Synthesis and electrocatalytic water oxidation by electrode-bound helical peptide chromophore-catalyst assemblies

Article abstract of DOI:10.1021/ic5011488

Artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells requires the assembly of a chromophore and catalyst in close proximity on the surface of a transparent, high band gap oxide semiconductor for integrated light absorption and catalysis. While there are a number of approaches to assemble mixtures of chromophores and catalysts on a surface for use in artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells, the synthesis of discrete surface-bound chromophore-catalyst conjugates is a challenging task with few examples to date. Herein, a versatile synthetic approach and electrochemical characterization of a series of oligoproline-based light-harvesting chromophore-water-oxidation catalyst assemblies is described. This approach combines solid-phase peptide synthesis for systematic variation of the backbone, copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) as an orthogonal approach to install the chromophore, and assembly of the water-oxidation catalyst in the final step. Importantly, the catalyst was found to be incompatible with the conditions both for amide bond formation and for the CuAAC reaction. The modular nature of the synthesis with late-stage assembly of the catalyst allows for systematic variation in the spatial arrangement of light-harvesting chromophore and water-oxidation catalyst and the role of intrastrand distance on chromophore-catalyst assembly properties. Controlled potential electrolysis experiments verified that the surface-bound assemblies function as water-oxidation electrocatalysts, and electrochemical kinetics data demonstrate that the assemblies exhibit greater than 10-fold rate enhancements compared to the homogeneous catalyst alone.

Full text of DOI:10.1021/ic5011488

Article
pubs.acs.org/IC
Synthesis and Electrocatalytic Water Oxidation by Electrode-Bound
Helical Peptide Chromophore−Catalyst Assemblies
Derek M. Ryan,§ Michael K. Coggins,§ Javier J. Concepcion,‡ Dennis L. Ashford, Zhen Fang,
Leila Alibabaei, Da Ma, Thomas J. Meyer,* and Marcey L. Waters*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
S
* Supporting Information
ABSTRACT: Artificial photosynthesis based on dye-sensitized
photoelectrosynthesis cells requires the assembly of a chromophore
and catalyst in close proximity on the surface of a transparent, high
band gap oxide semiconductor for integrated light absorption and
catalysis. While there are a number of approaches to assemble
mixtures of chromophores and catalysts on a surface for use in
artificial photosynthesis based on dye-sensitized photoelectrosynthesis
cells, the synthesis of discrete surface-bound chromophore−catalyst
conjugates is a challenging task with few examples to date. Herein, a
versatile synthetic approach and electrochemical characterization of a
series of oligoproline-based light-harvesting chromophore−water-
oxidation catalyst assemblies is described. This approach combines
solid-phase peptide synthesis for systematic variation of the backbone,
copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) as an orthogonal approach to install the chromophore, and assembly
of the water-oxidation catalyst in the final step. Importantly, the catalyst was found to be incompatible with the conditions both
for amide bond formation and for the CuAAC reaction. The modular nature of the synthesis with late-stage assembly of the
catalyst allows for systematic variation in the spatial arrangement of light-harvesting chromophore and water-oxidation catalyst
and the role of intrastrand distance on chromophore−catalyst assembly properties. Controlled potential electrolysis experiments
verified that the surface-bound assemblies function as water-oxidation electrocatalysts, and electrochemical kinetics data
demonstrate that the assemblies exhibit greater than 10-fold rate enhancements compared to the homogeneous catalyst alone.
Several strategies have been investigated for the assembly of
both chromophores and catalysts on metal-oxide surfaces,
including codeposition,2,3 electropolymerization,4,5 and self-
assembly.6 However, a limitation of these approaches is a lack
of control of chromophore−catalyst spacing, orientation, and
distance from the surface due to the lack of discrete assembly
architectures. One strategy to address the issue of separating
the oxidized catalyst from the semiconductor surface is to
synthesize chromophore−catalyst assemblies that link the light-
harvesting chromophore and water-oxidation catalyst while
maximizing the separation distance from the catalyst to the
semiconductor surface to slow back electron transfer.1,7−9 To
date, several chromophore−catalyst assemblies have been
reported that use Ru(II) polypyridyl chromophores for efficient
light absorption with various direct linkage strategies to a
remote water-oxidation catalyst.10−17 The chromophores can
be synthetically tailored to include phosphonated bipyridine
(pbpy) ligands, which facilitate attachment to oxide surfa-
ces.12−14,16,17 The need to develop a new synthetic approach
for each new system, however, has inhibited systematic studies
INTRODUCTION
■
Artificial photosynthesis based on dye-sensitized photoelec-
trosynthesis cells (DSPECs) is an attractive route for the
storage of solar energy in chemical bonds. The development of
efficient DSPECs would help to alleviate current issues arising
in solar energy supply and storage arising from the inability of
conventional PV devices to function at night or under poor
lighting conditions. Chemical fuels from DSPECs can be stored
for later use on demand by combustion or in fuel cells.
