Molecular mechanism of reductive dehalogenation by P450 enzymes: possible importance of dissociative electron attachment for biosensor applications.

The molecular mechanism of enzymatic processes induced by cytochrome P450 is of importance for biosensor applicationsi as well as for construction of biological interfaces via immobilization of enzymes in the condensed environment.ii These interfaces are known to be able to model the drug metabolism under electrochemical conditions.iii The electron exchange plays a key role in the P450 catalytic cycleiv responsible for reductive dehalogenation of environmental pollutants, including herbicides both in vivo and in electrochemical experiments.v It is worth mentioning that enzymatic active centers are hydrophobic in nature, thus containing a very few water molecules.vi Additionally, the substrate molecule is bound in the active site by weak non-covalent bonds, thus keeping its native electronic structure. Therefore, resonance dissociative electron attachment (DEA) to isolated molecules in vacuo can serve as a model of enzymatic processes under reductive conditions as shown in case of some widely used non-steroidal anti-inflammatory drugs.vii The present work reports formation of transient negative ions and their decay for the halogenated herbicides atrazine and bromoxynil studied by means of electron transmission spectroscopy (ETS) and DEA spectroscopy. Dehalogenation of atrazine and bromoxynil is found to be the dominant decay of the temporary molecular negative ions formed at very low (thermal) energies of the incoming electrons. It is concluded that formation of negative ions by electron donation in enzymatic active centers followed by their dissociation along the sigma bond can be considered as a unifying mechanism of the initial step of reductive dehalogenation catalyzed by P450 enzymes. The present findings attract attention on the role of the resonance electron attachment mechanism in electron-driven enzymatic processes. The work was supported by RFBR grants (17-03-00196 and 15-29-05786) and by the Italian Ministero dell\u2019Istruzione, dell\u2019Universit\ue0 e della Ricerca. i Bistolas, N., Wollenberger, U., Jung, C., Scheller, F. W. Biosens. Bioelectron. 2005, 20(12), 2408-2423. ii Manoli, K., Magliulo, M., Mulla, M.Y., Singh, M., Sabbatini, L., Palazzo, G., Torsi, L. Angew. Chem. Int. Ed. 2015, 54(43), 12562-12576. iii Iwuoha, E.I., Joseph, S., Zhang, Z., Smyth, M.R., Fuhr, U., de Montellano, P.R.O. J. Pharm. Biomed. Anal. 1998, 17(6), 1101-1110. iv Shumyantseva, V.V., Bulko, T.V., Archakov, A.I. J. Inorg. Biochem. 2005, 99(5), 1051-1063. v Rotko, G., Roma\u144czyk, P.P., Andryianau, G., Kurek, S.S. Electrochem. Commun. 2014, 43, 117-120. vi Poully, J.C., Nieuwjaer, N., Schermann, J.P. Phys. Scrip. 2008, 78(5), 058123. vii Pshenichnyuk, S.A., Modelli, A. J. Chem. Phys. 2012, 136(23), 234307

Electron Acceptor Properties of DDT and Molecular Mechanism of Its Toxicity

This Chapter reports the empty-orbital electronic structure and experimental data of dissociative electron attachment (DEA) to the gas-phase molecules DDT (1,1'-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene)) and its principal metabolite DDE (1,1-bis-(4-chlorophenyl)-2,2-dichloroethene), which possesses good electron-withdrawing abilities. Electron transmission spectroscopy (ETS) is employed to study formation of temporary negative ions (TNIs) by electron addition to vacant molecular orbitals of neutral molecules. Fragments formed by dissociation of TNIs, in kinetic competition with extra electron detachment, were detected by means of DEA spectroscopy. The experimental findings are supported by density functional theory (DFT) calculations. According to ideas put forward by James Lovelock in the early sixties of the XX century, these findings play a vital role to describe toxic effects produced in vivo by free radical formation followed by deactivation of enzymes by chemicals with high electron affinity, as in the case of the model toxicant carbon tetrachloride (CCl4). The present DEA results indicate that, in close analogy with CCl4, a chlorine negative ion is effectively eliminated from DDT \u201cactivated\u201d by capture of an extra electron. Provided that a similar process can take place in the cellular ambient under conditions of excess negative charge, this would gives rise to formation of DDE and the second most abundant metabolite DDD (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl) ethyl]benzene) through generation of intermediate dechlorinated radicals. Two possible mechanisms for attachment of an extra electron to DDT in cells are discussed. In the first one, where e\u2013 derives from the mitochondrial electron transport chain, DDD formation is associated with hydrogen atom abstraction from neighboring lipids by [DDT \u2013 Cl]\u2022 neutral radicals, eventually initiating chain reactions of peroxidation of biomembranes. In the second one, e\u2013 can originate from the catalytic cycle of cytochrome P450, where dechlorinated radicals can be tightly bound to an active center causing \u201csuicide deactivation\u201d of P450 enzymes