The biological reduction of N2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E0. Illumination for sequential 1-h periods at 233 K under 1 atm of N2 led to a cumulative attenuation of E0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E2) and four-electron (E4) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E6 and E8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe–Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

The biological reduction of dinitrogen (N2) to ammonia (NH3) and protons to dihydrogen (H2) requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase.1–3 Electron accumulation on the MoFe protein active site FeMo-cofactor leads to the formation of discrete En (n = 0, 1, 2, etc., electrons) states (Fig. 1), which are the intermediates of the N2 reduction mechanism.4,5 Important insights about the properties of E-states and the mechanism of nitrogenase N2 reduction have come from interrogating Fe protein–MoFe protein complexes (both wild-type and variant proteins with amino acid substitutions) trapped during steady-state turnover.5–14 The resulting compositions of paramagnetic, even numbered E-states have been analyzed by electron paramagnetic resonance (EPR) spectroscopy, leading to a mechanistic model of the N2 reduction reaction (Fig. 1).5,15

FIG. 1.

Top: scheme of the light-driven process of electron delivery in CdS:MoFe protein complexes. Bottom: simplified Lowe–Thorneley kinetic scheme of N2 reduction by nitrogenase.16 Sequential delivery of electrons to the MoFe protein results in the reduction of the catalytic site FeMo-cofactor and the formation of intermediate En-states in the catalytic reduction of N2 to NH3 and H2.

FIG. 1.

Top: scheme of the light-driven process of electron delivery in CdS:MoFe protein complexes. Bottom: simplified Lowe–Thorneley kinetic scheme of N2 reduction by nitrogenase.16 Sequential delivery of electrons to the MoFe protein results in the reduction of the catalytic site FeMo-cofactor and the formation of intermediate En-states in the catalytic reduction of N2 to NH3 and H2.

Close modal

Electron delivery to the MoFe protein and catalysis of the N2 reduction reaction can also be achieved with CdS nanorods and quantum dots (referred to as CdS QD, Fig. 1) replacing the Fe protein.17,18 In this system, photoexcitation rates of QDs can be used to control electron flux into the MoFe protein. Herein, controlled illumination of QD:MoFe protein complexes in the frozen state was used to populate E-states in the pre-steady state. Illumination-dependent reduction of the MoFe protein was tracked by monitoring the reduction of the resting state, E0, to EPR active, even numbered E-states, which were quantified and mapped onto the Lowe–Thorneley kinetic scheme.

Photochemical reduction of the MoFe protein was tested with two QD preparations that differed in diameter. The ability of QDs to photochemically activate the MoFe protein was tested in reactions prepared under 1 atm of N2 and illuminated at T = 298 K to measure NH3 production (see Figs. S1 and S2 and Table S1 of the supplementary material).19 Under these conditions, the complexes catalyzed NH3 production with a turnover frequency (TOF) of 2.3–3.7 ± 1.0 mol NH3 (mol MoFe protein)−1 min−1, within the range previously measured for other CdS:MoFe protein complexes.17,18 The variation in TOF exemplifies the sensitivity to properties of CdS nanocrystals, the illumination intensity of the reactions, and the quantum efficiency of electron transfer (QEET).17,18,20–22

We have demonstrated that MoFe protein reaction intermediates can be trapped by illuminating CdS:MoFe protein complexes in the frozen state, T ∼ 230–260 K, which allows electron delivery from CdS to the MoFe protein while minimizing turnover and formation of reaction products (i.e., NH3 and H2).19,23,24 Here, CdS:MoFe protein complexes were tested under a series of reaction conditions to identify parameters that led to both attenuation of the resting state, E0, and formation of higher E-states (>E2) (Fig. 2). Two different concentrations of NaCl were used to create a high (200 mM) or low (5 mM) ionic strength based on binding studies of CdS QDs and the MoFe protein by microscale thermophoresis favoring low salt conditions.25 Reactions were prepared in an EPR tube under a pN2 of either 1 or 3 atm to test the pN2 effect on E-states and equilibrated, in the dark, to a reaction temperature of 233 or 298 K in the integrating sphere (see the supplementary material for details). A comparison of the EPR spectra of the reactions before and after illumination (Fig. 2) showed that the greatest E0 attenuation and highest level of E4(2N2H) were observed under 2 W of 405 nm light at 233 K at a NaCl concentration of 5 mM [Figs. 2(a) and 2(b)].