The principal components behind an efficient DPSEC device
are a transparent, high band gap oxide semiconductor and a
surface-bound, light-harvesting chromophore−catalyst assem-
bly for integrated light absorption and catalysis.1 Light
absorption and injection by the chromophore is followed by
intra-assembly electron transfer activation of the catalyst.1At a
photoanode for water oxidation, the water-oxidation catalyst is
activated by intra-assembly electron transfer to the oxidized
chromophore as a means of transferring the redox equivalents
necessary for water oxidation. The transfer rate of the oxidative
equivalent from the oxidized chromophore to the catalyst needs
to effectively compete with the rate of back electron transfer
from the semiconductor surface in order to drive water
oxidation.
Received: May 16, 2014
Published: July 21, 2014
© 2014 American Chemical Society
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Figure 1. Chemical structures of metallopeptides 1−3, with A (red sphere) representing chromophore [Ru(pbpy)2(L)]2+ and B (blue sphere)
representing water-oxidation catalyst [Ru(Mebimpy)(bpy)OH2]2+.
of the effect of structural variation on the optimization of
assembly properties.
There is little kinetic difference between assemblies, consistent
with rapid intra-assembly electron transfer even through five
intervening proline spacers.
An alternative to the direct linking between chromophore
and catalyst is to use a modular approach with a rigid scaffold
for linking the chromophore and catalyst. Such an approach has
been used before for multiple chromophore assemblies, but not
for combinations of chromophores and catalysts. In our effort
to apply our previous approach for multiple chromophore
assemblies to chromophore−catalyst assemblies, we found that
a new synthetic strategy was required. We report here the use
of a rigid oligoproline peptide as a modular scaffold that
incorporates both a light-harvesting chromophore and water-
oxidation catalyst using a combination of solid-phase peptide
synthesis (SPPS),18 the copper-catalyzed alkyne−azide cyclo-
addition reaction (CuAAC, or “click” reaction) (Figure 1), and
late-stage catalyst assembly.19−22 Importantly, we found that
neither SPPS nor CuAAC is compatible with the water-
oxidation catalyst. Instead, attachment of the bpy ligand to the
scaffold, followed by formation of the catalyst in the final step,
is required. A significant advantage of this strategy is the ease of
preparation, the ability to vary both chromophores and
catalysts, and the ability to control the interspatial distance
between the chromophore and catalyst by incorporating a
predetermined number of proline spacers. Finally, while
oligoprolines are used as the scaffold in this work, the scaffold
itself can also be varied using different natural or unnatural
amino acid building blocks, making the entire assembly
modular in nature.
EXPERIMENTAL SECTION
■
General Methods. All chemical reagents and solvent were
purchased from commercial vendors and used without further
purification. Synthetic protocols for cis-Fmoc-4-N3-Pro, 4,4′-bis-
(diethylphosphonomethyl)-2,2′-bipyridine, and 2,6-bis(1-methyl-
benzimidazol-2-yl)pyridine are provided in the Supporting Informa-
tion. Planar fluorine-doped tin oxide (FTO) glass slides (Rs = 7−8 Ω)
were purchased from Sigma-Aldrich. Prior to assembly binding, each
planar FTO electrode was washed twice with neat CH3OH, followed
by 0.1 M HNO3, water, and finally dried. GC analyses were performed
on a Varian 450-GC with molecular sieve columns and a thermal
conductivity detector. 1H and 13C NMR spectra were recorded at 400
and 100 MHz, respectively. High-resolution and low-resolution mass
spectra were obtained using a Bruker Biotof instrument. Electro-
chemical measurements were performed with a model CHI660D
electrochemical workstation (CH instruments) and a three-electrode
system consisting of a planar fluorine-doped tin oxide working
electrode (typical area 1.0−1.2 cm2), a platinum wire counter
electrode, and either a Ag/AgCl (3.5 M NaCl) or an SCE reference
electrode without iR compensation. All potentials are referenced to the
normal hydrogen electrode. Milli-Q water (>18 MΩ) was used for the
preparation of all buffers and solutions. Surface binding of each
assembly on planar FTO was performed by immersion of a clean
planar FTO glass slide in aqueous loading solutions containing 500
μM assembly and 0.1 M HClO4. Surface coverage values (Γ) were
calculated by determining the charge passed (Q) under the Rub(III/II)
CV wave and the area of the electrode (A) using the equation Γ = Q/
nFA, where n is the number of electrons passed (1 e−) and F is
Faraday’s constant. UV−vis spectra of each surface immobilized
assembly were recorded on a Thermo Scientific NanoDrop 2000
spectrometer. Controlled potential electrolysis experiments were
conducted with a closed three-compartment electrolysis cell with
∼10−12 mL of total electrolyte volume and ∼3 mL of total headspace
above the working electrode cell compartment that had been
deoxygenated for 30 min with N2. Samples of each assembly in
water (5 μL of 150 μM complex) were applied to the sample stage,
and spectra were collected at 25 °C. Circular dichroism spectra were
recorded on an applied photophysics chirascan plus steady-state
The synthesis and structural characterization of two
polyproline chromophore−catalyst assembles provide a basis
for exploring the effect of proline spacer units between the
chromophore and catalyst on assembly properties. In this
Article, we describe surface binding of these assemblies to the
surface of oxide electrodes and an investigation of electro-
catalytic water oxidation. Our results point to an increase in the
rate of water oxidation for both polyproline assemblies at pH
1.0 by more than an order of magnitude compared to
[Ru(Mebimpy)(bpy)(H2O)]2+ under identical conditions.
