Proton-Coupled Electron Transfer Reduction of a Quinone by an

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Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative Na Song, Christopher J Dares, Matthew V. Sheridan, and Thomas J. Meyer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08176 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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The Journal of Physical Chemistry

Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative Na Song, Christopher J. Dares, Matthew V. Sheridan and Thomas J. Meyer*

Department of Chemistry, University of North Carolina at Chapel Hill, North Carolina 27599-3290, United States.

Supporting Information Placeholder ABSTRACT: The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and nanoITO electrodes under acidic conditions in 1:1 CH3CN/H2O (V:V). On FTO, reversible 2e-/2H+ reduction of FTO|-FMN to FTO|-FMNH2 occurs with the pH and scan rate dependence expected for a 2e-/2H+ surface-bound couple. Addition of tetramethylbenzoquinone (Me4Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with kH = (2.6 ± 0.2) × 106 M–1 s– 1 . Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area nanoITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure.

Here we report on the derivatization of oxide electrodes by a phosphate flavin derivative, flavin mononucleotide (FMN), and the redox properties of the couple on oxide electrode surfaces. We also report on the reactivity of the derivatized electrodes toward electrocatalytic reduction of tetramethylbenzoquinone (Me4Q) to its hydroquinone (E1/2 = 0.48 V vs. NHE)11 both on FTO (fluorine-doped SnO2) electrodes and on high surface area nanoparticle films of nanocrystalline Sn(IV)-doped In2O3 (nanoITO). The structures of the surface-bound riboflavin derivative, riboflavin-5'monophosphate, FMN- and its reduced form, FMNH2- are shown in eq 1.

INTRODUCTION Flavins are important biological cofactors, integral to the catalysis of key redox reactions in metabolism, notably in the critical energy recycling of the NAD+/NADH couple.1, 2, 3, 4, 5 There is an extensive background electrochemical literature on this couple.6,

(1)

7, 8, 9

Proton coupled electron transfer (PCET) with its significant implications for biological function and energy conversion is one of the most active research areas.10, 11, 12, 13 In earlier studies, we reported on an important role for PCET in the redox interconversions of key redox cofactors–tyrosine,14 tryptophan,15 cysteine,16 quinone/hydroquinone,17 and a NADH analog18 by use of solution-based electron transfer mediators. We have also documented an important role for surface catalysis of both water oxidation19, 20, 21, 22 and amino acid oxidation23 by related mediators surfacebound to metal oxide electrodes.

EXPERIMENTAL SECTION Reagents and Materials. All commercial chemical reagents were used as received except as noted. All solutions were freshly prepared with deionized water provided by a MilliQ purification system (Synthesis A10). Methods. Electrochemical measurements were performed with a CH Instruments CH-660D electrochemical workstation at room temperature (22 ± 1 °C). A three-electrode configuration was applied in a three-compartment cell with a Fluorine-doped tin oxide (FTO, 1 cm2) or nanocrystalline tin-doped indium tin oxide (nanoITO, 1 cm2) on glass (Hartford Glass;

ACS Paragon Plus Environment


i 2kobsΓ 0 = FS 1+ exp[ F (E − E 0 )] RT

ip FνΓ 0 = FS 4RT

(2)

Figure 1. CVs of a FTO|-FMN electrode (1 cm2) in 1:1 CH3CN/H2O (V:V) at scan rate 100 mV/s for 50 repetitive scan cycles. [HClO4] = 0.1 M, I = 0.25 M (NaClO4),

As shown in Figure 2, the scan-rate dependence of the peak current for the surface-bound flavin couple from 0.01–0.2 V/s is linear as expected for a nondiffusional, surface-bound couple.26 Over this range in scan rates, variation in the peak-to-peak separation, ∆EP = (Ep,a – Ep,c), was ≤ 0.060 V, consistent with a kinetically inhibited, but reversible surface PCET couple and the reductive component shown in eq 1. -1×10-4

(3)

i 8RTkobs (4) = ip Fν {1+ exp[ F (E − E 0 )]} RT i is the experimentally measured current, Γ0 is the surface concentration of the catalyst, F is the Faraday constant, S is the electrode surface area, E0 is the E1/2 value for the FTO|-FMN/FMNH2 couple, ip is the FMN reduction peak current in the absence of quinone, and kobs is the observed rate constant for Me4Q reduction.