FIG. 2.

Effect of CdS preparation, buffer, and illumination temperature and time on photoreduction of QD:MoFe protein complexes. EPR spectra of the S = 1/2, g = 2.0 region are shown identifying signals of the FeMo-co E0 and E4(2N2H) states and oxidized P-cluster (P+) intermediate. Reaction conditions: (a) 50 mM HEPES, pH 7.5, 5 mM NaCl, 5 mM DT, 5 mM MPA, QD1 (sample analyzed in more detail below); 1 atm pN2; 2 h of illumination at 233 K. (b) 50 mM HEPES, pH 7.5, 5 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 2 h of illumination at 233 K. (c) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 3 atm p15N2; 2 h of illumination at 233 K. (d) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 30 min illumination at 293 K. (e) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 4 W illumination, 1 h at 233 K. (f) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 3 atm pN2; 4 W illumination, at 233 K for 1 h (dotted trace) or 2 h (solid trace). Spectra were collected at P = 1 mW and T = 12 K prior to illumination (gray traces) and post illumination (black traces). All signal intensities were normalized to the E0 signal intensity at g = 2.01 prior to illumination.

FIG. 2.

Effect of CdS preparation, buffer, and illumination temperature and time on photoreduction of QD:MoFe protein complexes. EPR spectra of the S = 1/2, g = 2.0 region are shown identifying signals of the FeMo-co E0 and E4(2N2H) states and oxidized P-cluster (P+) intermediate. Reaction conditions: (a) 50 mM HEPES, pH 7.5, 5 mM NaCl, 5 mM DT, 5 mM MPA, QD1 (sample analyzed in more detail below); 1 atm pN2; 2 h of illumination at 233 K. (b) 50 mM HEPES, pH 7.5, 5 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 2 h of illumination at 233 K. (c) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 3 atm p15N2; 2 h of illumination at 233 K. (d) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 30 min illumination at 293 K. (e) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 1 atm pN2; 4 W illumination, 1 h at 233 K. (f) 50 mM HEPES, pH 7.5, 200 mM NaCl, 5 mM DT, 5 mM MPA, QD2; 3 atm pN2; 4 W illumination, at 233 K for 1 h (dotted trace) or 2 h (solid trace). Spectra were collected at P = 1 mW and T = 12 K prior to illumination (gray traces) and post illumination (black traces). All signal intensities were normalized to the E0 signal intensity at g = 2.01 prior to illumination.

Close modal

The EPR spectra of samples prepared with either QD1 or QD2 [Figs. 2(a) and 2(b)], were collected after sequential 1 h illumination periods (full spectra, Fig. S3) and simulated using EasySpin26 to identify and quantify E-state populations (S = 3/2 region, Fig. 3; S = 1/2 region, Fig. 4). At t = 0 in the dark, the QD:MoFe protein spectrum consisted of a nearly uniform E0 signal at g = 4.33, 3.63, and 2.01,6,23 with small fractions (2.3% and 6.4%) of signals assigned to the E2(2H)1c and E2(2H)1d states, respectively (Table S2). These may have formed from background room light excitation and CdS mediated reduction of the MoFe protein. The result indicates that freezing of the complexes had little to no effect on the integrity of the MoFe protein.

FIG. 3.

