Photocatalytic reduction of artificial and natural nucleotide co-factors with a chlorophyll-like tin-dihydroporphyrin sensitizer

Article abstract of DOI:10.1021/ic401611v

An efficient photocatalytic two-electron reduction and protonation of nicotine amide adenine dinucleotide (NAD⁺), as well as the synthetic nucleotide co-factor analogue N-benzyl-3-carbamoyl-pyridinium (BNAD ⁺), powered by photons in the long-wavelength region of visible light (λ_irr > 610 nm), is demonstrated for the first time. This functional artificial photosynthetic counterpart of the complete energy-trapping and solar-to-fuel conversion primary processes occurring in natural photosystem I (PS I) is achieved with a robust water-soluble tin(IV) complex of meso-tetrakis(N-methylpyridinium)-chlorin acting as the light-harvesting sensitizer (threshold wavelength of λ_thr = 660 nm). In buffered aqueous solution, this chlorophyll-like compound photocatalytically recycles a rhodium hydride complex of the type [Cp*Rh(bpy)H]⁺, which is able to mediate regioselective hydride transfer processes. Different one- and two-electron donors are tested for the reductive quenching of the irradiated tin complex to initiate the secondary dark reactions leading to nucleotide co-factor reduction. Very promising conversion efficiencies, quantum yields, and excellent photosensitizer stabilities are observed. As an example of a catalytic dark reaction utilizing the reduction equivalents of accumulated NADH, an enzymatic process for the selective transformation of aldehydes with alcohol dehydrogenase (ADH) coupled to the primary photoreactions of the system is also demonstrated. A tentative reaction mechanism for the transfer of two electrons and one proton from the reductively quenched tin chlorin sensitizer to the rhodium co-catalyst, acting as a reversible hydride carrier, is proposed.

Full text of DOI:10.1021/ic401611v

Article
pubs.acs.org/IC
Terms of Use
Photocatalytic Reduction of Artificial and Natural Nucleotide Co-
factors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer
Kerstin T. Oppelt,† Eva Woß,† Martin Stiftinger,‡ Wolfgang Schofberger,† Wolfgang Buchberger,‡
̈
̈
and Gunther Knor*,†
̈
̈
†Institute of Inorganic Chemistry, and Institute of Analytical Chemistry, Johannes Kepler University Linz (JKU), A-4040 Linz,
‡
Austria
S
* Supporting Information
ABSTRACT: An efficient photocatalytic two-electron reduction and proto-
nation of nicotine amide adenine dinucleotide (NAD+), as well as the synthetic
nucleotide co-factor analogue N-benzyl-3-carbamoyl-pyridinium (BNAD+),
powered by photons in the long-wavelength region of visible light (λirr > 610
nm), is demonstrated for the first time. This functional artificial photosynthetic
counterpart of the complete energy-trapping and solar-to-fuel conversion
primary processes occurring in natural photosystem I (PS I) is achieved with a
robust water-soluble tin(IV) complex of meso-tetrakis(N-methylpyridinium)-
chlorin acting as the light-harvesting sensitizer (threshold wavelength of λthr
=
660 nm). In buffered aqueous solution, this chlorophyll-like compound
photocatalytically recycles a rhodium hydride complex of the type [Cp*Rh-
(bpy)H]+, which is able to mediate regioselective hydride transfer processes.
Different one- and two-electron donors are tested for the reductive quenching of
the irradiated tin complex to initiate the secondary dark reactions leading to nucleotide co-factor reduction. Very promising
conversion efficiencies, quantum yields, and excellent photosensitizer stabilities are observed. As an example of a catalytic dark
reaction utilizing the reduction equivalents of accumulated NADH, an enzymatic process for the selective transformation of
aldehydes with alcohol dehydrogenase (ADH) coupled to the primary photoreactions of the system is also demonstrated. A
tentative reaction mechanism for the transfer of two electrons and one proton from the reductively quenched tin chlorin
sensitizer to the rhodium co-catalyst, acting as a reversible hydride carrier, is proposed.