RESULTS AND DISCUSSION Kinetics with FTO Electrodes. Cyclic voltammetric (CV) scans of FMN-derivatized electrodes were preformed in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), (Figure 1). A chemically reversible wave appears for the FMN/FMNH2 couple at E1/2 = 0.12 V vs. NHE, Eq 1. The surface-bound FTO|-FMN/FMNH2 couple is stable on the oxide surface for extended periods and, as shown in Figure 1, for multiple scan cycles with no evidence of loss from the surface at a scan rate of 100 mV/s for 50 repetitive scan cycles.

-5×10-5

-8×10-5

0.20 V/s 0.10 V/s 0.05 V/s 0.02 V/s 0.01 V/s

-6×10-5 Current, A

sheet resistance = 15 Ω/sq) working electrode, Ag/AgCl (3 M NaCl, 0.21 V vs NHE) reference electrode, and platinum wire counter electrode. The ionic strength (I) was adjusted to 0.25 M with added NaClO4. Unless otherwise stated, all electrochemistry was performed in 1:1 CH3CN/H2O. FTO or nanoITO slides were loaded with flavin mononucleotide (FMN), by immersion in 0.5 mM solutions in methanol for 24 h to give the surfacederivatized electrodes FTO|-FMN. The slides were removed, rinsed with methanol, water, and dried under a steam of nitrogen. UV-visible spectra were recorded on an Agilent Technologies model 8453 diode-array spectrophotometer with ChemStation software. Foot-of-the-Wave Analysis. The cyclic voltammetric data were analyzed by the methods developed by Savéant group. For heterogeneous system, the equation used to analyze the electrocatalytic rates here were derived by combining the eq 2 of reference 24 and the eq 3 of reference 25, as shown in eq 4.

Current, A

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0 5×10-5 1×10-4 0.4

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0.3 0.2 0.1 0.0 Potential, V vs NHE

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0 0.00 0.05 0.10 0.15 0.20 0.25 Scan Rate, V/s

Figure 2. As in Figure 1, scan rate dependence for the FTO|-FMN/ FMNH2 couple at 0.01, 0.02, 0.05, 0.1, and 0.2 V/s, electrode size: 1 cm2.

The acid dependence of the FTO|-FMN/FMNH2 couple was investigated under the same conditions by CV measurements over the pH range 0.6-2.5. As shown by the data in Figure S1, E1/2 varies with pH by –0.058 ± 0.002 V consistent with a 2e-/2H+ process and reduction of FTO|-FMN to FTO|-FMNH2, eq 1. FMN adsorption isotherms on FTO were measured by soaking the slides for 24 h in methanol solutions of FMN with concentrations of 0.0125, 0.025, 0.05, 0.25, 0.5 and 0.75 mM. The results of a surface binding study on FTO as a function of FMN concentration in methanol (Figure 3) were analyzed by using the Langmuir adsorption relation in the form, Γ = Γmax((K[FMN])/(1 + K[FMN])), with Γmax the maximum surface loading in mol⋅cm-2 and K the surface loading constant.27 Analysis of the data in Figure 3 gave Γmax = 6.1 × 10–11 mol cm–2 with K = 9.0 × 104 M–1. In subsequent experiments, electrodes were loaded from solutions 0.5 mM in FMN in methanol to


ensure complete monolayer coverage on the FTO electrode surfaces. 8×10-11

Γ, mol/cm2

6×10-11 4×10-11 2×10-11 0

0

2×10-4 4×10-4 6×10-4 8×10-4 [FMN], M

FTO|-FMNH2 + Me4Q → FTO|-FMN + Me4QH2 (6) The CV data were analyzed by a foot-of-the-wave procedure24, 25 for heterogeneous system at potentials well below the peak current to avoid complications from local polarization and concentration depletion effects by use of eq 4. In Figure 5a are shown a series of CVs at FTO|-FMN electrodes as a function of added quinone. Figure 5b shows plots of the catalytic enhancement ratio (i/ip) as a function of {1 + exp[F(E- E0)/RT]}–1. Values of kobs were obtained from the limiting slopes by use of eq 4. As shown by the plot of kobs vs. [Me4Q] in Figure 5c, electrocatalysis at the electrode is first order in [Me4Q].

Figure 3. Surface binding of FMN on FTO to give FTO|FMN after soaking periods of 24 h in methanol as determined by electrochemical measurements in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), electrode size: 1 cm2.