EPR spectra of the S = 3/2, g = 4.0 region of QD1:MoFe protein complexes before and after 3 h of illumination. EPR spectra (black) and full simulations (gray). The gx and gy-values for signal components of the resting state E0 (red) and conformers of the two-electron reduced E2 states: E2(2H)1b (blue), E2(2H)1c (light blue), and E2(2H)1d (cyan) (Table S2). EPR spectra collected at P = 1 mW, T = 3.8 K. Reaction conditions, EPR parameters, and spectral simulation details are described in the supplementary material, QD1.

FIG. 3.

EPR spectra of the S = 3/2, g = 4.0 region of QD1:MoFe protein complexes before and after 3 h of illumination. EPR spectra (black) and full simulations (gray). The gx and gy-values for signal components of the resting state E0 (red) and conformers of the two-electron reduced E2 states: E2(2H)1b (blue), E2(2H)1c (light blue), and E2(2H)1d (cyan) (Table S2). EPR spectra collected at P = 1 mW, T = 3.8 K. Reaction conditions, EPR parameters, and spectral simulation details are described in the supplementary material, QD1.

Close modal
FIG. 4.

EPR spectra and simulations of the S = 1/2, g = 2.0 region of QD1:MoFe protein complexes before and after 3 h illumination. EPR spectra (black) and full simulations (gray). Signal components shown for E0 (red), E4(2N2H) (green), and signals assigned to E6 and E8 conformers9 (light and dark purple) (Table S3, full simulation Fig. S4). The table intensities of double integrated peak areas for the E6 and E8 signals are from simulation of 3.6–3.8 and 12 K spectra of QD1:MoFe protein complexes (numbers in parentheses are from QD2:MoFe protein complexes, Fig. S5). Spectra collected at P = 1 mW and T = 3.8 and 12 K. Reaction conditions, EPR parameters, and spectral simulation are described in the supplementary material, QD1.

FIG. 4.

EPR spectra and simulations of the S = 1/2, g = 2.0 region of QD1:MoFe protein complexes before and after 3 h illumination. EPR spectra (black) and full simulations (gray). Signal components shown for E0 (red), E4(2N2H) (green), and signals assigned to E6 and E8 conformers9 (light and dark purple) (Table S3, full simulation Fig. S4). The table intensities of double integrated peak areas for the E6 and E8 signals are from simulation of 3.6–3.8 and 12 K spectra of QD1:MoFe protein complexes (numbers in parentheses are from QD2:MoFe protein complexes, Fig. S5). Spectra collected at P = 1 mW and T = 3.8 and 12 K. Reaction conditions, EPR parameters, and spectral simulation are described in the supplementary material, QD1.

Close modal

After illumination of the QD1:MoFe protein (and QD2:MoFe protein, Fig. S3) complexes for sequential 1 h periods at 233 K, there was a time-dependent decrease in the intensity of the E0 signal and appearance of new S = 3/2 and S = 1/2 signals (Figs. 3 and 4, and S3). Simulations of the EPR spectra were used to resolve E-state signals based on comparison to defined Mo-nitrogenase signals (Figs. 3 and 4 and S4 and Tables S2 and S3).6,8,12,27–29 In addition to E0, there were signals in the S = 3/2 region (Fig. 3) that match to E2(2H)1b, E2(2H)1c, and E2(2H)1d, which are conformers of the two-electron reduced E2 state.12,23 These E2 states have been proposed to differ in the position of a bridging hydride12 and vary in population depending on enrichment conditions. E2(2H)1d has been observed as a major species in CdS:MoFe protein reactions when electron flux was limited by combining low excitation rates in the absence of a hole scavenger.23 In contrast, the E2(2H)1b and E2(2H)1c states are observed in Mo-nitrogenase reactions under high electron flux turnover and have been implicated as intermediates in the N2 reduction reaction.8,11

