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Title: Femtosecond broadband time-resolved fluorescence and transient absorption spectroscopic study on excited state dynamics : methylated adenine, halogenated cytosine and halogenated uracil and their derivatives
Authors: Wong, Ka Wa
Degree: Ph.D.
Issue Date: 2021
Abstract: A numerous of modified nucleobases are found in a human cell, some are obtained from the ultraviolet (UV) induced chemical or enzymatic reaction in cell and then further initiate the carcinogenesis. Besides the biological function difference between the canonical nucleobases and the modified nucleobases, their excited state dynamics, the basic information to understand the photostability, are also different from canonical nucleobases. Recently, the importance of modified nucleobases raises but their studies about excited state dynamics are still fragmented, therefore their excited state dynamics are still unclear. To help in understanding the excited state dynamics of the modified nucleobases, we here selected three modified nucleobases systems for case studies. They are (i) N6-methylated adenine (Ade) and its derivatives, (ii) C5- halogenated deoxycytidine (dCyd) and (iii) C5-halogenated uridine (Urd) and deoxyuridine (dUrd). These three cases were studied by a combined method of steady-state spectroscopy and femtosecond broadband time-resolved spectroscopy upon different excitation wavelengths in a wide range of solvents. (i) The study of N6-methylated Ade and its N9-derivatives provides solid evidence that the blue fluorescence of dual fluorescence is originated from two La (ππ*) states and the red fluorescence is from a twisted intramolecular charge transfer (TICT) state. The origin of these three states is directly from the same Franck-Condon (FC) region. Irrespective of the involvement or not of the TICT state, which is a precursor of the triplet state, the formation of triplet state still occurs. (ii) The investigation of C5-halogenated dCyd supports that their excited state dynamics mainly consist of two bright ππ* species. The C5-halogenation effect is similar to C5-methylation, especially in restricting the molecular structure into planar conformation. This restriction increases the energy barrier from the ππ* state to the conical intersection (CI), resulting in two bright ππ* species. The C5-iodinated dCyd is the only one with a significant triplet state which is ascribed to the heavy atom effect. (iii) The study of C5-halogenated Urd and dUrd discovers that the triplet state generally occurs but except for deoxy-5-iodouridine (5IdUrd). All the molecules examined obtain a closely lying ππ*/ nπ* region where the ππ* and nπ* states are close to each other in energy. This region is susceptible to both C5-substituent and solvents. The results in these cases provide new insights into the excited state dynamics and the effect of substituent and solvent for modified nucleobases. It can help to estimate the photostability of the modified nucleobase and the modified nucleobase carrying deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). This information helps further investigation on the special phenomenon from the DNA and RNA carrying a modified nucleobase.
Subjects: Nucleosides
Hong Kong Polytechnic University -- Dissertations
Pages: xxxvii, 307 pages : color illustrations
Appears in Collections:Thesis

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