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circular dichroism spectrometer. Samples of complexes were diluted to
25 μM in either 0.1 M HClO4, pH 3.0, 100 mM phosphate buffer, or
pH 7, 100 mM phosphate buffer. Each spectrum was collected at 25
°C.
product was purified by LH-20 sephadex size-exclusion chromatog-
raphy, yielding the desired metallopeptide assembly 3. Product was
confirmed by observing fragmentation in the ESI-MS, as outlined in
Scheme S1 (Supporting Information). ESI-MS (80% MeCN:H2O, 1%
CO2H2) for 14 [M + Na]+ calcd. 986.77, found 986.5. ESI-MS (80%
MeCN:H2O, 1% CO2H2) for 15 [M − H + Na]+ calcd. 1051.02,
found 1051.93. ESI-MS (80% MeCN:H2O, 1% CO2H2) for 16 [M +
2CO2H2]+ calcd. 844.19, found 844.94.
General Protocols for Solid-Phase Peptide Synthesis.
Oligoproline peptides were synthesized on Rink-amide resin using
Fmoc-protected amino acids. Fmoc-proline (commercially available)
or cis-Fmoc-4-N3-Pro (9) were coupled in the appropriate positions.
The amide coupling reagents 2-(1H-benzotriazole-1-yl)-1,1,3,3-
tetramethyl-uronium hexafluorophosphate (HBTU, 4 equiv) and
hydroxybenzotriazole (HOBt, 3.9 equiv) were mixed with the
appropriate amino acid along with N,N,N-diisopropylethyl amine
(10 equiv) in DMF and coupled with resin. After each coupling step, a
20% piperidine in DMF solution was used to deprotect the N-terminal
Fmoc groups. All peptides were capped with acetic anhydride prior to
cleavage. Peptides were then cleaved from resin with 95:2.5:2.5 TFA/
triisopropylsilane/water. Solvent was removed, and product was
precipitated with diethyl ether. The product was filtered and purified
by reverse phase HPLC using an Atlantis Prep OBD C-18
semipreparative column, with a gradient of 0−100% solvent B over
60 min, where solvent A was 95:5 water/MeCN, 0.1% TFA, and
solvent B was 95:5 MeCN/water, 0.1% TFA. Purified samples were
lyophilized, and the peptide sequences were confirmed by ESI-MS.
General Protocol for the Copper(I)-Catalyzed Azide−Alkyne
Cycloaddtion (CuAAC) Reaction. For both on-resin and solution-
phase CuAAC reactions, the following protocols were used. Cu(II)SO4
(4 equiv) and sodium ascorbate (4 equiv) were stirred in 10 mM
phosphate buffer (pH 8) for 5−10 min. The alkyne (1 equiv) was then
added and stirred for 5 min. The solution was then added to the azide-
containing peptide, which was either on resin or in solution. The
CuAAC reaction mixture was stirred for 12−24 h. The on-resin
CuAAC reaction was washed several times with DI H2O and DMF to
remove excess copper and unreacted ligand and then cleaved from the
resin as described above. Solution-phase reactions were desalted by
sephadex size-exclusion chromatography. The peptides were then
purified by HPLC, as described in the peptide protocols.
Ac-(N3-Pro1)-Pro2-Pro3-(bpy-Pro4)-Pro5-Pro6-CO2NH2 (7). This
peptide was synthesized using the general protocols for solid-phase
peptide synthesis described above with the following exceptions. Upon
coupling cis-Fmoc-4-N3-Pro, the resin was submitted to CuAAC
conditions in the presence of ligand 6. Following the click reaction, the
peptide sequence was completed, acetyl-capped, and cleaved. The
peptide was purified by HPLC as previously described and lyophilized.
ESI-MS [M]+ calcd. 975.46, found 975.48.
Synthesis of Ac-(Chromophore A-Pro1)-Pro2-Pro3-(bpy-
Pro4)-Pro5-Pro6-CO2NH2 (8). Solution-phase CuAAC of chromo-
phore A and 7 was conducted using the previously described CuAAC
protocols. Upon completion of the reaction, excess copper and
unreacted ligand were removed by sephadex size-exclusion chroma-
tography. The peptide was purified by RP-HPLC, yielding 8. ESI-MS
[M + 2Na]2+ calcd. 1003.343, found 1003.48. [M + K + H2O]3+ calcd.
672.25, found 672.15.
