Investigations of the Efficient Electrocatalytic Interconversions of Carbon Dioxide and Carbon Monoxide by Nickel-Containing Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 85

injected at -0.3 V, during CO oxidation, the current drops: the reaction is more
rapid for CODH I Ch . Injecting CN instead at the limit of very negative potential, at
which CO 2 reduction is observed, then scanning in a positive direction, CN binds
once the electrode potential becomes more positive than -750 mV, resulting in
complete inhibition of CO 2 reduction (within a narrow potential window). Con-
tinuing the scan to more positive potential, it is clear that CO oxidation is
completely blocked. The activity remains at zero during the remaining cycle,
until the electrode potential passes down through -0.6 V, at which point the CO 2
reduction activity starts to increase strongly. The rapid release of cyanide, when the
electrode potential is taken below -0.6 V, is an observation that is unique to PFE,
because conventional kinetic assays rarely explore such highly reducing conditions
and lack the ability to record the activity simultaneously as the potential is scanned.
The data are interpreted in terms of the differing dominance of the two major
redox states of the C-cluster, C red1 , and C red2 , as the potential is varied. At potentials
above -0.6 V, C red1 dominates and CN is tightly bound, blocking catalysis. At
electrode potentials below -0.6 V, C red2 becomes the dominant catalytic species,
and recovery of activity that occurs during the scan in this very negative potential
region reveals that CN is released. During the experiment, which is carried out at a
very slow scan rate, the cyanide is lost as HCN which is removed in the gas flow;
hence the effect diminishes with time. The preferential binding to C red1 perfectly
complements the EPR data which showed that only the spectrum of this state is
affected by CN [ 37 ]. The kinetics of binding and release of CN are measured
quantitatively by chronoamperometric experiments, in which the time dependence
of the increase or decrease in current is recorded at a constant potential. Some
exemplary experiments are shown in Figure 7 .
The top panels of Figure 7 show the rates at which CN reacts to inhibit CO
oxidation and CO 2 reduction by CODH II Ch . The lower panels show a more
complicated series of injections and potential steps, to compare the rates of
reductive reactivation of CODH I Ch and CODH II Ch upon stepping from +0.14 V
to -0.76 V. Reductive release of CN from CODH II Ch is much faster and occurs
within the step time.
5.2 How Class IV Carbon Monoxide Dehydrogenases
Respond to Cyanate
Inhibition by cyanate (NCO ), an analogue of CO 2 , shows the opposite trend to that
observed for CN : it binds tightly at potentials below -0.5 V but is released at
higher potentials (Figure 8 )[ 18 , 19 ]. The effect is such that CODH functions as a
unidirectional catalyst in the presence of cyanate, and CO oxidation is virtually
unaffected apart from the requirement for a small overpotential before the activity
climbs. The sharp potential dependence shows that NCO reacts selectively with