Electroenzymatic C-C bond formation from CO2

Article abstract of DOI:10.1021/jacs.8b02319

Over the past decade, there has been significant research in electrochemical reduction of CO₂, but it has been difficult to develop catalysts capable of C-C bond formation. Here, we report bioelectrocatalysis of vanadium nitrogenase from Azotobacter vinelandii, where cobaltocenium derivatives transfer electrons to the catalytic VFe protein, independent of ATP-hydrolysis. In this bioelectrochemical system, CO₂ is reduced to ethylene (C₂H₄) and propene (C₃H₆), by a single metalloenzyme.

Full text of DOI:10.1021/jacs.8b02319

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Electroenzymatic C-C bond formation from CO2
Rong Cai, Ross D. Milton, Sofiene Abdellaoui, Terry Park,
Janki Patel, Bassam Alkotaini, and Shelley D. Minteer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02319 • Publication Date (Web): 02 Apr 2018
Downloaded from http://pubs.acs.org on April 2, 2018
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Electroenzymatic C-C bond formation from CO2
Rong Cai, Ross D. Milton,† Sofiene Abdellaoui, Terry Park, Janki Patel, Bassam Alkotaini and Shelley
D. Minteer*
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Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, Utah 84112, United States
Supporting Information Placeholder
ABSTRACT: Over the last decade, there has been significant
research in electrochemical reduction of CO2, but it has been
difficult to develop catalysts capable of CꢀC bond formation.
Here, we report bioelectrocatalysis of vanadium nitrogenase from
Azotobacter vinelandii, where cobaltocenium derivatives transfer
electrons to the catalytic VFe protein, independent of ATPꢀ
hydrolysis. In this bioelectrochemical system, CO2 is reduced to
ethylene (C2H4) and propene (C3H6), by a single metalloenzyme.
Due to everꢀincreasing carbon dioxide (CO2) emissions and
environmental concerns, electrochemical CO2 reduction to
commodity chemicals, or electrofuels, such as formate
(HCOOꢀ) and methane (CH4) is a prominent theme among
electrochemical researchers in attempts to close the anthropoꢀ
genic carbon cycle. The conversion of CO2 to hydrocarbons
has been of interest for many decades;1 however, the forꢀ
mation of hydrocarbons from CO2 involving multiple hydroꢀ
genations, electron transfers and CꢀC coupling remains chalꢀ
lenging.2 Chemical processes are used predominantly in indusꢀ
try for the hydrogenation of CO2 and carbon monoxide (CO),
Figure 1. a) Crystal structure of vanadium nitrogenase and FeVꢀ
co from Azotobacter vinelandii, depicting the transient association
of the VnfH Fe protein with the catalytic VnfDKG VFe protein.
The crystal structure of the VFe protein was adapted from PDB
file 5N6Y and the Fe protein for molybdenum nitrogenase (NifH)
was used to depicted the VnfH Fe protein, whose structure has not
yet been determined (PDB file 4WZA). b) Bioelectrocatalytic
turnover of VFe nitrogenase mediated by derivatives of cobaltoꢀ
cene/cobaltocenium.
such as the Sabatier or FischerꢀTropsch reactions, although
many require high temperatures (453~673K) and high presꢀ
sures (≥ 7 bar).3
While many promising electrocatalysts are currently being
explored for such reactions, the majority of aqueous systems
are plagued by poor selectivity over the hydrogen evolution
reaction (HER), with molecular hydrogen (H2) often being the
4
major product. Perhaps the most promising CO2 reduction
reaction (CO2RR) catalysts are derived from Cu.2, 5 In contrast
to metallic and molecular catalysts, multiple enzymatic sysꢀ
tems have been employed for bioelectrocatalytic CO2RR.4b
Carbon monoxide dehydrogenase (CODH) is a metalloenzyme
that interconverts CO and CO2, where direct bioelectrochemisꢀ
try has been reported.6 Nicotinamide adenine dinucleotideꢀ
dependent (NAD) enzymes have also been utilized for CO2,
where an enzymatic cascade was able to reduce CO2 to methaꢀ
nol (CH3OH).7 Moꢀ and Wꢀdependent formate dehydrogenases
do not employ diffusive NAD cofactors (thereby negating the
need to separate/refresh the cofactor from the subꢀ
strate/products), and as such, their bioelectrochemical properꢀ
ties have also been exploited for the reduction of CO2 to forꢀ
mate (HCOOꢀ).8
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Figure 2. Representative cyclic voltammograms of VFe bioelectrocatalysis of H+ reduction using (a) Cc(CO2H)2, (b) Cc(CO2H) or (c) Cc
to mediate electrons between the electrode and the VFe protein. In all cases, the dashed lines were recorded in the absence of the VFe
protein and the solid lines represent the bioelectrocatalytic response obtained following the addition of VFe protein to the electrolyte.