The four electron reduced state (E4) is a key intermediate in the N2 reduction reaction by nitrogenase. Formation of E4(4H), which coordinates two bridging hydrides, is required for N2 binding and activation that results in formation of the E4(2N2H) state (see Fig. 1). E4(4H) has been trapped in the MoFe protein under turnover reactions with the Fe protein,8,28,29 and it has a distinctive S = 1/2 EPR signal at g = 2.15, 2.00, and 1.96 (Table S3).11,29,30 Weak features identified between g = 2.12 and 2.19 and centered at ∼g = 2.15 are consistent with the presence of E4(4H) for QD1:MoFe protein complexes (Fig. S4, Table S3). These signals showed some variability between samples and were more resolved upon illumination for the sample prepared with QD2 (Fig. S4, Table S3). Reductive elimination of the two hydride ligands from E4(4H) as a H2 molecule is a key step for N2 binding and formation of the E4(2N2H) intermediate (Fig. 1).5,8,29 E4(2N2H) has a distinctive S = 1/2 signal at g = 2.09, 1.99, and 1.97 (Table S3),29 which was evident following illumination of QD1:MoFe protein (and QD2:MoFe protein) complexes (Fig. 4 and S4). In-line with its formation, spectral simulation after 3 h illumination at 233 K showed an EPR signal at g = 2.094, 2.00, and 1.975 that matches the g-values and Topt ∼12 K of the E4(2N2H) signal (Fig. 4, Table S3).

In addition to the E4 state signals in the g = 2.0 region, there were overlapping peaks at g = 2.088–2.11 with distinct temperature dependencies evident from comparing spectra collected at 12 vs 3.6 K (Fig. 4). These spectra were simulated with signals at g = 2.11, 2.00, and 1.975 and g = 2.088, 2.00, and 1.975. For both samples analyzed, the g = 2.088 signal was more clearly dominant at 3.6–3.8 K. For the sample prepared with QD2, the g = 2.11 signal became more intense than the g = 2.088 signal at 12 K (Fig. S5). Slight differences in the behavior of the QD1 spectra relative intensities was observed; however, the g = 2.088 signal was less resolved for this sample compared to that of QD2. Overall, the two signals show similar g-values and similar type temperature-dependencies to a pair of signals at g = 2.11, 2.01, and 1.98 and g = 2.09, 2.01, and 1.98 that were previously observed in the MoFe protein variant H195Q/V70A trapped under turnover with hydrazine (N2H4) or diazene (N2H2).9 In that study, the g = 2.09 signal was dominant at lower temperatures (T = 3.8 K), whereas the g = 2.11 signal was dominant at higher temperatures (T = 15 K). Combined ENDOR and HYSCORE studies that used isotopically labeled 15N-substrates assigned the signals to conformers of a E6 or E8 state formed after reduction and cleavage of the N–N bond to a –NH3 ligand (Table S3).9,31 The similarities between the g = 2.09/2.11 signals in these two systems suggest that illumination of QD:MoFe protein reactions in the frozen state can also trap later E-state intermediates, such as the E6 and E8 states observed in the MoFe-protein under reduction by the Fe protein.

Using the signal assignments summarized above, the populations of S = 3/2 and S = 1/2 E-states in the reduced MoFe protein were determined by simulation and spin-quantification of the 3.8 and 12 K EPR spectra, respectively (Tables S2 and S3). Figure 5 summarizes the change in populations over the illumination period and maps the final populations onto the Lowe–Thorneley kinetic model. Starting from E0, 3 h of illumination led to attenuation of ∼75% spin mol−1 of the resting state, E0, MoFe protein population. A large fraction of the attenuated E0 population (31% at t = 1 h; 41% at t = 2 h; 49% at t = 3 h) can be accounted for by reduction to odd-numbered, EPR-silent E-states, i.e., E1, E3, E5, and E7. The populations of EPR-active E2, E4, E6, and E8 states varied in proportion according to the reduction level. This is evident by the highest fraction being E2(2H)1b + E2(2H)1c that accounted for 21% total spin mol−1 MoFe protein at t = 1 h and 18% at t = 3 h. The combined amount of E2(2H)1c and E2(2H)1b at all timepoints is similar (18%-21%) but the fractional amounts change over time. This likely reflects differences in the backward relaxation kinetics of H2 release12 or kinetics of reduction to higher E-states (Fig. 5, bottom). The trend in the accumulation of E4(2N2H) was nearly linear and increased from 3% to 4.4% of the total spin population. The formation of E2, E4(4H), and E4(2N2H) signifies that performing photochemical reactions at 233 K supports proton and electron transfer, as well as N2 binding and reduction, by the MoFe protein. A low population of E6 and E8 state signals was detected in the 12 K spectrum, increasing from 1.5% to 2.1% spin mol−1 MoFe protein over the 1–3 h period of illumination.

