Investigation On The Relationship Between The Excited State Intramolecular Proton Transfer And Antioxidation Of Hydroxyl-type Compounds

Hydroxyl-type compounds,such as flavonoids and anthraquinones,have been widely used in the clinical medicine due to their antioxidant activity.How to improve their antioxidant activity has also attracted great attention of researchers.Some flavonoids and anthraquinones can generate the keto and enol configurations through the photoisomerization of excited state intramolecular transfer?ESIPT?.It is not clear whether the photoinduced ESIPT process can improve the antioxidant activity of these compounds.In this paper,the ESIPT process and antioxidant activity of hydroxyflavones and anthraquinones were studied to provide reference for improving the antioxidant activity of these compounds by using the femtosecond transient absorption spectroscopy combined with the density functional theory?DFT?and time-dependent density functional theory?TD-DFT?methods.The main contents are as follows:?1?The effects of atomic electronegativity on the ESIPT process of3-hydroxyflavone and its derivatives?3-HF,3-HTF and 3-HSeF?were studied via DFT and TD-DFT methods.At the same time,the antioxidant activity of the three compounds was studied with different parameters in theory.The results show that the intramolecular hydrogen bond strength of the three compounds in S₁ state are enhanced,which is conducive to the occurrence of ESIPT process.However,3-HTF has the lowest proton transfer barrier in S₁ state,which indicates that 3-HTF is most likely to undergo ESIPT process under light excitation.In addition,the ionization potentials and energy gap values from 3-HF to 3-HTF and 3-HSeF decreased gradually,indicating that the antioxidant activity of the three compounds gradually increased with the weakened atomic electronegativity,and the three compounds have higher antioxidant activity in the configuration after proton transfer.?2?The ESIPT property and antioxidant activity of 3-hydroxy-4'-methoxyflavone?3H4M?were studied by using the femtosecond transient absorption spectroscopy,DPPH free radical scavenging experiment and DFT method.Experimental and theoretical results show that 3H4M is more likely to produce ESIPT process under the light excitation.DPPH free radical scavenging experiment indicates that 3H4M exhibits the higher antioxidant activity under the laser irradiation than that under the dark condition.In addition,the energy gaps of the enol and keto configurations of 3H4M in S₀ and S₁ states shows that the keto configuration of3H4M after ESIPT process has higher antioxidant activity,suggesting that the occurrence of ESIPT process under light excitation can improve the antioxidant activity of 3H4M.?3?Using the femtosecond transient absorption spectroscopy,DPPH free radical scavenging experiment and DFT method,the relationship between ESIPT process and antioxidant activity of 2,6-DHAQ,1,5-DHAQ and 1,5-DAAQ was studied.Experimental and theoretical results show that both 1,5-DHAQ and 1,5-DAAQ tend to undergo the intramolecular proton transfer in the excited state,and the ESIPT process of 1,5-DAAQ?⁴86.9 fs?is significantly slower than that of 1,5-DHAQ?²70.5 fs?.In addition,the ionization potentials of the three compounds in S₁ state is lower than that in S₀ state,which indicates that the antioxidant activity of the three compounds will be enhanced under light excitation.Compared with the enol configuration,1,5-DHAQ and 1,5-DAAQ have stronger antioxidant activity in the keto configuration.Under the light excitation,1,5-DHAQ can also improve its antioxidant activity by transforming the enol configuration into the keto configuration through a faster ESIPT process.?4?Using the femtosecond transient absorption spectroscopy,DPPH free radical scavenging experiment and DFT method,the ESIPT behavior and antioxidant activity of the inclusion of 1-HAQ in three cyclodextrins??-,?-and?-CD?were studied.The transient absorption spectra results show that the time scales of ESIPT process for the three inclusion compounds increase from 1-HAQ/?-CD to 1-HAQ/?-CD and1-HAQ/?-CD,and the ESIPT rate slows down.In addition,the antioxidant activity of the three inclusion compounds increases from 1-HAQ/?-CD to 1-HAQ/?-CD and1-HAQ/?-CD.The results indicate that along with the increase of the internal cavity size of CD,the ESIPT rate of 1-HAQ gradually slows down,and the antioxidant activity gradually increases,and there is a negative correlation between the ESIPT rate and the antioxidant activity.