Measurements were performed at 2 mV/s in a stationary solution of 100 mM MOPS buffer (pH 7.0), under anoxic conditions (O2 < 1
ppm). The working electrode was a glassy carbon electrode with a geometric surface area of 0.071 cm2.
In contrast to the CO2RR, the nitrogen reduction reaction
(NRR) of molecular nitrogen (N2) to ammonia (NH3) has
gained attention in the realm of bioelectrocatalysts, where the
only enzyme known to reduce molecular nitrogen (N2) to NH3
has recently been interfaced with electrodes for applied and
mechanistic evaluation.9 Nitrogenases are essential enzymes
whose primary physiological function is to catalyze the reducꢀ
tion of N2 to NH3 at the expense of ATPꢀhydrolysis, allowing
some microbes to grow diazotrophically.10 All nitrogenases are
a biꢀenzyme cascade that consist of a reducing component
protein (Fe protein) and one of three catalytic proteins, which
are characterized by their molybdenumꢀiron (MoFe)ꢀ, vanadiꢀ
umꢀiron (VFe)ꢀ or ironꢀonly (FeFe)ꢀdependent catalytic cofacꢀ
tors.11 MoFe nitrogenase is commonly recognized as the “conꢀ
ventional” nitrogenase, while VFe nitrogenase (VFe) and Fe
nitrogenase (FeFe) are considered as alternative nitrogenases
that are expressed under Mo starvation.12
VFe nitrogenase was first isolated in 1986 by Hales et al.13
VFe nitrogenase is encoded by the vnf operon, which contains
genes for its own Fe protein (VnfH) alongside the catalytic
VFe protein. In contrast to the MoFe nitrogenase heteroꢀ
tetramer (a dimer of dimers), VFe nitrogenase is a heterohexꢀ
amer (a dimer of trimers) containing an additional subunit
encoded by the vnfG gene (VnfD2K2G2, Figure 1a).12 It is
assumed that VFe nitrogenase follows a similar catalytic
mechanism to that of MoFe nitrogenase, whereby the reduced
Fe protein binds to the catalytic VFe protein and a singleꢀ
electron transfer (ET) from the Fe protein is coupled to the
hydrolysis of ATP, before dissociation of the transient VFeꢀFe
protein complex.14 Despite these assumed similarities, the
recentlyꢀdetermined Xꢀray crystal structure of VFe nitrogenase
also revealed the presence of an unusual bridging ligand at the
FeVꢀcofactor (currently thought to be a µꢀ1,3ꢀcarbonate, Figꢀ
ure 1a).15 Excitingly, the wildꢀtype VFe nitrogenase has also
been demonstrated to be able to undergo FischerꢀTropsch
chemistry at ambient pressure and temperature, producing up
to C3 products from CO in vivo (in Azotobacter vinelandii) and
in vitro.14, 16
MoFe nitrogenase bioelectrocatalysis. Finally, we demonstrate
that this bioelectrocatalytic VFe system can reduce CO2 to
ethylene (C2H4) and propene (C3H6), bypassing the requireꢀ
ment of CO as the substrate and forming CꢀC bonds. Imꢀ
portantly, significant quantities of C>1 products (CꢀC bond
formation) have not been observed for wildꢀtype MoFe and
FeFe nitrogenases.9e We highly anticipate that this bioelectroꢀ
catalytic approach to contacting VFe nitrogenase will be esꢀ
sential in revealing further mechanistic details that may lead to
improve biological and molecular electrocatalysts for CO2RRs
and CꢀC bond formation.