-6.5×10-4

-1.5×10-3

Current, A

-9.0×10-4

Current, A

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-1.0×10-3

0.0 0.4 40 30

-1.5×10-4 i/ip


(a)

-5.0×10-4

FTOFMN + 1 mM Me4Q FTO 1 mM Me4Q FTOFMN

-4.0×10-4

1.0×10-4 0.4

2 mM Me4Q 1 mM Me4Q 0.5 mM Me4Q 0.1 mM Me4Q

-0.1

Figure 4. CVs of FTO|-FMN in the absence of Me4Q (blue); FTO (green) and FTO|-FMN (red) in the presence of 1 mM Me4Q at a scan rate of 0.1 V/s in 1:1 CH3CN/H2O(V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), electrode size: 1 cm2.

Given the well-established and significant hydride donor reactivity of the reduced flavin,28 electrochemical measurements were extended to the reduction of the model quinone, tetramethylbenzoquinone (Me4Q) The current response at an underivatized FTO electrode with 1 mM added Me4Q was negligible (Figure 4). At the derivatized FTO|-FMN electrode, the FTO|FMN/FMNH2 couple appears at E1/2 = 0.12 V vs. NHE with clear evidence for electrocatalysis with added Me4Q. Under these conditions, the peak current (ip) increased with [Me4Q] over the concentration range 0.1 to 2 mM, Figure 5a. The appearance of electrocatalysis is consistent with surface reduction of FTO|-FMN to FTO|-FMNH2 followed by |-FMNH2 reduction of the quinone and regeneration of the oxidized form of the catalyst, eqs 5-6. FTO|-FMN + 2e- + 2H+ → FTO|-FMNH2 (5)

2 mM Me4Q 1 mM Me4Q 0.5 mM Me4Q 0.1 mM Me4Q


-0.1

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21

20 10 0 0.00

28

(b)

kobs, s–1

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14 7

0.25

0.50

0.75

1.00

1/(1+exp(F(E-E0)/RT))

0 0.0000 0.0006 0.0012 0.0018 0.0024 [Me4Q], M

Figure 5. a) cyclic voltammograms of FTO|-FMN in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4) at [Me4Q] = 0.1, 0.5, 1 and 2 mM, scan rate 0.1 V/s. b) plot of i/ip vs. {1 + exp[F(E- E0)/RT]}-1. c) plot of kobs vs. [Me4Q], electrode size: 1 cm2.

The influence of acid on reduction of the quinone was also investigated over the range 0.01 M to 0.25 M by varying [HClO4] at a fixed ionic composition of 0.25 M with added NaClO4. As shown by the results in Figure 6a, single scan CVs are [H+] dependent with the peak current increasing and peak potential decreasing as the concentration of acid was increased. As shown in Figure 6c, application of the foot-of-thewave analysis to the CV wave forms with 1 mM Me4Q show that the electrocatalytic current increases linearly with [H+] consistent with a rate limiting step first order in acid and the rate constant expression, kobs/[Me4Q] = k0 + kH [H+], with kH = (6.5 ± 0.2) × 104 M–1 s–1 and k0 = (4.0 ± 0.3) × 103 M–1 s–1.



pH 0.6 pH 1.0 pH 1.5 pH 2.0 pH 2.5

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-4×10-4 -2×10-4 0 0.4

40

20 10 0 0.0

0.2

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1/(1+exp(F(E-E0)/RT))

1.0

-0.1

2.4×104

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pH 0.6 pH 1.0 pH 1.5 pH 2.0 pH 2.5

kobs/[Me4Q], M–1 s–1

30 i/ip


(c)

4

1.8×10

1.2×104 6.0×103 0.0 0.0

0.1

0.2

0.3

[H+], M

Figure 6. FTO|-FMN in 1:1 CH3CN/H2O (V:V) in the presence of 1 mM Me4Q, pH 0.6–2.5 (HClO4), I = 0.25 M (NaClO4), scan rate 0.1 V/s, electrode size: 1 cm2.

