Study On The Dynamics Of Photosensitive Damage To Biomolecules And Steric Hindrance Effect

In recent years,photosensitive damage to biomolecules has attracted much attention as one of the important mechanisms of biological damage.However,there is still a lack of systematic study on the kinetics of photosensitive damage reactions,including proton coupled electron transfer mechanism,steric hindrance effect,the influence of active reaction sites,etc.In this thesis,based on time-resolved electron paramagnetic resonance（TR-EPR）and transient absorption spectroscopy（TAS）techniques,combined with theoretical calculations,the kinetic processes of photosensitive damage reactions have been deeply studied.The kinetic mechanism of p-benzoquinone（PBQ）to L-tryptophan（Trp）,the reaction sites and steric hindrance effect of acenaphthenequinone（ACQ）to dipeptides,and the intramolecular kinetic processes of dipeptides have been unveiled.On the basis of previous studies,it was further discovered that there was also steric hindrance effect on the process of triplet energy transfer.The detailed results are as follows:1.The kinetic mechanism of photosensitive damage reactions of PBQ to Trp in homogeneous solutionIn this section,the photoinduced chemical reaction of PBQ and Trp was investigated in homogeneous solution using TR-EPR and TAS techniques.Under photoexcitation of PBQ in N₂-saturated EG–H₂O homogeneous solution at 355 nm,the ³PBQ^*produced via intersystem crossing（ISC）in ethylene glycol aqueous（EG-H₂O）solution followed by the H-atom transfer（HAT）from EG to ³PBQ^*was a significant process in competition with the non-radiative transition of ³PBQ^*,which was clearly observed in the transient absorption spectra and chemically induced dynamic electron polarization spectra.When Trp was added into the PBQ/EG-H₂O solution,a new decay channel of ³PBQ^*was produced that reacted with Trp to form a p-benzoquinone anion radical(PBQ^·-)and a tryptophan cationic radical(Trp^·+),indicating that the photoinduced chemical reaction mechanism was the electron transfer.By fitting the decay dynamic curves,the rate constant of ³PBQ^*quenched by Trp in homogeneous solution was determined as 6.8×10⁸ M^-1 s^-1,which was close to the diffusion-controlled rate.2.Photosensitive damage of dipeptides:mechanism and influence of structureBased on time-resolved transient absorption spectroscopy technology,with tyrosine（Tyr）serving as the core structure,two classic dipeptides with double（tryptophan-tyrosine,Trp-Tyr）and single（tyrosine-alanine,Tyr-Ala and alanine-tyrosine,Ala-Tyr）active reaction sites were constructed,and the underlying photodamage mechanisms of ACQ to dipeptides were investigated carefully.According to the experimental results,the proton-coupled electron transfer processes between ACQ and numerous Trp-Tyr reaction sites have independent reaction properties.The bimolecular quenching rate(k_q=3.65×10⁹ M^-1 s^-1)value is roughly equivalent to the sum of the rates of two amino acid monomers(ACQ/Trp:2.11×10⁹ M^-1 s^-1;ACQ/Tyr:1.55×10⁹ M^-1 s^-1).Meanwhile,a novel intramolecular dynamic channel between Trp/N^·-Tyr and Trp-Tyr/O^·was observed for the first time（Trp/N^·-Tyr→Trp-Tyr/O^·）.The kinetic mechanisms of photosensitized damage reactions between ACQ and two dipeptide isomers（Tyr-Ala and Ala-Tyr）have been studied,and theoretical calculations were combined to optimize the ground state configurations of dipeptides.It has been demonstrated that there was steric hindrance effect on the proton coupled electron transfer processes between photosensitizers and peptides,which was one of the key factors determining the quenching rates of bimolecules.3.The steric hindrance effect of intermolecular energy transfer between Os-1 and different acceptorsThere is steric hindrance effect on the processes of intermolecular proton coupled electron transfer reactions.However,it is still a fundamental scientific issue at present that whether there is steric hindrance effect on the processes of triplet energy transfer as bimolecular reactions.In this section,a new type of osmium complex（Os-1）was used as a photosensitizer,the kinetic processes of triplet energy transfer have been investigated in the solution of Os-1 and six acceptors with different spatial structures by using a time-resolved transient fluorescence spectrometer.The rates of triplet energy transfer of different acceptors were measured and analyzed.Combined with density functional theory,the configurations of ground states of acceptors and the distribution of triplet spin density surfaces were optimized.Ultimately,the correlation between steric hindrance and the rates of triplet energy transfer was demonstrated.