We previously reported that the cobaltocene/cobaltocenium
(Cc+ PF6ꢀ) redox couple (E1/2 = ꢀ0.958 vs. SHE) was able to
support mediated electron transfer (MET) to MoFe nitrogenꢀ
ase in the absence of the Fe protein/ATPꢀhydrolysis, where the
bioelectroreduction of 2H+ to H2, NO2 to NH3 and N3 to NH3
was observed. We evaluated whether Cc could act as an elecꢀ
tron mediator to VFe nitrogenase in addition to 2 other Cc
derivatives, 1,1'ꢀdicarboxyꢀcobaltocenium (Cc(CO2H)2), 1ꢀ
carboxyꢀcobaltocenium (Cc(CO2H)) (Figure 1b). The cobaltoꢀ
cene derivatives have moreꢀpositive reduction potentials than
the original cobaltocene mediator. We hypothesized that these
cobaltocene derivatives could still mediate electron transfer
between the VFe protein and an electrode for carbon dioxide
reduction products (formal potentials for CO2/CH4, 2CO2/C2H4,
and 3CO2/C3H6 are calculated as ꢀ0.24, ꢀ0.34, and ꢀ0.31 V vs.
SHE at pH = 7), while minimizing the potential difference
(overpotential) required for bioelectrocatalysis. Additionally,
the P cluster is estimated to be located ~12 Å within the VFe
protein (from the surface), whereas the FeVꢀco is estimated to
be located at ~15 Å. This makes the possibility of contacting
VFe by direct electron transfer unlikely where physiologicalꢀ
lyꢀrelevant rates of biological electron transfer commonly
occur across distances of less than 14 Å.17
ꢀ
ꢀ
Initially, we evaluated the possibility of mediating electron
transfer between an electrode and the VFe protein by these Cc
derivatives under anaerobic, stationary conditions (Figure 2).
Since wildꢀtype nitrogenases revert to 100 % H+ reduction in
the absence of other substrates, proton reduction was initially
explored and the absence of VFe served as the control experiꢀ
ments. In the case of all three Cc derivatives, an apparent
reductive catalytic response was observed following the addiꢀ
tion of VFe (52 µg) to the electrolyte. Interestingly, these
initial results indicated that the apparent onset potential for
Here, we show that a mediated electrochemical approach can
be used to interface VFe nitrogenase with an electrode, whereꢀ
by bioelectrocatalytic turnover can be realized in the absence
of the VnfH Fe protein and ATPꢀhydrolysis. Initially, we
report the use of 2 additional cobaltocene electron mediators
to the previously employed unsubstituted cobaltocene used for
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bioelectrocatalysis can be shifted more positively by ~200 mV
H2, CO2 was included as a substrate to evaluate the product
distribution of VFe nitrogenase in a bioelectrochemical setting.
Directly after the addition of VFe, a significant increase in
reductive current was observed, reflecting the bioelectrocataꢀ
lytic reduction of H+ in the absence of CO2. Importantly, no
reductive signal was observed following the addition of denaꢀ
tured VFe under these experimental conditions. A further
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when Cc(CO2H)2 is used in the place of the Cc electron mediaꢀ
tor first explored with the MoFe protein, with a reductive
catalytic current appearing at an approximate onset of ꢀ0.65 V
vs. SHE. Offering a promising onset potential of bioelectrocaꢀ
talysis, the Cc(CO2H)2 derivative was selected for subsequent
analysis.
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Figure 3. Amperometric iꢀt trace for Cc(CO2H)2ꢀmediated bioeꢀ
lectrocatalytic reduction of CO2 by VFe nitrogenase. The experiꢀ
ments were performed in a stirred 2 mL solution of 100 µM
Cc(CO2H)2 in MOPS buffer (pH 7.0, 100 mM) at an applied
potential of ꢀ0.86 V vs. SHE, using a customꢀmade Hꢀcell with a
headspace of 7 mL. Initially (green trace), denatured VFe protein
(1.3 mg) was added at 200 s, followed by active VFe protein (1.3
mg) at 500 s. Finally, 1 mL of CO2 was added at 1000 s. The
experiment (black trace) was performed with only denatured VFe
protein (1.3 mg) added at 500 s followed by 1 mL CO2 at 1000 s.
