Ultrafast Excited State Dynamics Of Biliverdin Dimethyl Ester Coordinate With Zinc Ions

Biliverdin has a wide biological distribution in nature organism.It is the final product in heme catabolism in birds and insects It's also the precursor of bilirubin in animals.Biliverdin is mainly presented in bile and it is the main chemical component that causes greenish color seen in bruises.Usually,blood biliverdin in patients with liver disease is high and accumulation of biliverdin and bilirubin in the circulatory system and tissues is the main cause of jaundice.In addition,as a linear tetrapyrrole,biliverdin is also one of the chromophores of in fluorescence proteins.Biliverdin can form complex with transition metal ions and its structure is more rigid compared with free biliverdin in solution.Therefore,it may show similar emission behavior as biliverdin in protein environment.Studying the excited state dynamics of biliverdin and its metal complexes can enable us to further understand their fluorescence mechanism,which may provide guidance for exploring new red fluorescence proteins.Because the spectral properties of biliverdin dimethyl ester?BVE?are basically the same as biliverdin,and the coordination sites between BVE and metal is less than those of BV,it is easier to analysis the spectroscopy in BVE-metal complex.In this thesis,we focused the study on excited state dynamics of BVE-Zn²⁺complexes.The fluorescence intensity of BVE itself in solution is very weak,and the quantum yield?QY?is less than 0.01%?However,after complex with Zn²⁺,fluorescence quantum yield is increased by 60-80 times measured by our ultraviolet-visible absorption spectra and fluorescence spectra.BVE can form a stable coordination complex with Zn²⁺with 1:1 stoichiometry in alcoholic solvents.However,it can further complex with Zn²⁺with 1:2 stoichiometry in DMSO besides the 1:1 complex.For the1:1 BVE-Zn²⁺complex,the fluorescence quantum yield is measured and fluorescence lifetime is determined in ethanol,n-propanol and DMSO using TCSPC technology.Therefore,the corresponding radiative and non-radiative coefficients are solved.Comparing three sets of data,it is found that the structural change of BVE complex in different hydrogen bonding network environments will affect its fluorescence QY.Meanwhile,the reason why Zn²⁺enhances BVE fluorescence is explained through comprehensively analyzing the steady-state and time-resolved fluorescence spectra.Femtosecond transient absorption spectroscopy technique was used to study the excited state dynamics of BVE and its Zn²⁺complexes.The excited state relaxation model of the 1:1 BVE-Zn²⁺complex in alcohol and DMSO is proposed.In addition,we also used the above method to study the fluorescence behavior of the 1:2 BVE-Zn²⁺complex in DMSO.The results show that the fluorescence of 1:2BVE-Zn²⁺complex is quenched compared with that of 1:1 complex and the quantum yield is only 0.36%.The difference in the steady-state spectra between the 1:1 and 1:2complexes is very similar to the difference between Pr and Pfr states of phytochromes.Based on this result,we speculate that the structure of the 1:2 complex is similar to that of the Pfr state chromophore.Femtosecond transient absorption spectroscopy technique was also applied to study the excited state dynamics of the 1:2 BVE-Zn²⁺complex and the excited state relaxation model is also proposed.