Study On The Redox Mechanism Of Vitamin K By In-Situ FT-IR Spectroelectrochemistry

Vitamin K,also known as clotting vitamin,is an essential substance for the synthesis of coagulation proteins in the liver and has important physiological functions.In fact,vitamin K is a derivative of 2-methyl-1,4-naphthoquinone.Both 2-methylnaphthoquinone(MQ)and VK1 having the function of promoting blood coagulation,can be used to treat hemorrhagic diseases caused by vitamin K deficiency.Both of them are important quinone compounds needed by human body and play an important physiological role.At the same time,they are also important electrochemical active substances,and vitamin K can be dissolved in the lipids of mitochondrial membrane,which plays the role of electron transfer.Therefore,the electrochemical reduction mechanism of MQ and VK1 in different solvents was investigated by cyclic voltammetry,rapid scanning infrared spectroscopy,cyclic voltabsorptometry and derivative cyclic voltabsorptometry at the molecular structure level.The main experimental work is summarized as follows:1.Temperature is one of the important factors affecting electrochemical reaction.In order to carry out electrochemical experiments and study the adsorption phenomenon on the electrode surface at higher temperature,a high temperature thin-layer cell(HTC)is constructed for in-situ IR spectroelectrochemistry.It works very well in the temperature range from room temperature to 373K(depend on boiling point of solvent)with temperature control precision within ±0.5K to apply for room-temp and high-temp in-situ IR spectroelectrochemistry.The cell is facile cleaning and convenient manipulation and can be utilized in aqueous solution and non-aqueous system.The electrochemical and IR performance were characterized with K3[Fe(CN)6]in aqueous and with benzoquinone(BQ)in ionic liquid BMIMPF6 by cyclic voltammetry and in-situ IR spectroelectrochemistry at various temperature.Experimental results suggest that the cell have excellent electrochemical performance within wider range of temperature,and good signal-noise ratio for IR absorption peaks during CV scan.In conclusion,HTC have promising application in electrochemical process that temperature plays an important role.2.Electrochemical mechanism of 2-methylnaphthoquinone in different solvents(1)The electrochemical characteristics of MQ was investigated in dimethyl sulfoxide(DMSO)and acetonitrile by cyclic voltammetry(CV)and in-situ FTIR spectroelectrochemistry.The electrochemical performance of MQ were distinct in different solvent when the scan range is from OV to-1.8V.Combining with cyclic-voltabsorptometry(CVA),derivative cyclic voltabsorptometry(DCVA)and high-temperature CV,results indicated MQ was reduced to anionic radical MQ·-firstly in DMSO,and then converted to dianion MQ2-,which was two-step one-electron transfering process.Nevertheless,MQ in acetonitrile dimerization have taken place accompanied by electrochemical reduction,in which anionic radical MQ·-and dianion MQ2-formed the dimer and further ruduced at more negative potential.(2)In DMSO-ethanol,DMSO-acetonitrile and acetonitrile-ethanol mixed solventFirst,in the mixture of acetonitrile-ethanol,The CV curve of MQ has changed significantly.When ethanol content increases gradually,the three pairs of redox peak potentials are all moving in a positive direction.Compared with the first pair of peaks,the other two pairs of redox peak potentials have a large positive shift amplitude.The electrochemical mechanism of MQ in acetonitrile-ethanol mixed solvent was studied by electrochemical technique and in-situ FTIR spectroelectrochemistry.The experimental results show that the electrochemical mechanism of MQ in acetonitrile-ethanol mixed solvent is EECE mechanism,which is similar to that in acetonitrile solvent.Secondly,in the mixed solvent of DMSO-ethanol and DMSO-acetonitrile,MQ exhibited similar electrochemical behavior in the above two solvents when cyclic voltammetry was carried out in the range of 0-1.8V potential.When the concentration of ethanol or acetonitrile increases to a certain value,the third pair of small peaks appear on the CV behind the original two pairs of peaks,and the three pairs of redox peak potentials shift positively.The results show that the addition of another solvent has a significant effect on the redox peak of the latter two pairs.It is found that the electrochemical reduction mechanism of MQ in the mixed solution is the same as that in acetonitrile solution by analyzing the CVA and DCVA.Finally,by comparing the experimental results of the gradual change of peak potential of MQ in the above five different solvents and the strong infrared absorption peak of 3000cm-1 in the 3D IR spectra,it is inferred that:hydrogen bonds exist in in acetonitrile-ethanol,DMSO-ethanol,and DMSO-ethanol.3.Electrochemical Mechanism of Vitamin K1 in different solvents(1)In dimethyl sulfoxide and acetonitrile systemIn two aprotic proton solvents,VK1 shows two pairs of reversible redox peaks in DMSO,but VK1 exhibits three oxidation peaks and three reduction peaks in acetonitrile solvent,and the peak potential and peak shape are completely different from the electrochemical behavior in DMSO.The infrared spectra were further analyzed by using cyclic voltabsorptometry and derivative cyclic voltabsorptometry.It was found that VK1 was reduced to anion radical VK1·-in the first step and to divalent anion VK12-in the second step in DMSO.This process is a continuous two-step single-electron transfer process.In acetonitrile solvent,dimerization occurs at the same time when VK1 is reduced:the anion radical VK1·-and divalent anion VK12-were dimerized,and the dimer was reduced in the subsequent electrochemical process.However,the dimer is very stable and does not oxidize at the corresponding position of the reduction peak,but only at the positive potential.(2)In mixture of acetonitrile-ethanol,dimethyl sulfoxide-ethanolThe electrochemical redox of VK1 has also three oxidation peaks and three reduction peaks in acetonitrile-ethanol mixed solvent.The information given by IR 3D is similar to that of acetonitrile solvent.The results show that the electrochemical reduction mechanism of VK1 in acetonitrile-ethanol is the same as that in acetonitrile.However,due to the influence of proton donor,all the electrochemical peaks move in a positive direction because of the hydrogen bond,and the second pair of peaks deviate most obviously.In addition,When the concentration of ethanol increases to a certain extent,hydrogen bonds and related phenomena also exist in DMSO-ethanol solution.Finally,all the experimental results show that VK1 has hydrogen bonding in the above mixed solvents.