Because of their excellent stability properties and a
preference for mediating multielectron transfer processes,12−14
tin porphyrins are very attractive candidates for the
sensitization of artificial photosynthetic reactions based on
earth-abundant components. Some of their water-soluble
derivatives have been successfully involved in pioneering
studies on the photogeneration of hydrogen under visible-
light irradiation.15,16 The photochemical formation of different
aromatic ring-reduced hydroporphyrin species is a common
observation under these conditions.17,18 In this context, the
occurrence of tin chlorins (SnC), which are stable 2,3-
dihydroporphyrin derivatives of their porphyrin parent
compounds (SnP), has been considered as an undesirable
side reaction, which could no longer be coupled to hydrogen
production.15,19 In combination with a suitable hydride transfer
mediator, however, the chlorin species SnC, which exhibits
attractive chlorophyll-type spectral features, including an
enhanced red light-absorption capability, becomes accessible
as a photocatalyst for the accumulation and transfer of
hydrogen equivalents. This novel approach allows the harvest-
ing of low-energy photons for a more efficient solar energy
1. INTRODUCTION
Protonated nicotine amide adenine dinucleotide (NADH)
produced in the course of photosynthetic energy conversion
can be considered as the biological equivalent of solar hydrogen
as a fuel.1 As a versatile two-electron reductant, this co-factor is
an essential component for many biocatalytic and bioinspired
substrate transformation processes, requiring competent
electron input for chemical bond formation.2−4 For example,
the assimilation of atmospheric CO2 to produce liquid carbon-
based fuels such as methanol can be achieved via an enzymatic
reduction sequence based on NADH.5 A variety of biomimetic
asymmetric hydrogenation reactions also critically depends on
the availability of this class of redox co-factors.6,7 For many of
these potential applications, however, a permanent addition of
the native co-factor as a sacrificial reductant remains costly and
impractical. Therefore, a sustainable recycling method for
nicotine amide adenine dinucleotide (NAD+) and related
model compounds based on a renewable energy source is a
highly desirable feature.8−11 Here, we report about the efficient
nonenzymatic regeneration of native NADH and the functional
nucleotide co-factor analogue BNADH (protonated N-benzyl-
3-carbamoyl-pyridinium) powered by red light, using a water-
soluble tin(IV)chlorin complex as a novel chlorophyll-like
photosensitizer for solar chemistry.
Received: June 25, 2013
Published: September 27, 2013
© 2013 American Chemical Society
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Scheme 1. Photochemical Reduction of Water-Soluble Tin Porphyrin Complexes (SnP) To Generate the Two-Electron
Reduced Tin(IV)-meso-tetrakis(N-methylpyridinium)chlorin Derivatives (SnC) upon Green Light Irradiation under Aerobic
Conditions
Scheme 2. Photocatalytic System for the Reduction of BNAD+ Applying Tin(IV)-meso-tetrakis(N-methylpyridinium)-chlorin
(SnC) as a Red-Light Responsive Multielectron Transfer Sensitizer Able To Recycle the Selective Hydride Transfer Mediator
[Cp*Rh(bpy)H]+ in Neutral Aqueous Solution
conversion, while, at the same time, solving the putative
difficulties of a gradual chlorin accumulation. The axial
coordination sphere of the central metal of such photo-
sensitizers may be easily varied independently for further
modifications of the systems such as immobilization or
coupling to other functional subunits including co-catalysts,
depending on the nature of the substituents X.13,14,20−24
Moreover, in a wavelength-controllable process, the readily
accessible tin porphyrin precursor complexes SnP, which can be
considered as the long-term stable resting states of this class of
multielectron transfer photosensitizers under ambient con-
ditions, can be conveniently transformed in situ to the
corresponding hydrogenated chlorin species SnC (Scheme
1).25 Here, we show that the metallochlorin complexes
obtained may then be selectively excited with red light for
photocatalytic (B)NADH formation (Scheme 2).
chloro ligands bound to the high-valent central metal of tin(IV)
porphyrins have been shown to become labile against
hydrolysis.29 For our investigations with water-soluble tin
porphyrin derivatives, the actual nature of the axially bound
ligands X was therefore additionally studied via solid-state 119Sn
NMR of the isolated compounds (Figure 1), where only one
resonance occurred, at −588.3 ppm. A comparison of the
chemical shift of this signal with the literature data for the
closely related tetraphenylporphyrin complex Sn(TPP)Cl2 in
CDCl3 (δ = −589 ppm) leads to the conclusion that, in our
SnP samples, two chloride ligands are attached to the tin(IV)
central atom.30,31 More importantly, the corresponding 119Sn
NMR chemical shift was also observed in neutral aqueous
solution of the SnP compound at 298 K. This finding clearly
indicates that, at neutral pH, the complex is not immediately
hydrolyzed to form a dihydroxo tin species with X = OH. In
order to further study the conditions required for such an axial
ligand exchange, subsequently, 10 μL of KOH (c = 2 mol/L)
were added, and a broadened and downfield-shifted (de-
shielded) 119Sn resonance occurred at −585.5 ppm. At this
stage of the reaction, the increased line width of the 119Sn
resonance peak might reflect a higher asymmetry around the
119Sn metal center and, therefore, indicate the occurrence of a
stepwise exchange process of the axial chloro ligands against
hydroxyl groups, as observed previously by mass spectroscopy
with related main group metalloporphyrin complexes.32 After
the addition of further 10 μL of the concentrated KOH
solution, finally a chemical shift at −579.1 ppm was reached.
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of the Tin
Porphyrin Precursor Complex. The starting material
tin(IV)-meso-tetrakis(N-methylpyridinium)-porphyrin (SnP
with X = Cl, Scheme 1) was prepared and characterized
according to the reported literature procedure.26 Depending on
the specific synthesis and purification conditions of such
compounds, different metalloporphyrin derivatives containing
an undesired mixture of axially bound ligands X such as chloro
and hydroxy groups may sometimes be formed and
isolated.27,28 Moreover, under reductive conditions, the axial
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porphyrin complex with X = Cl remained essentially constant.