FIG. 5.

Top: plots of the changes in the E-state populations in the QD1:MoFe protein complexes following illumination at 233 K. E0 and E2 populations are obtained from simulations of 3.8 K spectra (Fig. 3, Table S2). E4(4H), E4(2N2H), E6, and E8 are derived from simulations of the 12 K spectra (Fig. 4, Table S3). Spin mol−1 MoFe protein values are normalized to the initial E0 spin population at t = 0. Bottom: model of the Lowe–Thorneley E-state intermediates with an increase (+) or decrease (−) in the fractional population of E-states after t = 3 h of illumination at 233 K (gray font).

FIG. 5.

Top: plots of the changes in the E-state populations in the QD1:MoFe protein complexes following illumination at 233 K. E0 and E2 populations are obtained from simulations of 3.8 K spectra (Fig. 3, Table S2). E4(4H), E4(2N2H), E6, and E8 are derived from simulations of the 12 K spectra (Fig. 4, Table S3). Spin mol−1 MoFe protein values are normalized to the initial E0 spin population at t = 0. Bottom: model of the Lowe–Thorneley E-state intermediates with an increase (+) or decrease (−) in the fractional population of E-states after t = 3 h of illumination at 233 K (gray font).

Close modal

Overall, this work demonstrates that nanocrystal-MoFe protein complexes can be used in combination with illumination in the frozen state to populate higher E-states of the FeMo-cofactor catalytic cycle in a single sample. Combining the approach developed here with inhibitory studies and MoFe protein mutagenesis can enable future studies directed at understanding specific determinants of E-state equilibrium in mechanism. These approaches may help to support greater enrichment of higher E-states (i.e., E6 and E8) that have been challenging to isolate in nitrogenase under N2 reduction conditions and determination of E-state kinetics of nitrogenase, which have proved challenging to address with the natural biochemical system.

The supplementary material section includes materials and methods with descriptions of CdS Quantum Dot (QD) synthesis, MoFe protein preparation, photochemical 15NH3 production assays, nuclear magnetic resonance spectroscopy, nuclear magnetic resonance spectroscopy analysis, electron paramagnetic resonance spectroscopy, and EPR simulations and E-state population analysis. It also contains Fig. S1—UV–Vis absorption spectrum normalized at the first exciton peak of QDs, Fig. S2—1H-NMR spectra of QD2:MoFe protein reactions, Fig. S3—EPR spectra of QD:MoFe protein complexes, Fig. S4—EPR spectra of the S = 1/2, g = 2.0 region of QD:MoFe protein complexes, Fig. S5—EPR spectra collected at 3.6 and 12 K and simulations of the S = 1/2, g = 2.0 region of QD2:MoFe protein complexes, Table S1—fluorometric and 1H-NMR measurements of NH3 production by QD:MoFe protein reactions, Table S2—EPR signals and spin concentrations for S = 3/2 region for Fe protein:MoFe protein and QD:MoFe protein reactions, and Table S3—EPR signals of S = 1/2 region for Fe protein:MoFe protein and QD:MoFe protein reactions.

