 nm and 282.4 nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short time photodissociation dynamics of cytidine have multidimensional characters with motion predominantly along the nominal C14=N₅/C11=O stretch mode (ν15, 1629cm^-1), the nominal the cytosine ring stretch+CH₂ scissor in the ribose ring mode (ν17, 1506 cm^-1), the nominal the cytosine ring stretch +CH wag in the ribose ring mode (ν33, 1224 cm^-1). We have compared the vibrational modes of cytosine and cytidine, and intense modes of cytidine are more or less coupled by the vibrations of the ribosyl substituent. It shows that vibronic coupling of ground state can influence the short-time structural dynamics of cytidine.