The H/D kinetic isotope effect for Me4Q reduction was investigated at FTO|-FMN derivatized electrodes in 1:1 D2O/CD3CN (V:V) solutions, 0.1 M in HClO4 and I = 0.25 M (NaClO4) with 1 mM added Me4Q. In D2O mixtures, E1/2 for the FTO|-FMN/FMNH2 couple was anodically shifted by 0.045 V (Figure S2) due to an equilibrium isotope effect. By using the foot-ofthe-wave analysis, the H2O:D2O kinetic isotope effect (KIE) was kH2O-CH3CN/kD2O-CD3CN = 1.5 for the reduction of Me4Q at FTO|-FMN (Figure S3). Based on the electrochemical measurements, reduction of Me4Q by the reduced, surface-bound FMN derivative is dominated by a pathway first order in both quinone and added acid. By inference, this pathway arises from initial protonation of the quinone, eq 7, followed by rate limiting hydride transfer from FTO|-FMNH2 to Me4QH+, eq 8, followed by rapid proton loss from FTO|-FMNH+, eq 9. There is precedence for a related proton-assisted pathway in the reduction of quinone by the NADH analog N-benzyl1,4-dihydronicotinamide (BNAH).18 FTO|-FMN + 2e- + 2H+→ FTO|-FMNH2 (5) Me4Q + H+ = Me4QH+ (7) + FTO|-FMNH2 + Me4QH → FTO|-FMNH+ + Me4QH2 (8) + + FTO|-FMNH → FTO|-FMN + H (9) Kinetics with nanoITO Electrodes. The oxideattached reactivity chemistry of -FMNH2 was also extended to high surface area, nanoparticle films of tin-doped indium tin oxide (nanoITO) for possible applications in electrocatalysis. In these experiments, 2.5 µm thick nanoITO slides were loaded with FMN

by immersion in 0.5 mM solutions in methanol for 24 h with a surface coverage of 2.5×10-8 mol/cm2, 1.7 × 10–9 mol/cm2/µm, based on the integrated charge under the FMN wave at 0.12 V vs. NHE. Surface binding of the phosphate-derivatized flavin on the nanoparticle oxide was also stable for extended periods. Repetitive CV scans on the derivatized oxide in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), show that surface-bound FMN is stable for at least 300 scans through the FTO|-FMN/FMNH2 couple, Figure S4. As can be seen from the CV scans in Figure S5, in the cavities of the porous nanoITO oxide electrode, capacitive effects and internal oxide film dynamics lead to large peak-to-peak splitting for the nanoITO|FMN/FMNH2 couple. Figure S5 shows scan rate dependent data from 0.005 – 0.050 V/s. As on the planar electrode, a linear relationship exists between peak current and scan rate as expected for a surface-bound couple with the peak-to-peak separation, ∆EP = (Ep,a – Ep,c), increasing from 75 mV at 5 mV/s to 240 mV at 50 mV/s. As shown in Figure 7, with 8.4 mM added Me4Q, there is clear evidence for electrocatalytic reduction of the quinone at nanoITO|-FMN following reduction to nanoITO|-FMNH2. The wave shape is revealing, consistent with a contribution from electrocatalytic quinone reduction at Ep,c = 0.06 V vs. NHE followed by the reductive component for the nanoITO|FMN/FMNH2 couple at Ep,c = 0 V vs. NHE at a scan rate of 0.01 V/s. The latter provides evidence for an important role for diffusion of the quinone into and within the pores of the oxide in competition with hydride transfer. For the catalytic wave, the peak current increases linearly with increasing [Me4Q] (Figure 8a) with a first order dependence on concentration as established by foot-of-the-wave analysis of single sweep CV scans. -2.0×10-3 -1.4×10-3 Current, A

-8×10-4

Current, A

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nanoITOFMN + 8.4 mM Me4Q nanoITO 8.4 mM Me4Q nanoITOFMN

-8.0×10-4 -2.0×10-4 4.0×10-4 0.5

0.3 0.1 -0.1 Potential, V vs NHE

-0.3

Figure 7. CVs at nanoITO|-FMN blue and with 8.4 mM added Me4Q at nanoITO (green) and nanoITO|-FMN (red) at a scan rate of 0.01 V/s in 1:1 CH3CN/H2O(V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), electrode size: 1 cm2.


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(a)

8.4 mM 5.7 mM 3.8 mM 2.0 mM 1.0 mM

-1.4×10-3 Current, A

ASSOCIATED CONTENT Supporting Information. Experimental details and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

-8.0×10-4 -2.0×10-4

AUTHOR INFORMATION 4.0×10-4 0.5 6

4

8.4 mM 5.7 mM 3.8 mM 2.0 mM 1.0 mM

0.3 0.1 -0.1 Potential, V vs NHE 6

(b)

-0.3

Corresponding Author

[email protected]

(c)

ACKNOWLEDGMENT 4

This research was supported by the National Science Foundation under grant CHE-1362481, supporting N.S..

kobs, s–1

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i/ip 2

2

REFERENCES 0 0.0

0.2

0.4

0.6

0.8

1/(1+exp(F(E-E0)/RT))

1.0

0 0.000 0.002 0.004 0.006 0.008 0.010 [Me4Q], M

Figure 8. nanoITO|-FMN in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4), [Me4Q] = 1.0, 2.0, 3.8, 5.7 and 8.4 mM, scan rate 0.01 V/s, electrode size: 1 cm2.