At pH >10, the spectroscopic titration experiments resulted in a
small bathochromic shift of the Soret-band to 424 nm, which,
consistent with our proton NMR data, is interpreted as the
formation of the dihydroxo species with X = OH. In acidic
solution (at pH 4−6), we could verify a reversible protonation
step of the dichloro-substituted tin(IV) complex of the meso-
tetrakis(N-methylpyridinium)-porphyrin cation as indicated by
a hypsochromic shift of the Soret band, which has been
previously described and tentatively interpreted in the literature
as a process occurring at one of the central porphyrin nitrogen
atoms.34
Taking together all these results, we have evidence that the
photocatalyst precursor SnP studied in our system is initially
carrying axial chloro ligands (X = Cl), since the pH value in all
experiments carried out in buffered aqueous systems was kept
between pH 7 and pH 9 in order to maintain the stability of the
investigated nucleotide co-factors. Under the photocatalytic
reaction conditions reported here, however, the intermediate
formation of aromatic ring-reduced species will certainly allow
an exchange of axially bound chloride with other ligands such as
hydroxide.29 Interestingly, even a direct inner-sphere inter-
action with the rhodium co-catalyst in the axial coordination
sphere of the tin complex cannot be excluded, as will be
discussed later.
2.2. Formation of the Tin Chlorin Photosensitizer.
Upon visible-light irradiation in the presence of suitable
electron donors (Scheme 1), the water-soluble tin(IV)-complex
of meso-tetrakis(N-methylpyridinium)-porphyrin (SnP) can be
selectively reduced under ambient conditions to generate the
tin(IV)-meso-tetrakis(N-methylpyridinium)-(2,3-dihydropor-
phyrin) or the chlorin derivative SnC (Scheme 1). This clean
and rapid transformation, which can be driven to completeness,
is easily followed by UV−vis spectroscopy and characterized by
the occurrence of several isosbestic points (see the Supporting
Information). Upon irradiation with green light in the presence
of dioxygen, a photochemical quantum yield of φ = 4 × 10−3
was obtained for the chlorin formation process, using 0.01 M
EDTA as an electron donor (298 K; light-emitting diode
(LED) wavelength of 525 nm). A more efficient route to obtain
the chlorin species is the photochemical reduction with sodium
sulfite added as an electron donor in basic aqueous solution,
which is also carried out under ambient conditions. Following
this approach, a very promising quantum yield of up to φ =
0.34 could be achieved for the in situ generation of the
photosensitizer SnC. Sulfate was identified as the oxidation
product of the sacrificial two-electron donor sulfite (for more
details, see the Supporting Information).
Figure 1. 119Sn NMR spectra of tin(IV)-meso-tetrakis(N-methylpyr-
idinium)-porphyrin (SnP) in the solid state and in H2O:D2O = 9:1
with increasing pH values.
The sharp peak at −579.1 ppm indicates the complete
exchange of two axial chloro ligands by hydroxide ions.30,31
Additional 1H NMR experiments in neutral aqueous solution
also showed no evidence for axially attached hydroxide ligands,
since there were no proton resonances found between 0 and
−10 ppm.28 Only under more-alkaline conditions (pH 12,
Figure 2) could a gradual evolution of a single proton signal at
Figure 2. 1H NMR spectra of tin(IV)-meso-tetrakis(N-methylpyr-
idinium)-porphyrin (SnP) dissolved in H2O/D2O = 9:1 at pH 12.1.
The increase of the single proton signal at −3.85 ppm indicates the
gradual exchange process of axial chloride by hydroxide with time.
As expected for this class of compounds,35−37 reducing one
of the pyrrole rings of the SnP precursor complex to form the
metallochlorin (2,3-dihydroporphyrin) derivative SnC results in
characteristic effects on the absorption band pattern in the
visible spectrum (see Figure 3).
The tin chlorin compound displays a strong absorption peak
in the red spectral region (Qy-band), and several less
pronounced absorptions at wavelengths of <600 nm, which
resembles the spectrum of native chlorophyll derivatives such as
Chl b (also shown in Figure 3). With regard to the SnP
precursor, this enhanced red-light-harvesting feature of the
corresponding tin chlorin complex SnC is also reflected by the
deep green color of the photosensitizer in solution. Such a
sufficiently red-shifted threshold wavelength (here, with an
absorption onset at λthr = 660 nm or E = 1.88 eV) can be
−3.85 ppm be observed, indicating the occurrence of an
exchange process from chloride to hydroxide ligation. Despite
of the enormous stability typical for high-valent tin
porphyrins,14,33 further addition of KOH finally led to a
demetalation of the SnP compound. As a consequence, the
119Sn signal at −579.1 ppm fully vanished, while at the bottom
of the NMR tube, a solid gray precipitate was formed, which
was not further characterized.
The pH-dependent exchange processes of axial ligands X in
SnP in aqueous solution were also studied by ultraviolet−visible
light (UV-vis) spectroscopy (see the Supporting Information).