Figure 4. Product distribution of the bioelectrocatalytic CO2RR
by VFe nitrogenase, demonstrating the production of CH4 (blue),
C2H4(green) and C3H6 (grey). Control experiments were perꢀ
formed with VFe that had been denatured by exposure to air for
30 minutes. Error bars represent one standard deviation (n = 3).
increase (~150 %) in the reductive signal was observed folꢀ
lowing the addition of 1 mL CO2 in the presence of the native
VFe, indicating the bioelectrocatalytic reduction of CO2. Once
again, no bioelectrocatalytic signal was observed when denaꢀ
tured VFe was added in the place of VFe.
Additional control experiments were performed by the seꢀ
quential addition of an equivalent amount of catalyticallyꢀinert
bovine serum albumin (BSA) in 2 mM sodium dithionite (to
account for the buffer and reducing agent in which VFe was
purified) or denatured VFe (where VFe was exposed to air for
30 min) before adding active VFe (Figure S1). Interestingly,
increasingly reductive currents were observed following the
sequential additions of BSA (in 2 mM DT) and denatured VFe,
although the reductive response increased further following
the addition of active VFe. As a catalyticallyꢀinert protein,
BSA does not contribute towards any reductive current. Our
original bioelectrochemical study of MoFe nitrogenase highꢀ
lighted that an apoꢀMoFe protein variant (deficient in FeMoꢀ
co and containing only the P cluster) yielded a diminished
reductive current implying that the P cluster has minimal
bioelectrocatalytic activity, thus, we hypothesize that the reꢀ
ductive current observed in the denatured VFe control is due
to persistent bioelectrocatalytic activity of the P cluster for H+
reduction (Figure S2).9a Further, premature FeMoꢀco assembly
proteins are reported to be able to undergo the acetylene reꢀ
duction reaction (which is synonymous to the NRR and is used
to prove nitrogenase activity) and the observed electrochemiꢀ
cal activity may be due to partiallyꢀdisassembled P clusters or
FeVꢀco.18
Finally, bulk bioelectrosynthesis and determination of the
product distribution of the CO2RR was performed to confirm
the bioelectrocatalytic activity of VFe, by gas chromatography
(Figure 4). Bulk bioelectrosynthesis experiments were carried
out in a customꢀmade sealed Hꢀcell. Control experiments were
performed with denatured VFe and blank experiments were
performed with active VFe protein in the absence of CO2. The
products were detected and quantified after the passage of 4 C
of charge (ca. 20 h) at ꢀ0.86 V (vs. SHE). Triplicate experiꢀ
ments revealed the production of 25 ± 5 nmol C2H4 and 42 ± 7
nmol C3H6 per µmol VFe (corrected to blank experiments and
statically significant at the 95% confidence level), demonstratꢀ
ing that the bioelectrocatalytic reduction of CO2 by the VFe
nitrogenase protein results in the reductive formation of CꢀC
bonds, presenting an alternative approach to the electrosyntheꢀ
sis of C≥2 products. Interestingly, no carbon monoxide (CO)
was detected in this system, which may indicate that the initial
reduction of CO2 may be rateꢀlimiting where C≥2 hydrocarbon
formation from CO2 is not favorable and presumably takes
place via the CO produced from CO2 reduction.19
In conclusion, we report that the VFe protein of vanadium
nitrogenase can be electronically contacted by the use of Cc
derivatives as electron mediators, where VFe nitrogenase
electrochemistry has not yet been reported. While the producꢀ
tion of C≥2 hydrocarbons by VFe nitrogenase has been demonꢀ
strated, we report that an electrochemical approach can be
used to regenerate reducing equivalents in the place of the
To further validate the bioelectrocatalytic activity of VFe
nitrogenase, steadyꢀstate amperometric analyses were conꢀ
ducted with Cc(CO2H)2 at an applied potential of ꢀ0.86 V vs.