Controlled Potential Electrolysis. Controlled potential electrolysis of 2.6 mM Me4Q was carried out at an FTO|-FMN/FMNH2 electrode at 0.11 V vs NHE in 1:1 CH3CN/H2O (V:V), [HClO4] = 0.1 M, I = 0.25 M (NaClO4). Based on integrated current-time plots, n ~ 2 for overall reduction of the quinone. Re-oxidation of the reduced solution occurred with 90% of the charge passed during the reduction. Both CV and UV-visible measurements were consistent with reversible reduction of the quinone to the hydroquinone followed by reoxidation to the quinone, Figures S6 and S7.

CONCLUSIONS The results reported here are significant in demonstrating the translation of the redox properties of an important biological redox cofactor to the surfaces of oxide electrodes. The redox reactivity of the cofactor is maintained on these surfaces with reversible conversion between FMN and FMNH2 occurring on both FTO and nanoparticle nanoITO electrodes. The results of a mechanistic study on the reduction of Me4Q to its hydroquinone by FTO|-FMNH2 are consistent with reactivity dominated by prior protonation of the quinone and rate limiting hydride transfer. Although microscopically complex, protonation of the quinone and hydride transfer avoid high energy 1e- intermediates and give the hydroquinone product directly.13 The ability to translate surface reactivity to high surface area nanoITO electrodes is notable possibly presaging future applications in analysis and electrocatalysis.

1. Carrillo, N.; Ceccarelli, E. A., Open Questions in Ferredoxin-NADP+ Reductase Catalytic Mechanism. Eur. J. Biochem. 2003, 270 (9), 1900-1915. 2. Pang, J.; Hay, S.; Scrutton, N. S.; Sutcliffe, M. J., Deep Tunneling Dominates the Biologically Important Hydride Transfer Reaction from NADH to FMN in Morphinone Reductase. J. Am. Chem. Soc. 2008, 130 (22), 7092-7097. 3. Brinkley, D. W.; Roth, J. P., Determination of a Large Reorganization Energy Barrier for Hydride Abstraction by Glucose Oxidase. J. Am. Chem. Soc. 2005, 127 (45), 15720-15721. 4. Joosten, V.; van Berkel, W. J., Flavoenzymes. Curr. Opin. Chem. Biol. 2007, 11 (2), 195-202. 5. Medina, M., Structural and Mechanistic Aspects of Flavoproteins: Photosynthetic Electron Transfer from Photosystem I to NADP+. Febs J. 2009, 276 (15), 39423958. 6. Tan, S. L. J.; Webster, R. D., Electrochemically Induced Chemically Reversible Proton-Coupled Electron Transfer Reactions of Riboflavin (Vitamin B2). J. Am. Chem. Soc. 2012, 134 (13), 5954-5964. 7. Chatterjee, A.; Foord, J. S., Biological Applications of Diamond Electrodes; Electrochemical Studies of Riboflavin. Diamond Relat. Mater. 2009, 18 (5-8), 899903. 8. Rezaei-Zarchi, S.; Saboury, A. A.; Javed, A.; Barzegar, A.; Ahmadian, S.; Bayandori-Moghaddam, A., Nano-Composition of Riboflavin-Nafion Functional Film and its Application in Biosensing. J Biosciences 2008, 33 (2), 279-287. 9. Zhang, J.; Chi, Q.; Dong, S.; Wang, E., Orientation and Electrocatalysis of Riboflavin Adsorbed on Carbon Substrate Surfaces. J. Chem. Soc., Faraday Trans. 1996, 92 (11), 1913-1920. 10. Hammes-Schiffer, S., Proton-Coupled Electron Transfer: Moving Together and Charging Forward. J. Am. Chem. Soc. 2015, 137 (28), 8860-8871. 11. Warren, J. J.; Tronic, T. A.; Mayer, J. M., Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications. Chem. Rev. 2010, 110 (12), 6961-7001.



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Proton-Coupled Electron Transfer Reduction of a Quinone by an

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