In the pH range of 6−10, the 422-nm Soret-band absorption
maximum of the tin(IV)-meso-tetrakis(N-methylpyridinium)-
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mM) containing a catalytic amount of the SnC photosensitizer
(typically in the micromolar range) were mixed with an excess
of sacrificial electron donors such as TEOA or EDTA. The
concentrations of all components were kept low enough to be
able to follow the photocatalytic process spectroscopically
under continuous-wave irradiation without any further dilution
steps. As an additional redox mediator, the rhodium complex
[Cp*Rh(bpy)Cl]Cl was added. In aqueous solution,39 the
complex [Cp*Rh(bpy)H2O]2+ is formed (Scheme 2), which
has already been extensively studied as an electrocatalyst for
regioselective NAD+ hydrogenation.10 This selective catalytic
process involves the participation of a hydrido-rhodium
intermediate [Cp*Rh(bpy)H]+ exclusively generating the
enzymatically active 1,4-NADH form of the reduced co-
factor.10,39,40 Recently, it has been shown that such rhodium
complexes can also be incorporated in light-driven redox
systems based on different types of sensitizers.9,41−43 Here, we
are employing this type of redox mediator to enable a
photocatalytic reduction of the nucleotide co-factor model
compound BNAD+,44 which was added to the system as
chloride salt in buffered aqueous solution.
Figure 3. Comparison of the Q-band absorption pattern of the water-
soluble tin(IV)-meso-tetrakis(N-methylpyridinium)-chlorin complex
SnC (solid line) with the visible-light-harvesting features of native
chlorophyll b from natural photosynthetic antenna proteins, showing a
comparable threshold wavelength at ∼660 nm (denoted by the dashed
line).
In the presence of SnC as a sensitizer, a photocatalytic
accumulation of the two-electron reduced compound BNADH
was indicated by a new absorption band between 320 and 360
nm (ε = 7240), which continuously increased under steady-
state irradiation with visible light (Figure 4). No such reaction
was observed in darkness or without SnC or the rhodium
mediator present. After the photolysis experiments, lumines-
cence measurements showed a broad new emission band at 470
nm upon excitation of the sample at 340 nm, which could also
be verified by the corresponding excitation spectrum (Figure
4). These findings can also be ascribed to a successful
photochemical synthesis of BNADH.45
regarded as one of the key requirements for potential solar
energy conversion applications such as artificial photosyn-
thesis.2,38 It should also be mentioned, in this context, that, in
contrast to the natural chlorophylls, which display quite similar
spectral features but an adaptation to biological energy
conversion conditions, the biomimetic photosensitizers re-
ported here are characterized by an excellent water-solubility
and turned out to be extremely robust against demetalation and
degradation processes.
2.3. Photocatalytic Reduction of Nucleotide Co-factor
Analogues. The artificial photosynthetic production of N-
benzyl-1,4-dihydro nicotine amide (BNADH) was powered by
green- or red-light irradiation and carried out under an argon
atmosphere (see Scheme 2). For this purpose, buffered aqueous
solutions of the oxidized nucleotide co-factor model BNAD+ (1
In the experiment shown, the total increase of the product
absorption band corresponds to ∼15% conversion of the
oxidized BNAD+ co-factor initially added (1 mM). Therefore, it
Figure 4. Spectral changes observed during visible-light irradiation (λ > 530 nm, 298 K, 1 cm cell) of an anaerobic aqueous solution containing 1
mM BNAD+, [Cp*Rh(bpy)H2O]2+, EDTA, and a small amount of the tin chlorin complex SnC as the photosensitizer (6 × 10−6 M). Inset:
Luminescence and excitation spectra of the BNADH photoproduct accumulated in the course of the photocatalytic process (the asterisk marks
artifacts due to minor SnC reabsorption and fluorescence).
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is clear that the reaction is catalytic, both in terms of the tin
chlorin photosensitizer (6 μM, log ε = 4.25 at the Qy-band
maximum),25 as well as the rhodium co-catalyst present in
solution. From the spectroscopic data of the photolysis shown
in Figure 4, a lower limit for the average turnover number
(TON) of BNADH molecules produced per each photo-
sensitizer molecule (the minimum number of turnovers
completed up to this point)46 can be estimated as TON(SnC)
= 28. We also measured the quantum yield of this process using
selective red-light (Qy-band) excitation (see the Supporting
Information). A value of φ = 3 × 10−4 was obtained for the
BNADH formation reaction monitored by the increase of UV
absorption (298 K, LED: 623 nm). This value (calculated here
as a lower limit by assuming only one mole of incident photons
required per mole of product molecule formed)47 is in the
order of magnitude of other photocatalytic co-factor recycling
systems reported recently, which were powered by blue-light
radiation.43,48
insight into the oxygen-sensitive steps of the tin chlorin
formation sequence summarized in Scheme 1. In another set of
experiments, the photolysis was performed with the tin chlorin
photosensitizer SnC produced via photochemical reduction
with EDTA under ambient conditions prior to the addition of
other reagents to the sample and purging with argon. The third
variation of photocatalytic reaction conditions was based on the
use of anaerobic aqueous SnC solutions obtained from sulfite
reduced SnP already described in the previous sections.