SHE (Figure 3). In addition to the expected reduction of H+ to
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natural VFe partner, VnfH (Fe protein), presenting a novel
bioelectrochemical approach for the formation of CꢀC bonds.
We anticipate that this bioelectrochemical approach will be
utilized to reveal further mechanistic details surrounding the
formation of C≥2 hydrocarbons from CO2 by VFe, including
the determination of the full substrate/product distribution for
VFe bioelectrocatalysis.
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ASSOCIATED CONTENT
Supporting Information
9
9. (a) Milton, R. D.; Abdellaoui, S.; Khadka, N.; Dean, D. R.;
Materials, A. vinelandii growth, protein purification, activity
assays, electrochemical experimentation, gas chromatography
experimental, control experiments.The Supporting Information is
available free of charge on the ACS Publications website.
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Leech, D.; Seefeldt, L. C.; Minteer, S. D., Energy
&
Environmental Science 2016, 9 (8), 2550ꢀ2554; (b) Milton, R.
D.; Cai, R.; Abdellaoui, S.; Leech, D.; De Lacey, A. L.; Pita,
M.; Minteer, S. D., Angewandte Chemie (International ed. in
English) 2017, 56 (10), 2680ꢀ2683; (c) Khadka, N.; Milton, R.
D.; Shaw, S.; Lukoyanov, D.; Dean, D. R.; Minteer, S. D.;
Raugei, S.; Hoffman, B. M.; Seefeldt, L. C., Journal of the
American Chemical Society 2017, 139 (38), 13518ꢀ13524; (d)
Milton, R. D.; Cai, R.; Sahin, S.; Abdellaoui, S.; Alkotaini, B.;
Leech, D.; Minteer, S. D., Journal of the American Chemical
Society 2017, 139 (26), 9044ꢀ9052; (e) Hu, B.; Harris, D. F.;
Dean, D. R.; Liu, T. L.; Yang, Z.ꢀY.; Seefeldt, L. C.,
Bioelectrochemistry 2018, 120, 104ꢀ109.
AUTHOR INFORMATION
Corresponding Author
*minteer@chem.utah.edu
Present Addresses
†RDM current address: Department of Civil & Environmental
Engineering, Stanford University, James H Clark Center E250,
318 Campus Drive, Stanford, CA 94305, USA.
10.
(a) Burgess, B. K.; Lowe, D. J., Chemical reviews 1996,
96 (7), 2983ꢀ3012; (b) Dance, I., Chemical Communications
2013, 49 (93), 10893ꢀ10907.
Notes
There are no conflicts to declare. Electrochemical experiments
were performed against a saturated calomel electrode (SCE) and
11.
(a) Eady, R. R., Chemical reviews 1996, 96 (7), 3013ꢀ
3030; (b) Eady, R. R., Coordination Chemistry Reviews 2003,
237 (1), 23ꢀ30; (c) Hales, B. J.; Langosch, D. J.; Case, E. E.,
Journal of Biological Chemistry 1986, 261 (32), 15301ꢀ15306;
(d) Müller, A.; Schneider, K.; Knüttel, K.; Hagen, W. R., FEBS
letters 1992, 303 (1), 36ꢀ40.
reported against the standard hydrogen electrode (SHE), corrected
20
by the following relationship: E(SCE) = E(SHE) ꢀ 0.242 V
.
ACKNOWLEDGMENT
The authors would like to thank Dr. Ian McAlexander (Deꢀ
partment of Chemistry, University of Utah) for assistance
with gas chromatography experiments and the Department of
Energy SBIR with Fulcrum Bioscience for funding.
12.
Sippel, D.; Schlesier, J.; Rohde, M.; Trncik, C.;
Decamps, L.; Djurdjevic, I.; Spatzal, T.; Andrade, S. L.; Einsle,
O., JBIC Journal of Biological Inorganic Chemistry 2017, 22
(1), 161ꢀ168.
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Hales, B. J.; Case, E. E.; Morningstar, J. E.; Dzeda, M.
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