Synthesis of Ac-(Chromophore A-Pro1)-Pro2−5-(Catalyst B-
Pro6)-Pro7−9-CONH2 (4). The peptide was synthesized as described
in the general methods. The water-oxidation catalyst B was installed as
previously for 2. Briefly, the peptide portion (1 equiv) and
[Ru(Mebimpy)OTf2]2 (0.5 equiv) of 3 were dissolved in a 95%
ethanol:water mixture and reacted in a microwave oven at 150 °C for
20 min. Completion of the reaction was confirmed by UV−vis
spectroscopy, and the solvent was removed by rotary evaporation. The
product was purified by LH-20 sephadex size-exclusion chromatog-
raphy, yielding the desired metallopeptide assembly 4. Product was
confirmed by observing fragmentation in the ESI-MS. ESI-MS (80%
MeCN:H2O, 1% CO2H2) for 17 [M + Na]+ calcd. 986.5, found
986.77. ESI-MS (80% MeCN:H2O, 1% CO2H2) for 15 [M − H +
Na]+ calcd. 1051.02, found 1051.93. ESI-MS (80% MeCN:H2O, 1%
CO2H2) for 16 [M + 2CO2H2]+ calcd. 844.19, found 844.94.
Synthesis of Ac-(Chromophore A-Pro1)-Pro2−6-(Catalyst B-
Pro7)-Pro8−9-CONH2 (5). The peptide was synthesized as described
in the general methods. The water-oxidation catalyst B was installed as
previously for 2. Briefly, the peptide portion (1 equiv) and
[Ru(Mebimpy)OTf2]2 (0.5 equiv) of 3 were dissolved in a 95%
ethanol:water mixture and reacted in a microwave oven at 150 °C for
20 min. Completion of the reaction was confirmed by UV−vis
spectroscopy, and the solvent was removed by rotary evaporation. The
product was purified by LH-20 Sephadex size-exclusion chromatog-
raphy, yielding the desired metallopeptide assembly 5. Product was
confirmed by observing fragmentation in the ESI-MS. ESI-MS (80%
MeCN:H2O, 1% CO2H2) for 18 [M]+ calcd. 986.5, found 986.77. ESI-
MS (80% MeCN:H2O, 1% CO2H2) for 15 [M − H + Na]+ calcd.
1051.02, found 1051.93. ESI-MS (80% MeCN:H2O, 1% CO2H2) for
16 [M + 2CO2H2]+ calcd. 844.19, found 844.94.
RESULTS AND DISCUSSION
■
System Design. Chromophore−catalyst metallopeptide
assemblies were designed to investigate their potential for
electrocatalytic water oxidation (Figure 1). We chose to use a
polyproline scaffold as it is known to adopt a relatively rigid
helical structure (PPII helix) in water that aligns the i and i + 3
positions on the same side of the helix at a distance of
approximately 9 Å, providing a means for controlling the
distance between the chromophore and catalyst.23−28 Metal-
lopeptide 1 is a control peptide that has been previously
reported by our laboratory and bears the known phosphonated
light-harvesting chromophore A at the N-terminal position (i
position).29 The target was to synthesize metallopeptide 2,
containing chromophore A at the same N-terminal position (i)
and water-oxidation catalyst [Ru(Mebimpy)(bpy)OH2]2+
(B)30−32 in the fourth position (i + 3), thus positioning the
chromophore and catalyst on the same side of the helix, in close
proximity, as supported by MD simulations of a similar peptide
containing two chromophores at i and i + 3 positions.29 In
peptide 3, the chromophore and catalyst are placed at the i and
i + 6 positions with a distance between attachment points of
approximately 18 Å.29
Synthesis of Ac-(Chromophore A-Pro1)-Pro2-Pro3-(Catalyst
B-Pro4)-Pro5-Pro6-CO2NH2 (2). Peptide 8 (1 equiv) and [Ru-
(Mebimpy)OTf2]2 (0.5 equiv) were dissolved in a 95% ethanol:water
mixture and reacted in a microwave oven at 150 °C for 20 min.
Completion of the reaction was confirmed by UV−vis spectroscopy,
and the solvent was removed by rotary evaporation. The product was
purified by LH-20 sephadex size-exclusion chromatography, yielding
the desired metallopeptide assembly 2. ESI-MS (80% MeCN:H2O, 1%
CO2H2) [M]4+ calcd. 604.55, found 604.30. [M + 2Na + H2O]2+
calcd.1248.86, found 1248.61. [M]2+ calcd. 1208.98, found 1208.61.
Synthesis of Ac-(Chromophore A-Pro1)-Pro2−4-(Catalyst B-
Pro5)-Pro6−9-CONH2 (3). The peptide was synthesized as described
in the general methods. The water-oxidation catalyst B was installed as
previously for 2. Briefly, the peptide portion (1 equiv) and
[Ru(Mebimpy)OTf2]2 (0.5 equiv) of 3 were dissolved in a 95%
ethanol:water mixture and reacted in a microwave oven at 150 °C for
20 min. Completion of the reaction was confirmed by UV−vis
spectroscopy, and the solvent was removed by rotary evaporation. The
Synthesis. Metallopeptide 1 had previously been synthe-
sized by a combination of solid-phase peptide synthesis and the
CuAAC catalyzed click reaction.29 The peptide portion of 1 was
synthesized on Rink amide resin using standard fluorenylme-
thoxycarbonyl (Fmoc) protection and deprotection protocols.