An experiment starting directly from photoexcited SnP in the
presence of EDTA as the electron donor is shown in Figure 5.
Frequently, the formation of the reduced nicotinic acid
moiety of nucleotide co-factors and related model compounds
such as BNADH is only characterized by the type of diagnostic
electronic spectral features reported above. Typically, an
appearance of the optical absorbance band at 340 nm and
sometimes also the corresponding luminescence signal at ∼470
nm are utilized for the quantification of NADH derivatives in
bioanalytical assays. Under abiotic conditions, however,
following these spectroscopic signatures alone could be
misleading, since other reaction products such as biocatalyti-
cally inactive isomeric forms or undesired co-factor dimeriza-
tion products with similar spectroscopic features might also be
obtained. Therefore, the usefulness of potential NADH
regeneration systems is sometimes verified with an appropriate
enzymatic assay, where the reduced co-factor is consumed and
thus indirectly monitored via substrate conversion. We also did
so within the case of artificial photosynthetic NADH
production, and the results are presented in the next sections.
For an unambiguous direct quantification of the accumulated
model compound BNADH, a combination of different
analytical methods was chosen here. It has been shown
previously that the regioselective reduction of synthetic
nucleotide co-factor substitutes such as BNAD+ to the
corresponding 1,4-dihydro form can be analyzed using high-
performance liquid chromatography (HPLC), nuclear magnetic
resonance (NMR), and electrospray ionization−mass spectros-
copy (ESI-MS).48,49 Therefore, we also decided to apply an
additional high-performance liquid chromatography/high-reso-
lution mass spectroscopy (HPLC/HRMS)-based detection
method for the confirmation and characterization of the
obtained photoreduction products. The details of these further
control experiments are described in the Supporting
Information.
Figure 5. UV−vis and NIR spectral changes of a solution containing
SnP, NAD+, EDTA, and [Cp*Rh(bpy)H2O]2+ under N2 in 0.1 M
phosphate buffer pH 7.4, using a 150 W xenon lamp light source with
a 530-nm cutoff filter. The initially increasing absorbance at 840 nm
indicates that tin-phlorin species50 are formed first under these
conditions. After a few minutes, the chlorin complex SnC and also
higher reduced tetrahydroporphyrin species accumulate. Irradiation of
these compounds then starts to accelerate the reduction of NAD+ to
NADH visible at 340 nm. The results shown correspond to a turnover
number of TON > 72 for co-factor photoreduction based on the initial
amount of SnP.
Note that the rhodium complex added as an additional redox
mediator shows a characteristic absorption pattern in the 300−
320 nm range. At the beginning of the reaction, a rapid
depletion of the metalloporphyrin Soret bands and Q-bands at
423 and 556 nm, and the rising of a broad new NIR-band with
a maximum at 840 nm is observed. In agreement with the
typical behavior reported for other tin porphyrins in the
literature,19 these spectral variations indicate that, in the first
reaction sequence, the air-sensitive meso-hydrated tin(IV)-
phlorin (5,24-dihydroporphyrin) derivative19,50 is formed as a
primary reduction product of SnP in deaerated solution, which
then rearranges to form the oxygen-stable metallochlorin
species SnC. In aerobic solution, only the overall SnC
formation process can be monitored under steady-state
irradiation conditions, according to Scheme 1 (see the
Supporting Information).
It turned out that several additional cross-reactions are
possible, which could be explored separately by a selective
excitation of the steady-state chromophore mixture present in
solution using different cut-off filters or LED-light sources,
providing nearly monochromatic wavelengths. When, for
example, a 530-nm cutoff filter is used as shown in Figure 5,
the Q-bands of SnP are also excited, and a product-consuming
back reaction of NADH competing with the sacrificial electron
2.4. Photocatalytic Reduction of Natural NAD+ Co-
factors. The application of the artificial photosynthetic co-
factor recycling system described above was extended to study
also the visible-light-driven reduction of natural NAD+ under
slightly different conditions, which were systematically varied to
get more insight into the mechanistic details of the process.