Funding was provided by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This work was authored, in part, by the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DEAC36-08GO28308. The U.S. Government and the publisher, by accepting the article for publication, acknowledge that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. The authors gratefully acknowledge the NREL NMR facility and Dr. Renee Happs for assistance with NMR spectroscopy measurements of 15NH3 in photochemical reactions.

The authors have no conflicts to disclose.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

L.M.P., G.E.V., B.C., Z.-Y.Y., and D.W.M contributed equally to the work.

Lauren M. Pellows: Data curation (equal); Formal analysis (supporting); Methodology (supporting); Resources (equal); Writing – review & editing (supporting). Gregory E. Vansuch: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (lead); Writing – original draft (lead); Writing – review & editing (equal). Bryant Chica: Conceptualization (equal); Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). Zhi-Yong Yang: Formal analysis (supporting); Methodology (supporting); Resources (equal); Writing – review & editing (supporting). Jesse L. Ruzicka: Data curation (equal); Formal analysis (equal); Methodology (equal); Project administration (supporting); Resources (equal); Supervision (supporting); Writing – original draft (equal); Writing – review & editing (equal). Mark A. Willis: Data curation (equal); Formal analysis (supporting); Investigation (supporting); Methodology (supporting); Resources (equal); Writing – review & editing (supporting). Andrew Clinger: Data curation (supporting). Katherine A. Brown: Conceptualization (equal); Investigation (supporting); Methodology (supporting); Writing – review & editing (supporting). Lance C. Seefeldt: Conceptualization (equal); Funding acquisition (equal); Investigation (supporting); Project administration (equal); Resources (equal); Writing – review & editing (supporting). John W. Peters: Conceptualization (equal); Funding acquisition (equal); Methodology (supporting); Project administration (equal); Resources (supporting); Supervision (equal); Writing – review & editing (equal). Gordana Dukovic: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (supporting). David W. Mulder: Data curation (equal); Formal analysis (lead); Investigation (equal); Methodology (equal); Project administration (supporting); Supervision (supporting); Writing – original draft (equal); Writing – review & editing (equal). Paul W. King: Conceptualization (equal); Formal analysis (supporting); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (lead); Writing – original draft (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

1H-NMR

proton nuclear magnetic resonance

atm

atmosphere

ATP

adenosine triphosphate

CdS

cadmium sulfide

e

electron

EPR

electron paramagnetic resonance

E0, E2, E4, E6, E8

catalytic intermediates of the MoFe protein

FeMo-co

MoFe protein iron molybdenum cofactor

Fe protein

nitrogenase iron protein

h

hour

h+

hole

H

proton

H2

hydrogen gas

kEA

rate constant of electron accumulation

K

kelvin

MoFe protein

nitrogenase iron molybdenum protein

mW

milliwatt

N2

nitrogen gas

NH3

ammonia

P

power

P-cluster

MoFe protein [8Fe–7S] cluster

QEET

quantum efficiency of electron transfer

QD

quantum dot

t

time

T

temperature

W

watt

1H-NMR

proton nuclear magnetic resonance

atm

atmosphere

ATP

adenosine triphosphate

CdS

cadmium sulfide

e

electron

EPR

electron paramagnetic resonance

E0, E2, E4, E6, E8

catalytic intermediates of the MoFe protein

FeMo-co

MoFe protein iron molybdenum cofactor

Fe protein

nitrogenase iron protein

h

hour

h+

hole

H

proton

H2

hydrogen gas

kEA

rate constant of electron accumulation

K

kelvin

MoFe protein

nitrogenase iron molybdenum protein

mW

milliwatt

N2

nitrogen gas

NH3

ammonia

P

power

P-cluster

MoFe protein [8Fe–7S] cluster

QEET

quantum efficiency of electron transfer

QD

quantum dot

t

time

T

temperature

W

watt

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Published open access through an agreement with National Renewable Energy Laboratory

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