Quantum Chemical Investigations On The Excited States Of Flavin-related Compounds In Natural And Artificial Systems

Many species on earth have the ability to utilize the earth's magnetic field for orientation and navigation. The mechanism of the magnetic sensing is proposed to be the"radical pair mechanism"based on a protein named cryptochrome, which is a blue-light photoreceptor. The photonic excitation of a flavin cofactor within cryptochrome causes sequential electron transfer between the flavin cofactor and three conserved tryptophan residues, leading to the formation of the radical pair which is sensitive to the earth's magnetic field. Therefore, the excitation of flavin is essential to this process. Carrying on quantum chemical research on the excited states of flavin related molecular systems could play important role in understanding the mechanisms of electron transfer in cryptochrome, and provide preliminary theoretical instructions to the bio-mimic design of artificial magnetic-sensitive molecules.In the present paper, the quantum chemical methods are employed to study the ground-state structure, the vertical excited state, and the absorption spectrum of several flavin compounds in nature and artificial systems. Ground-state structures, electronic structures, electronic coupling, and electron transfer mechanisms of flavin and its derivatives in these systems are investigated in detail.The main contents are as follows:Firstly, the vertical excited statesof riboflavin and lumiflavin are investigated by employing the CIS method and the TD-DFT method. The basis set 6-31g(d) 6-311g(d) 6-31+g(d) 6-311+g(d) and PCM model are used in the calculation. By comparing the calculated and experimental UV-visible absorption spectroscopy spectra, we found that the calculated results from TD-DFT(B3LYP)/6-31+g(d) with the PCM model are consistent with the experimental observations. Meanwhile, the CIS method is only suitable for qualitative research.Secondly, a flavin model in nature protein environment is built with reference to Arabidopsis Thaliana CRY-1 crystal structure. The changes of the ground state structure and the electronic structure of the flavin model together with hydrogen bonds fromed between flavin and protein environment are studied based on the optimized conformation which is obtained with the DFT(B3LYP) method. In Addition, different flavin-tryptophan models are constructed systematically.The charge transfer (CT) states of these models are calculated with the CIS method for qualitativeinvestigation of two possible electron transfer mechanisms. The FADH+ mechanism is supported in our results.Thirdly, an artificial flavin compound (flavin-phenothiazine) is investigated in this section. After the optimization of the ground state structure using the DFT(B3LYP) method, we calculated the vertical excited states of this compound in vaccum and in acetonitrile environment at TD-DFT(B3LYP)/6-31+g(d) level. Its electronic structure and electron transfer mechanisms are analyzed, and the existence of super-exchange mechanism is verified by calculating the electronic coupling value. Thus, we proposed a principle of designing similar biomimetic moleculars based on this mechanism.Finally, flavin-tryptophan (FT) artificial compound is designed. The vertical excited state of FT in water is calculated with TD-DFT(B3LYP)/6-31+g(d) method, and the UV-visible absorption spectroscopy is predicted. The photoinduced electron transfer mechanism of FT and its electronic coupling is also investigated in this section.