The peptide was synthesized using Fmoc-proline (Pro), with
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a
Scheme 1. Synthesis of Complex 2
a
Synthesis conditions: (i) Fmoc-Pro (or Fmoc-4-N3-Pro), HBTU, HOBt, DMF, DIPEA. (ii) CuAAC = Cu(II)SO4, sodium ascorbate, DMF. (iii)
TFA, H2O, TMS. (iv) Cu(II)SO4, sodium ascorbate, 10 mM phosphate buffer (pH 8). (v) [Ru(Mebimpy)(OTf)2]2 ethanol:water (3:1), microwave
130 °C.
the exception of the N-terminal position in which a Fmoc-cis-4-
N3-Pro was incorporated. Following completion of the peptide,
chromophore A was clicked to the backbone, giving the desired
metallopeptide 1.
All attempts at coupling the free amine on the peptide to a
derivatized Mebimpy ligand bearing a carboxylic acid led to
exceptionally low yields (<5%) and complex mixtures. It was
hypothesized that the open coordination site on the catalyst
was responsible for coupling reagent or product degradation,
resulting in poor amide coupling yields.
On the basis of these results, and due to the orthogonality
and reliability of the CuAAC reaction, a novel approach to the
synthesis of complex 2 was designed (Scheme 1). In this
scheme, the azide required for the attachment of the catalyst to
the peptide backbone was effectively protected by CuAAC
reaction with and alkyne-substituted uncoordinated 2,2′-
bipyridine ligand at the i + 3 position. This feature would
allow for elongation of the peptide backbone on solid phase
and inclusion of the N-terminal azide for the attachment of the
chromophore at the i position. The water-oxidation catalyst
could then be constructed by using the uncoordinated bpy
ligand on the peptide backbone in the final step of the
synthesis, thus avoiding both the click and the amide coupling
reactions in the presence of active catalyst.
Initial attempts to synthesize complex 2 followed a similar
strategy as complex 1, namely, utilizing CuAAC for the
sequential attachment of both the water-oxidation catalyst and
chromophore A. Peptide synthesis up to the first azidoproline
was completed, followed by an on-resin CuAAC reaction to an
alkyne-functionalized catalyst at the i + 3 position. Peptide
synthesis was then completed with azidoproline incorporated at
the N-terminal position. The peptide was then cleaved from the
resin, followed by the final CuAAC reaction to the
chromophore at the N-terminal azidoproline. In all cases,
these reactions only resulted in the formation of complex
mixtures containing unreacted starting material or unidentifi-
able products. An amide coupling strategy to the water-
oxidation catalyst was then explored through inclusion of cis-4-
NH2-Pro at the i + 3 position of the peptide, and the requisite
N-terminal azide for the CuAAC reaction to the chromophore.
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Figure 2. (A) Ground-state absorption spectra of 1 (blue) and 2 (green) in aqueous 0.1 M HClO4 and B (red) in 70% ethylene carbonate and 30%
water (1% triflic acid). All spectra were background-subtracted in their respective solvents and obtained at 25 °C. The additive spectrum of 1 and
catalyst B is shown in black. (B) CD spectra of metallopeptide 2 (25 mM) in pH 1 (0.1 M HClO4, red), pH 4 (10 mM phosphate, blue), and pH 7
buffer (10 mM phosphate, black).
The peptide backbone was sequentially assembled from the
C-terminus using standard Fmoc coupling conditions through
the i + 3 position, which acts as the site of attachment for the
water-oxidation catalyst. At this position, cis-Fmoc-4-N3-Pro
was incorporated into the peptide backbone, followed by an on-
resin CuAAC coupling to 4′-methyl-(2,2′-bipyridine)-4-prop-
argyl amide (6). Upon completion of the CuAAC reaction, the
peptide was further elaborated with two additional Fmoc-Pro
residues and N-terminated with cis-Fmoc-4-N3-Pro at the i
position. This was followed by acetyl capping of the peptide,
cleavage from the resin with TFA, and purification by HPLC.
The purified peptide 7 was submitted to a CuAAC reaction
with chromophore A, yielding 8. The metallopeptide was then
purified by HPLC and reacted with [Ru(Mebimpy)(OTf)2]2
under microwave irradiation to install the water-oxidation
catalyst possessing a triflate counterion that was immediately
hydrolyzed in situ to B, giving the desired complex 2.
The general route for synthesis of assembly 3 was identical to
that for 2 with the exception of inclusion of five proline spacers
instead of two (Figure 1). This peptide was synthesized to
investigate the effect of chromophore−catalyst spacing on the
rate of oxidative equivalent transfer. The ability to utilize this
methodology to rapidly vary the chromophore−catalyst
assemblies has a distinct advantage over methods involving
direct linkage of the chromophore and catalyst, for which a
unique synthesis for each assembly would be required.
