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Conduction Band of Hematite Can Mediate Cytochrome Reduction by Fe(II) under Dark and Anoxic Conditions
journal contribution
posted on 2020-03-04, 19:41 authored by Tongxu Liu, Ying Wang, Chongxuan Liu, Xiaomin Li, Kuan Cheng, Yundang Wu, Liping Fang, Fangbai Li, Chengshuai LiuWhile
it was recently reported that the conduction band of iron
minerals can mediate electron transfer between Fe(II) and different
Fe(III) lattice sites during Fe(II)-catalyzed mineral transformation,
it is unclear whether such a conduction band mediation pathway occurs
in the microbial Fe(II) oxidation system under dark and anoxic subsurface
conditions. Here, using cytochrome c (c-Cyts) as a model protein of microbial Fe(II) oxidation, the in vitro kinetics and thermodynamics of c-Cyts reduction by Fe(II) were studied. The results showed that the
rates of c-Cyts reduction were greatly enhanced in
the presence of the semiconducting mineral hematite (Hem, α-Fe2O3). The electrochemical experiments separating
Fe(II) and c-Cyts demonstrated that electrons from
Fe(II) to the electrode or from the electrode to c-Cyts could directly penetrate hematite, resulting in enhanced current.
Independent photochemical and photoluminescence experiments indicated
that c-Cyts could be directly reduced by the conduction
band electrons of hematite which were generated under light illumination.
In c-Cyts+Fe(II)+Hem, the redox potential of Fe(II)-Hem
was shifted from −0.15 to −0.18 V and that of c-Cyts+Hem changed slightly from −0.05 to −0.04
V. For the bulk hematite, Mott–Schottky plots illustrated that
the flat band was shifted negatively and positively in the presence
of Fe(II) and oxidized c-Cyts, respectively, and
the surface electron/charge density was higher in the presence of
Fe(II)/c-Cyts. As a consequence, the redox gradients
from adsorbed Fe(II) to adsorbed c-Cyts allow electron
transfer across the conduction band of hematite and facilitate c-Cyts reduction. This mechanistic study on conduction band-mediating
electron transfer could help interpret the role of semiconducting
minerals in the microbial Fe(II) oxidation process under dark anoxic
conditions.