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京都大学 / 新制・課程博士 / 博士(理学) / 甲第23723号 / 理博第4813号 / 新制||理||1689(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺嶋 正秀, 教授 林 重彦, 教授 渡邊 一也 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Repair mechanisms for damaged DNA are essential for the proliferation of nearly all forms of life. Although DNA is quite robust, the vital information-storing molecule can often be damaged from environmental exposures such as ultra-violet (UV) light. Exposure to UV light can result in various types of mutagens creating structural damages. One specific type of UV-induced damage is the creation of a cyclobutylpyrimidine dimer (CPD). This specific type of lesion can be efficiently repaired by the flavoenzyme DNA photolyase (PL). DNA photolyase is an ancient protein found across kingdoms and plays a crucial role in preventing mutagenesis and cell death. DNA photolyase is a monomeric flavoprotein that utilizes blue light to repair UV-induced CPD lesions in DNA via an electron transfer mechanism. All photolyases contain at least one flavin adenine dinucleotide (FAD) molecule as the catalytic cofactor responsible for initiating the electron transfer induced repair process. Flavin cofactors are intriguing because of their unique ability to donate one or two electrons. The conservation of FAD and the unique U-shaped configuration of FAD in PL led researchers to question if the adenine moiety of the FAD molecule was essential in the DNA repair mechanism and generated a spectral signature indicative of a radical adenine species. The importance of the adenine moiety could be linked to structural changes associated with environmental temperature. The rate of electron transfer is exponentially dependent on temperature and DNA photolyase is found in organisms which thrive in harsh environments that vary in temperature, pH, ionic strength etc. Photolyase presents a unique opportunity to study the adaptations that are required for proteins to function in extreme environments where temperature dependent processes should show dramatic differences. We have used ultrafast transient absorption spectroscopy to compare the similarities and differences in excited state dynamics of the FAD cofactor. Photolyase isolated from the hyperthermophilic archaea Sulfolobus solfataricus (SsPL) is compared to PL isolated from the mesophilic E. coli (EcPL). These results indicate differences in the dynamics of fully reduced flavin between enzymes as a function of temperature. We present evidence for charge separation in the FAD cofactor in the thermophilic enzyme previously seen in computation studies of photolyase. To investigate the excited state charge redistribution of flavin which is critical to its role in nature, the charge redistribution of the precursors to flavin biosynthesis were examined. Lumazine is a precursor in the biosynthetic pathway of flavins. As such, lumazine could have served as an enzymatic cofactor prior to flavins. Lumazine has been identified in biological processes, however it is not as prevalent as flavins. We utilize Stark spectroscopy to examine the charge redistribution in excited state lumazine to understand / Chemistry

Yes / An integrated Finite Element Method (FEM) model is proposed to investigate the dynamic seabed response for several specific pipeline layouts and to simulate the pipeline stability under waves loading. In the present model, the Reynolds-Averaged Navier-Stokes (RANS) equations are used to describe the wave motion in a fluid domain, while the seabed domain is described using the Biot’s poro-elastic theory. The interface between water and air is tracked by conservative Level Set method (LSM). The FEM and backward differentiation formula (BDF) are applied for spatial and temporal discretization respectively in the present model. One-way coupling is used to integrate flow and seabed models. The present model is firstly validated using several available laboratory experiments. It is then further extended to practical engineering applications, including the dynamic seabed response for the pipeline mounted on a flat seabed or inside a trench. The results show that the pipeline buried to a certain depth is better protected than that under partially buried in terms of transient liquefaction. / Energy Technology Partnership (ETP), Wood Group Kenny

Yes / Diffusion brazing has gained much popularity as a technique capable of joining dissimilar lightweight metal alloys and has the potential for a wide range of applications in aerospace and transportation industries, where microstructural changes that will determine the mechanical and chemical properties of the final joint must be controlled. This study explores the effect of Al2O3 nanoparticles on the mechanical and microstructural properties of diffusion brazed magnesium (AZ31) and aluminium (Al-1100) joints. The results showed that the addition of Al2O3 nanoparticle to the electrodeposited Cu coating increased the volume of eutectic liquid formed at the interface which caused a change to the bonding mechanism and accelerated the bonding process. When the Cu/Al2O3 nanocomposite coatings were used as the interlayer, a maximum bond strength of 46 MPa was achieved after 2 min bonding time while samples bonded using pure-Cu interlayers achieved maximum strength after 10 min bonding time. Chemical analysis of the bond region confirmed that when short bonding times are used, the intermetallic compounds formed at the interface are limited to the compounds consumed in the eutectic reaction.