Studies On The Mechanisms Of Protoncoupled Electron Transfer Associated With Biofuel Cell Electrode Reactions

Enzyme biofuel cells(EBFC)are environmentally friendly fuel cell,which use enzymes as catalysts to convert chemical energy into electric energy,and have a wide range of fuel sources.In recent years,the catalytic properties of the activity center(flavin)of glucose oxidases and the diversity of fuels of EBFC have attracted the interest of many scientists.Therefore,the detailed studies on the dehydrogenation reactions between flavin and organic molecules may promote the development of EBFC.In addition,the cathode region of EBFC will produce active hydrogens.The study on the transfer mechanism of active hydrogen in the protein is helpful to understand excess electron transfer mechanisms in the cathode region of EBFC.The detailed tasks are summarized in the following:(1)In order to explore the mechanisms of catalytic dehydrogenation reactions between the activity center(flavin)of glucose oxidase and small organic molecules,we investigate the dehydrogenation of 7,8,10-trimethylisollxazine(instead of flavin model,abbreviated as Fl)with alcohols,aldehydes and carboxylic acids at the M062X/6-31+g(d,p)level of theory.Two possible reaction pathways are revealed.One is a two-step reaction in which,a hydride(H-)is transferred from the small organic molecule to Fl via a single-proton-coupled double-electron transfer mechanism with a proton moving to the N5-atom of Fl and at the same time the small organic molecule binding to 4?C of Fl.Then,the small organic molecule releases a proton to the N1 of the Fl and the O-4?C bond is broken.The other is a concerted reaction in which,two hydrogen atoms of small organic fraction are transferred to Fl via a double-proton-coupled double-electron transfer mechanism with N5 and 4aC as the proton acceptors.After that,the proton of 4aC moves to the N1 position through a First-Third proton transfer mechanism.According to the comparison of the activation energies,we reveal that flavin-alcohol systems are inclined to occur through the the concerted mechanism and the reactions of flavin with aldehydes and carboxylic acids are prone to take place through the two-step mechanism.Furthermore,we also consider the effects of the protein environment and carbon nanotubes(sCNT)on the reactions between FAD and glucose/His505,which still have a high energy barrier.This may be attributed to the fact that the energy of highest occupied molecular orbitals(HOMOs)of the small organic molecules and glucose are too low and can not identical with the lowest unoccupied molecular orbital(LUMO)of flavin and the proton transfer not only drive electron transfer from the organic molecules to flavin but also increase the energy of HOMO of the organic molecules,which requires consume more energy.These results may be of guiding significance for the development of EBFC.(2)In this work,we explore regulation of the dehydrogenation of flavin and organic small molecules by graphene(GS)as an anode material in EBFC.The structures of the reactants,transition states and products involved in GS are optimized for the two-step and concerted reactions by the ONIOM method,and the corresponding molecular orbitals and energy barriers are analyzed.The effect of GS on the catalytic dehydrogenation reactions of flavin and small organic molecules is insignificant.However,GS can modulate the distribution of the highest occupied molecular orbitals(HOMOs)and change the energy barriers.GS reduces the energy barriers for the two-step reactions of flavin with ethanol,butyric acid and the concerted reaction of flavin with propanal.But GS increase the energy barriers of two-step reaction of flavin with propionaldehyde and the concerted reactions of flavin with alcohols,butyraldehyde and propionic acid.The different effects of GS on flavin with small organic molecule provide theoretical guidance for EBFC in which GS is used as a battery anode material.(3)The third part mainly investigate the influence of the protein microenvironment,depending on the different amino acids,on active hydrogen transfer from ammonium group of lysine to oxygen-atom(O-atom)of peptide backbone.A series of KH-XY peptide model(KH is a protonated lysine which captures an excess electron;X,Y represents two other amino acids)are optimized at the B3LYP/6-31+G(d,p)level.We reveal that hydrogen migration in the KH-XY model can occur via two different mechanisms according to the different micro-surroundings: one is that a proton and an electron are transferred from the ammonium group of the lysine to the O-atom of the adjacent peptide backbone in the same direction via a proton-coupled Rydberg-state electron transfer mechanism,the other is that a proton and an electron are transferred from the different donor,with proton transfer from the ammonium group of the lysine to the adjacent backbone amide oxygen,electron migration from the protonated imidazole ring of histidine to the adjacent peptide backbone.In addition,the factors,including the properties of amino acids X,Y,and the distances between hydrogen atom of ammonium group in lysine and hydrogen atom of Y amino acid side chain,regulate the active hydrogen transfer from side chain of lysine to peptide backbone.These results provide insight into the excess electron transfer in cathode of EBFC.