Long-wavelength irradiation was performed with a 150 W
xenon lamp equipped with suitable cutoff filters or with a high-
power LED setup (see the Supporting Information) and
approached in three different ways. First, experiments were
starting directly from the tin porphyrin precursor complex SnP
in nitrogen-saturated solution, which allowed us to gain more
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donor EDTA can take place, thus forming NAD+ and reduced
tin porphyrin species. Such a photoreduction of tin porphyrins
with NADH as a donor, which has been reported previously in
earlier work,16 can explain the apparent lag-phase of NADH
production in the systems starting with SnP instead of SnC as
the photosensitizer. Furthermore, depending on the type and
excess of sacrificial electron donor applied under photocatalytic
conditions, also a certain amount of a higher reduced
metalloporphyrin species carrying partially or fully protonated
tetrahydroporphyrin ligands may accumulate as an additional
side product besides the desired tin chlorin complex SnC. In
Figure 5, such a process is reflected by the rising Qy-band
pattern in the 600−650 nm region. While the SnC derivative
shows a maximum at ∼633 nm, the additional peak rising at
612 nm can be tentatively attributed to the presence of a
tin(IV)-complex carrying an adjacent tetrahydroporphyrin or
meso-tetrakis(N-methylpyridinium)-isobacteriochlorin type of
ligand.35,36 As will be shown below, however, at slightly higher
pH, this reversible pyrrole ring protonation process36 involving
the 7,8-position of the chlorin ligand can be suppressed to a
significant extent. Despite all of these inevitable complications
consuming some of the reduction equivalents, a new absorption
band at 340 nm is clearly rising under photostationary
conditions (Figure 5), which indicates the reduction of NAD+
to NADH. A lower limit of TON > 72 for the turnover number
achieved was calculated from the tin-porphyrin-derived
maximum amount of SnC photosensitizer that can be present
in the reaction mixture. This value was estimated from the
optical spectra, assuming the molar absorption coefficient of
NADH at 340 nm to be ε = 6300 mol−1 cm−1 (taken from
literature data).45
significant amounts of the initial porphyrin precursor SnP
visible (in the present case, up to 40%, upon estimation by the
residual absorbance at 556 nm). The remaining metal-
loporphyrin absorption bands usually disappear within 1 min
of steady-state photolysis in all observed cases, because of the
photochemical formation of different reduced porphyrin
species, as already discussed above.
During photolysis of a solution containing only 0.01 M
EDTA as a sacrificial donor, the reaction sensitized by SnC
typically reaches a plateau slightly above 20% of conversion of
the initial amount of NAD+ (see inset of Figure 6).
Interestingly, almost identical equilibrium conditions were
found, when the excitation wavelength was further red-shifted
(610 nm cutoff filter) to avoid an irradiation of any remaining
SnP. It could be assumed that, under the conditions of very low
excess of EDTA, as shown in Figure 6, the photocatalytic
process is presumably limited by the amount of the primary
reductant. Therefore, we decided also to vary the type and
concentrations of sacrificial electron donors. Indeed, when
larger amounts of EDTA were present, the product
accumulation proceeded faster and much higher conversion
yields of more than 70% NADH were readily achieved within a
single run (see Figure 7). A similar performance was also
Later experiments with SnC as the photocatalyst obtained
directly from the sulfite-reduced SnP species with either EDTA
or TEOA as a donor resulted in comparable or higher NADH
yields. As an example, in Figure 6, the photolysis of such a tin-
chlorin-containing aqueous solution with a very small excess of
EDTA as the electron donor is shown.51 There are still
Figure 7. UV-vis and NIR spectral changes of a solution containing 7.5
μM SnC obtained by sulfite reduction of SnP; 1 mM NAD+; 1 M
EDTA, and 7.7 × 10−5 M [Cp*Rh(bpy)OH]+ under argon in 0.1 M
phosphate buffer pH 8.8; 150 W Xe lamp with 610 nm cutoff filter;
298 K; 73% conversion to NADH and TON(SnC) = 97 are reached.
observed with TEOA as the electron donor under otherwise
identical conditions (Supporting Information). These findings
suggest that a more-pronounced reductive quenching of the
excited-state SnC sensitizer, thought to be responsible for
initiating the subsequent co-factor reduction process, is reached
with a larger excess of electron donor.
As an additional control experiment to further support the
successful accumulation of NADH, in some cases, the time-
dependent changes in luminescence spectra were also followed
in the course of the photocatalytic process (Figure 8). These
data are consistent with the co-factor conversion profiles
obtained from UV-vis spectroscopy.
From the linear region of the irradiation time profiles
typically observed within the first hours of exposure to red light
(Figure 9), an initial turnover frequency of TOF > 20 h−1 can
be estimated, which corresponds to a calculated space-time-
yield of 2.5 g L−1 d−1 of NADH in these cuvette-based
Figure 6. UV-vis spectral changes of a solution containing 7.5 μM SnC
obtained by sulfite reduction of SnP; 1 mM NAD+; 0.01 M EDTA and
7.7 × 10−5 M [Cp*Rh(bpy)OH]+ under argon in 0.1 M phosphate
buffer pH 8.8; 150 W Xe lamp with 590 nm cutoff filter; 298 K. Note
that, in contrast to Scheme 2, water ligands bound to the rhodium site
(pKa = 8.2)39 are deprotonated at pH 8.8. After 25 h of irradiation,
almost no more changes of the spectra are observed (the trace with the
highest amount of NADH was recorded at 48 h). Numbers shown for
% conversion are given relative to the total initial amount of NAD+.
TON(SnC) = 30.
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are possible when the relative concentration of the rhodium
mediator is increased. Therefore, we believe that our current
limit of ∼0.5 g L−1 NADH obtained from 1 mM NAD+ starting
solutions (Figure 7) could still be further improved in a similar
way.
