A Theoretical Study Of The Electron Excitation Spectra Of Bio-chromophores Of Interest

The studies of high efficiency photosynthesis mechanism play a crucial role indeveloping the novel pesticides targeting the photosynthesis systems, and rational usingsolar energy. Light-harvesting system and reaction center are the two fundamentalpigment-protein complexes in the plant photosynthesis system. To make clear theelementary processes of electronic transfer in photosynthesis, knowledge of theelectronic excited characters of light-harvesting system and reaction center is aprerequisite. The theoretical studies of the electronic excitations of light-harvestingsystem and reaction center can provide a sophisticated understanding and theassignments of UV/vis absorption and circular dichroism spectra of these two systems. Inthis thesis, the electronic excitation spectra of the model systems (light-harvesting systemof alga and photosynthetic reacting center (PSRC) of purple bacteria) of photosynthesishave been investigated by using modem molecular simulation methods. The presentthesis is organized as follows:The previous studies about light-harvesting system of alga and photosyntheticreacting center of purple bacteria have been reviewed in chapter one. Furthermore, thetheoretical methods used in this thesis have been introduced.In chapter two, the electronic ground and singlet vertical excited states of Free-BasePorphyrin (FBP) have been investigated by using TDDFT, ZINDO and INDO/S methods,the basis sets effects have been examined at TDDFT theoretical level. In order to furtherexamine the precision of TDDFT method, the UV/Vis spectra of tetraoxaporphyrindication (TOPDC) and Porphycene have been investigated by using TDDFT method. Thesolvent effects on excitation spectra also have been taken into account by usingpolarizable continuum model (PCM). TDDFT-PCM calculations lead to a closeragreement with the measured values of experimental absorption spectra in solutions,which demonstate the importance of considering the solvent effects for interpretingexperimental absorption spectra in solutions.In chapter three, the optical characteristics of UV/Vis spectra and CircularDichroism (CD) spectra ofαandβsubunit of C-phycocyanin (C-PC) were investigatedby using TDDFT method combined with the PCM model. Combining the MolecularDynamic (MD) Simulation and ONIOM methods, the effects of the torsion of thepyrrolic rings in phycocyanobilin (PCB) on the absorption maximum also have been taken into account in this chapter. The electronic excitation at the various levels of modelchemistry demonstrate that, protonation is most likely existing in three PCBs (α-84,β-84,β-155) in C-PC in environmental protein moiety, and the reaction pathway forprotonation of PCBs also have been verified.Using the super molecule cluster models, the excitation spectra of PSRC from Rps.viridis and Rb. sphaeroides have been investigated by using TDDFT method in chapterfour. The effects of the interactions of pigment-pigment on the electronic excitation alsohave been examined. The calculation results show that the interactions ofpigment-pigment play an important role in reasonably assigning the UV/Vis spectra andcircular dichroism (CD) spectra of PSRC from Rps. virids and Rb. sphaeroides. Bycomparing with TDDFT calculation, the agreeable assignment of the experimentalUV/Vis and CD spectra were achieved by using the ZINDO method.In chapter five, the effects of both C=O stretch and H-C-H scissor models on thephotoelectron spectrum of formaldehyde have been taken into account simultaneously.The potential energy surface of the ground state (1¹A₁) and the first three excited states(1²B₂, 1²B₁, 1²A₁) of formaldehyde have been scanned at SAC-CI-GR/D95* level. Byfitting the potential energy surfaces of these states, the fine structures of the experimentalphotoelectron spectrum of formaldehyde have been accurately assigned by using ab initiofranck-condon factor program.