Spectroscopic and Structural Characterization. Metal-
lopeptides 1 and 2 were characterized by UV−vis spectroscopy
and circular dichroism (CD) spectroscopy (Figure 2 and the
Supporting Information, Figures S1 and S2). Solution-phase
UV−vis measurements on 1 displayed a metal-to-ligand charge-
transfer absorption (MLCT) from 400−500 nm (Figure 2A).
The UV spectrum of 2 included a π → π* absorption from the
Mebimpy ligand at 325−370 nm and an MCLT band from
400−510 nm. Control experiments using a peptide fragment
bearing three proline residues and the water-oxidation catalyst
were also characterized by UV−vis spectroscopy. The spectra of
the water-oxidation catalyst displayed a red-shifted MLCT
absorption (450−550 nm) relative to the chromophore. When
the spectra of 1 and the water-oxidation catalyst were
superimposed, the resulting spectra closely resembled that
observed for complex 2. These results are consistent with
previously reported complexes assembled from a similar
Ru(pbpy)2(bpy)2+-derived chromophore and Ru(Mebimpy)-
(bpy)2+-derived catalysts.12−14 The spectrum of assembly 3 was
similar to the spectrum for assembly 2 (Figure S1, Supporting
Information).
To determine if 2 and 3 adopt the desired PP II helix in
solution, CD spectra of each as a function of pH were acquired.
PP II helices possess characteristic CD spectra with a minimum
at 205−210 nm and a maximum at 220 nm. The CD spectra of
1 with increasing pH clearly indicate a PP II helix at all pH
values, consistent with spectra previously reported for this
complex.23,24,33,34 The CD spectra of 2 and 3 with increasing
pH are also consistent with a well-folded PP II structure from
pH 1−7 (Figure 2 and Figure S2, Supporting Informa-
tion).25−28 The differences in the magnitude of the CD
spectrum at pH 1 relative to pH 4 and pH 7 may be due to a
slight decrease in folding, although contributions from this
effect do not appear in the electrochemical properties of the
assembly. In a previous study, it was demonstrated that a
related polyproline peptide bearing two light-harvesting
chromophores retained the PP II secondary structure when
bound to metal-oxide surfaces.23
Surface Electrochemistry. Cyclic voltammetry (CV)
experiments were conducted with phosphonate surface-bound
chromophore−catalyst assemblies 2 and 3 on planar fluorine-
doped tin oxide (FTO) electrodes with a Ag/AgCl (3.5 M
NaCl) reference electrode and a platinum wire counter
electrode. Assembly-bound FTO electrodes (FTO-2 and
FTO-3: FTO-[RuaII-Rub -OH2]4+) were prepared by soaking
II
electrodes in 0.1 M HClO4 stock solutions of each respective
assembly for 12 h, resulting in saturation coverage of ∼1 ×
10−10 mol/cm2 as determined by the integrated area under the
anodic (oxidative) component of Rub(III/II) CV waves for the
FTO-[RuaII-RubIII-OH2]5+/FTO-[RuaII-Rub -OH2]4+) couple at
II
0.713 V vs NHE (0.1 M HClO4, 0.4 M LiClO4). A
representative CV for FTO-2 at pH 1.0 (0.1 M HClO4) is
III
shown in Figure 3 and exhibits waves for the FTO-[RuaII-Rub
-
OH2]5+/FTO-[RuaII-Rub -OH2]4+, FTO-[RuaII-Rub O]4+/
II
IV
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exhibits a pH dependence of 56 mV/pH unit, consistent with
the 59 mV/pH unit slope predicted by the Nernst equation and
the 1e−/1H+ couple FTO-[RuaII-RubIII-OH]4+/FTO-[RuaII-
II
Rub -OH2]4+ with pKa = 2.24 for -RubIII-OH23+. The catalyst
Rub(IV/III) wave displays pH-dependent behavior from pH 1
to 2.24 with a slope of 124 mV/pH unit, consistent with a 1e−/
2H+ process and close to the predicted slope of 118 mV/pH
unit and the FTO-[Rua -Rub O]4+/FTO-[Rua -Rub
-
II
IV
II
III
OH2]5+ couple (Figure 4). From pH 2.4 to 10, the dependence
decreases to 68 mV/pH unit, consistent with the 1e−/1H+
IV
FTO-[RuaII-Rub O]4+/FTO-[RuaII-RubIII-OH]4+ couple.
IV
It is notable that the pH dependences of both the -Rub

O2+/-RubIII-OH2 and -Rub O2+/-RubIII-OH2+ couples are
higher than predicted based on Nernstian behavior, as observed
earlier for observations for surface-bound [Ru(bpy)3−n(PO3H2-
CH2-bpy)n]2+, with n = 1−3 and for [((PO3H2-CH2)-
bpy)2Rua(bpy-NH-CO-trpy)Rub(bpy)(OH2)]4+.14,37 Also, as
observed earlier, the nominally pH-independent chromophore
3+
IV
Figure 3. Cyclic voltammogram of FTO-2 at 250 mV/s (0.1 M
HClO4, 0.4 M LiClO4, 23 °C).