One of the most significant findings in our case, however,
besides the fact that the catalysis can be powered by red light, is
the excellent long-term stability of the multielectron transfer
sensitizer used. As displayed in Figure 10, the starting spectrum
Figure 8. Luminescence spectra of a solution containing 7.5 μM SnC;
1 mM NAD+; 15 w/v% TEOA and 7.7 × 10−5 M [Cp*Rh(bpy)OH]+
under argon in 0.1 M phosphate buffer pH 8.8; 150 W Xe lamp with
610 nm cutoff filter; increasing luminescence upon excitation at 340
nm exhibiting a maximum at λmax = 470 nm indicates the reduction of
NAD+ to NADH.
Figure 10. Typical UV-vis spectra of a photostationary mixture
recorded after successful NADH accumulation in a long-term
photolysis experiment performed with 15 w/v% TEOA as the electron
donor. As can be seen, the chlorin photosensitizer SnC is immediately
recovered by oxidation with air (solid line). The small peaks between
300 and 320 nm indicate that also the rhodium-based redox mediator
is still intact. Note that only the product absorption band at 340 nm
decreases significantly, because of the oxygen sensitivity of NADH.
of the green SnC complex is easily restored after driving the
NADH production process to the maximum yield of a single
long-term irradiation experiment. Purging the anaerobic
photostationary mixture of reduced metalloporphyrin deriva-
tives with air obviously leads to a reoxidation of all intermediate
photosensitizer reduction products back to the tin chlorin
derivative, as indicated by the reappearing Soret-band and Q-
band maxima at 430 and 636 nm (see Figure 10). Upon
subsequent addition of HCl, the overlapping 340-nm band of
the acid labile photoproduct NADH56 could be completely
removed to better analyze also the UV region of the spectrum,
where the rhodium mediator absorbs characteristically (not
shown here). These types of experiments support the belief that
the apparently photobleached SnC is still fully intact, even after
extended irradiation times with 40 h of exposure to light or
longer. Such a promising stability behavior of the key
components of this artificial photosynthetic system will
certainly open the stage for a repetitive series of long-term
NAD+ photoreduction cycles in order to increase the total yield
of the reduced co-factor. Corresponding experiments to
characterize the absolute TON limits46 of the catalysts applied
therefore still must be performed. However, such long-term
irradiation experiments were not yet within the scope of our
work, which was more focused on the mechanistic aspects of
this novel photocatalytic reaction sequence.
Figure 9. UV−vis and NIR spectral changes of a solution containing
7.5 μM SnC; 1 mM NAD+; 15 w/v% TEOA (1 M) and 7.7 × 10−5
M
[Cp*Rh(bpy)OH]+ under argon in 0.1 M phosphate buffer pH 8.8;
150 W Xe lamp with 610 nm cutoff filter; TON(SnC) = 56 after 5 h
(see the Supporting Information).
laboratory-scale experiments.52 Nevertheless, very promising
absolute product formation quantum yield values between 1%
and 2% were obtained for the artificial photosynthetic
formation of NADH. As an example, in Figure 9, the initial
reaction period of photocatalytic product accumulation with
SnC as a sensitizer, [Cp*Rh(bpy)H2O]2+ in its deprotonated
form as a redox mediator and TEOA as a sacrificial electron
donor is shown together with the spectral variations observed.
These data correspond to a NADH-production quantum yield
of φ = 0.014 (298 K, LED: 592 nm). In this context, it is
worthwhile to compare this value with the absolute quantum
efficiencies determined for the photoaccumulation of reduced
chemical species in natural photosynthetic systems, which are
typically reaching maximum values of φ = 0.01−0.06 in a
comparable spectral region.53−55
At the current stage, we have not yet tried to optimize the
NAD+ to [Cp*Rh(bpy)H2O]2+ ratio of the photocatalytic
reaction mixtures. It has been shown, however, in a recent
publication describing a related blue-light powered NADH
recycling system,43 that up to 100% conversion yields of NAD+
2.5. Coupling Artificial Photosynthesis of NADH to an
Enzymatic Process. The light energy stored in the form of
NADH equivalents can be utilized for powering other
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endergonic substrate conversion processes in subsequent “dark
reactions”. If any enzymatically accelerated steps must be
involved for this purpose,3,5 it is important to demonstrate that
the energy-rich nicotine amide compound provided, and the
reaction conditions selected, are indeed compatible with the
limiting constraints of such biochemical processes. As an
additional confirmation of the regioselective co-factor recycling
process, the photocatalytic 1,4-NADH synthesis reaction
reported in this work was, therefore, also coupled to a simple
enzymatic assay. For this purpose, we selected the NADH-
dependent yeast enzyme alcohol dehydrogenase (ADH, EC
1.1.1.1.) from Saccharomyces cerevisiae,57 which can be applied
to reversibly convert different carbonyl compounds such as
acetone, acetaldehyde, and butyraldehyde to the corresponding
alcohols.58 While interesting light-independent co-factor
recycling processes to provide chiral ketone reduction products
have also been reported in the literature,59 it should be pointed
out that we have chosen here a more simple reaction sequence
to serve merely as a clear proof of principle for our concept.
