Density Functional Theory Studies On The Excited-State Properties Of Bilirubin Molecule

Bilirubin is one of the main pigment in human bile and an important indicator of liver function,which is closely related to human health.Bilirubin combining with fluorescent protein represents a new type of fluorescent chromophore and has important applications in the field of biological imaging and biosensor.Due to the unique structure of bilirubin molecule,the research on the electronic structure and excited state property of bilirubin molecule has aroused wide interest among researchers.A series of experimental studies have shown that the dynamic behavior of the excited state of bilirubin molecule is closely related to the configuration change of the excited state.However,there is still a lack of systematic theoretical study on the fluorescence and luminescence mechanism of the bilirubin chromophore.Due to the lack of efficient and accurate electronic structure methods,the electronic structure and excited-state properties of bilirubin molecule are not characterized quantitatively and accurately.Herein,the vertical absorption energy and vertical emission energy of the lowest singlet excited state of bilirubin molecule are calculated by combining the implicit solvent model and the linear response time-dependent density functional theory.Compared to the experimental data and high-level RI-ADC?2?calculation,the prediction performance of a series of density functional methods is systematically investigated.The results show that the optimally-tuned range separated density functional method has the best overall performance and the minimum absolute and relative errors.This is obviously due to the fact that the suitable proportion of exact exchange included in density functionals can produce neither delocalized nor localized electronic structures.The vertical absorption energy and oscillator strength of the lowest singlet excited state of phycocyanobilin and tetracycline molecule are also calculated.Based on the produced wavefunction by the optimally-tuned method,the excited-state characteristics of the S₁ state of bilirubin molecule in chloroform,dimethyl sulfoxide and water indicate a hybrid local and charge transfer excitation,based on the quantitative characterization using hole-electron analysis and interfragment charge transfer method.This work can provide a theoretical basis for the study of excited-state dynamics and spectral properties of bilirubin molecules and the optimally tuned range-separated DFT method also provide a reliable and efficient theoretical tool to study the excited-state properties of other biomolecular systems in the future.The dissertation consists of four chapters and the content is as follows:The first chapter gives an overview of the structural features of bilirubin molecules and the research status at home and abroad.The second chapter introduces the development of the theory and computational chemistry,the development of density functional theory,and the theoretical basis of the optimally-tuned range separated density functional method.In Chapter 3,the vertical transition energy of bilirubin,phycocyanin and tetracycline molecules are calculated using a variety of density functional methods.The excited state properties were then analyzed based on the hole-electron analysis and the interfragment charge transfer method.Chapter 4 is a summary of the content of this thesis and an outlook on the next steps of the research.