II
IV
FTO-[Rua -RubIII-OH2]5+, and FTO-[RuaIII-Rub O]5+/
FTO-[RuaII-Rub O]4+ couples with half-wave potentials
IV
Rua(III/II) wave (FTO-[RuaIII-Rub O]5+/FTO-[Rua -
(E1/2) of 0.71, 1.16, and 1.24 V (vs NHE), respectively. The
peak current of each wave was found to vary linearly with the
scan rate (ν), as expected for surface-immobilized redox
couples.35 These values are consistent with those previously
reported for surface-bound [Ru(4,4′-(PO3H2)2(bpy)3]2+ and
[Ru(Mebimpy)(4,4′-(PO3H2)2bpy)(H2O)]2+.17,36 A reversible
IV
II
Rub O]4+) is slightly pH-dependent from pH 1 to 6.5,
IV
decreasing by 18 mV/pH due to local electric field
effects.14,38−41
Water Oxidation Electrocatalysis. To determine if the
surface-bound chromophore−catalyst assemblies were capable
of functioning as water-oxidation electrocatalysts, controlled
potential electrolysis (CPE) and scan-rate-dependent CV
measurements were conducted at pH 1 (0.1 M HClO4, 0.4
M LiClO4) using fully loaded FTO-2 and FTO-3 electrodes.
Sampling of the headspace from CPE measurements conducted
at an applied potential of 1.74 V vs NHE (570 mV
overpotential) and subsequent analysis by gas chromatogra-
phy/mass spectrometry revealed the formation of 8 μmol of O2
(∼8000 turnovers, 64% Faradaic efficiency, TOF = 0.28 s−1)
over 8 h of electrolysis, verifying that the surface-bound
assemblies are functional water-oxidation catalysts (Figure 5).
No evidence for electrolysis products other than O2 was
obtained in this study. The background detection of oxygen was
found to result from leakage of atmospheric O2 into the
wave for the FTO-[RuaIII-Rub O]6+/FTO-[RuaIII-Rub

V
IV
O]5+ couple is not observed due to the onset of water
oxidation.36 CVs of surface-immobilized assembly FTO-3
displayed nearly identical redox behavior to that of FTO-2,
which is expected given the structural and electronic similarities
between the two assemblies (Figure S3, Supporting Informa-
tion).
Oxidation of the aqua-based catalyst in the assemblies occurs
by sequential proton-coupled electron transfer (PCET) steps
II
2+
V
with oxidation of -Rub -OH2 to -Rub (O)3+, resulting in the
pH-dependent redox behavior shown in the pH−E1/2
(Pourbaix) diagram for FTO-2 in Figure 4. The catalyst
Rub(III/II) couple is pH-independent between pH 1 and 2.24,
consistent with the FTO-[RuaII-RubIII-OH2]5+/FTO-[RuaII-
Rub -OH2]4+ couple. From pH 2.24 to pH 10, this wave
II
Figure 4. Pourbaix diagram for FTO-2. Blue data points are E1/2 values for the FTO-Rua(II)-Rub(III/II) couple, red data points are for the FTO-
Rua(II)-Rub(IV/III) couple, and green data points are for the FTO-Rua(III/II)-Rub(IV)) couple.
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Inorganic Chemistry
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Figure 5. (Left) Electrolysis of FTO-2 at 1.74 V vs NHE at pH 1.0 (0.1 M HClO4, 0.4 M LiClO4), Γ = 1.1 × 10−10 mol/cm2. (Right) Gas
chromatographic response for O2 evolution during electrolysis (blue trace) and background response in the absence of catalyst (black trace).
electrolysis vessel over the extended period of each experiment
(Figure 5).
Scheme 2. Electrode-Immobilized Single-Site Water
Oxidation Mechanism32,44
The results of scan-rate-dependent CV measurements on
both FTO-2 and FTO-3 in 0.1 M HClO4 with 0.4 M LiClO4 at
room temperature were used to determine water oxidation rate
constants by application of eq 1.35 In eq 1, icat is the catalytic
III
peak current at 1.74 V, ip is the peak current for the -Rub
-
OH23+/-Rub -OH2 wave at E1/2 = 0.71 V vs NHE, kcat is the
catalytic rate constant, ν is the scan rate in mV/s, R is the ideal
gas constant, T is the temperature, ncat (=4) is the number of
electrons involved in water oxidation, np (=1) is the number of
II
2+
electrons involved in the oxidation of FTO-[RuaII-Rub -OH2]4+
II
to FTO-[RuaII-RubIII-OH2]5+, and F is Faraday’s constant.
icat/ip = (4RTncat/np F)kcat(ν)−1
2
(1)
Peak current ratios as a function of the inverse of the scan
rate according to eq 1 are shown in Figures S4 and S5
(Supporting Information). From these data, kcat values of
0.85(3) and 0.57(2) s−1 were determined for FTO-2 and FTO-
3, respectively. Steady-state current densities for FTO-2 and
FTO-3 from chronoamperometric experiments are both ∼0.1
mA/cm2, which is consistent with the close kcat values
determined from the CV kinetics measurements. Notably,
these experimental rate constants are more than an order of
magnitude higher than the value reported under the same
conditions for the [Ru(Mebimpy)(bpy)(H2O)]2+ water-
oxidation catalyst (0.027 s−1).30 These results point to a
redox mediator effect with the oxidized, Ru(III), chromophore
accelerating the rate of water oxidation. Related observations
have been reported in other chemically linked chromophore−
catalyst assemblies.14,37,42,43 The slight decrease in water
oxidation rate for FTO-3 is presumably due to a micro-
environmental effect with the remote catalyst further from the
electrode surface.
and O2 release and re-enters the catalytic cycle. As for the
surface-bound analogue shown in Scheme 2, [Ru(Mebimpy)-
(4,4′-(CH2PO3H2)2bpy)(H2O)]2+, there is also independent
evidence for the appearance of the proposed Ru−peroxide
intermediates in the chromophore−catalyst assembly reported
here.32,44 Following oxidative scans through the catalytic water
oxidation wave, new waves appear for Ru−peroxide couples at
E1/2 = 530 mV and 320 vs NHE. These waves were also
observed when the assembly-bound electrodes are dried in air.
The results of ultrafast photophysical measurements describing
the interfacial dynamics following excitation and injection will
be reported elsewhere.29
The results of the electrochemical experiments show that, in
both redox potentials and water oxidation rates, there are only
small differences between assemblies, FTO-2 and FTO-3,
which is not a surprise given the common catalyst. The
oxidation mechanism for related, single-site oxidants in solution
and on oxide surfaces is illustrated in Scheme 2 for the surface-
bound, single-site catalyst [Ru(Mebimpy)(4,4′-(CH2PO3H2)2-
bpy)(H2O)]2+.31,32 It involves stepwise PCET oxidation of
CONCLUSIONS
■
We describe here a novel synthetic approach applicable to the
rapid synthesis of a series of light-harvesting chromophore−
water oxidation assemblies, in which the chromophore is first
coupled to the backbone and the water-oxidation catalyst is
then assembled in the final step. The polypeptide backbone
adopts a polyproline type II helix structure, which serves as a
scaffold to align the two complexes along one face of the helix.
This represents a significant advantage over other molecular
-Rub -OH2 to -Rub O2+, followed by further oxidation to
II
2+
IV
-Rub (O)3+ and rate-limiting O atom transfer to a water
V
molecule to give a hydroperoxide intermediate, -RubIII-OOH2+.
The hydroperoxide intermediate undergoes further oxidation
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(4) Lapides, A. M.; Ashford, D. L.; Hanson, K.; Torelli, D. A.;
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assembly approaches in that key intra-assembly properties,
including both composition and spacing, can be varied
systematically and applied generally to other amide-linked
scaffolds for optimization of assembly properties. Electro-
chemical studies have elucidated redox potentials and their
associated pH dependences and demonstrated that they are
relatively unaffected by binding to the surface. Water-oxidation
catalysis is also maintained for the surface-bound assemblies
with the assembly-based catalysts undergoing water oxidation
unchanged through many turnovers. There is a notable order of
magnitude rate enhancement for water oxidation by the
assemblies compared to the substituent single-site, surface-
bound catalyst, providing evidence for a redox mediator effect.
Our results are important in demonstrating the use of standard
peptide synthesis coupled with click chemistry to prepare a
platform for a family of chromophore−catalyst assemblies for
potential DSPEC applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Additional synthetic details, UV/vis spectra, CD spectra,
representative cyclic voltammograms of FTO-3, and ic/ip
versus ν−1 plots for FTO-2 and FTO-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Harrison, D. P.; Norris, M. R.; Concepcion, J. J.; Fang, Z.; Templeton,
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: mlwaters@email.unc.edu (M.L.W.)
*E-mail: tjmeyer@unc.edu (T.J.M.).
■
Present Address
‡J.J.C. current address: Department of Chemistry, Brookhaven
National Laboratory, Upton, NY 11973.
Author Contributions
§D.M.R. and M.K.C. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(21) Kumin, M.; Sonntag, L.-S.; Wennemers, H. J. Am. Chem. Soc.
̈
This research was primarily supported by the UNC EFRC:
Center for Solar Fuels, an Energy Frontier Research Center
(EFRC), funded by the U.S. Department of Energy (DOE),
Office of Science, Office of Basic Energy Sciences, under Award
DE-SC0001011 supporting D.M.R., J.J.C., Z.F., L.A., and D.M.
Funding by the Center for Catalytic Hydrocarbon Function-
alization, an EFRC, funded by the U.S. DOE, Office of Science,
Office of Basic Energy Sciences, under Award DE-SC0001298
supporting M.K.C. is gratefully acknowledged. D.L.A. acknowl-
edges support from a fellowship from the Department of
Energy Office of Science Graduate Fellowship Program (DOE
SCGF), made possible, in part, by the American Recovery and
Reinvestment Act of 2009, administered by ORISE-ORAU
under Contract DE-AC05-06OR23100.
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