Nevertheless, several important aspects must be considered in
such a hybrid biophotocatalytic reaction system. The perform-
ance of any enzymes to be coupled is usually characterized by a
limited temperature and pH range and a substrate-dependent
variation in efficiency. Furthermore, the enzymatic process may
be severely disturbed by various substances present only under
abiotic reaction conditions. For example, the zinc-enzyme ADH
is well-known to be inhibited by Zn2+ chelating compounds
such as 2,2′-bipyridine (bpy) and EDTA, which are at least
partially present as functional components in all of the
photocatalytic systems applied here (Scheme 2). Moreover,
the natural function of several enzymes may turn out to be
significantly modified or even completely inactivated when the
biocatalysts are exposed to light under artificial reaction
conditions.2 Despite of these possible limitations, we could
successfully couple the photocatalytic NADH generation
process to a subsequent enzymatic alcohol production step
catalyzed by ADH. Different ways to provide the active
photosensitizer SnC, as discussed in the sections above, were
tested in the enzymatic assays. The obtained alcohols were
quantified by gas chromatography with acetonitrile added as an
internal standard. For analytical reasons, we started to monitor
the formation of isopropanol qualitatively from acetone as a
substrate using photochemically reduced NAD+ as a redox co-
factor and alcohol dehydrogenase as the biocatalyst accelerating
the coupled dark reaction. However, it is well-known that the
activity of ADH for the conversion of acetone is comparably
low.60 Even though it could be demonstrated that the expected
biophotocatalytic reaction sequence is working properly, the
best results achieved in the isopropanol production system
therefore resulted only in unsatisfactory turnover numbers. In
contrast, the application of our photochemical system to the
production of n-butanol from butyraldehyde (n-butanal) was
much more successful, and turnover numbers referred to the
limiting amount of tin chlorin photocatalyst of TON(SnC) >
100 could be readily achieved. A typical example for such an
enzyme-coupled photocatalytic experiment is shown in Figure
11. For this plot, the sample was irradiated in a 1-cm cuvette
sealed with a septum to directly compare these results with the
reaction conditions of the other experiments shown in the
previous sections. Within the first 90 min of irradiation, an n-
butanol concentration of ∼1 mM is built up, which corresponds
to a turnover number of TON > 130 of completed
photoreaction cycles up to this time. This lower limit value is
Figure 11. Time profile for the formation of n-BuOH in the NADH-
dependent photoenzymatic reduction of butyraldehyde using SnC as
the photosensitizer (see text).
based on the initial concentration of the multielectron transfer
sensitzer SnC and assuming a 1:1 ratio of NADH required for
each two-electron reduced aldehyde molecule.47
If NADH-based reduction equivalents are generated in the
system, the reversible enzymatic process catalyzed by ADH
should be able to reach the pH-dependent equilibrium between
the aldehyde and alcohol substrate.57 We assumed that, under
the chosen reaction conditions (see the Experimental Section),
the observed product formation rate should be limited by the
quantum yield of NADH generation in the absence of the
enzyme.
The reaction progress was followed by UV-vis spectroscopy
and gas chromatography (GC) measurements. In contrast to
the experiments described in the previous sections, the 340-nm
signal of NADH was no longer built up significantly upon
photolysis in the presence of ADH. The total amount of
enzyme in the system was set comparably high (∼210 U) in
order to guarantee sufficient catalytic activity also in the
solution containing an excess of free EDTA, which may affect
the active site of the ADH by competitive zinc-ion complex-
ation, as already mentioned previously.60 As can be seen
(Figure 11), the alcohol product formation rate rapidly
decreases within the first hour of irradiation, approaching an
equilibrium concentration. The initial rate of n-BuOH
production is ∼2 mM h−1 (33 U of net “photo-enzymatic”
activity), corresponding to a turnover frequency of TOF = 266
h−1, based on the SnC photosensitizer, which compared to the
maximum values of light-independent systems reported in the
literature is quite promising (TOF < 30 d−1).59 Interestingly,
this TOF value is also more than 10 times higher than the
typical range of values observed in the NADH accumulation
experiments described in the previous sections, where the loss
of stored chemical energy by back-reactions seems to be more
dominant. Although the absolute quantum yield determination
for monochromatic irradiation has not yet been carried out, it
can already be concluded from the data shown above (see
Figures 6 and 7) that the permanent product formation in the
presence of ADH is occurring with a mean quantum yield of φ
≈ 0.1 0.05, estimated for red-light irradiation, which is within
a reasonable order of magnitude, compared to the action
spectra of photosynthetic systems.55 We ascribe this enormous
improvement in efficiency to the effect of immediately utilizing
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Figure 12. (a) Molecular structure of the proposed tin(IV) chlorin-phlorin anion complex SnCH−, which, according to the literature,65 represents
the most plausible tautomeric form of a two-electron ring-reduced meso-protonated derivative of the chlorin photosensitizer SnC. (b) Possible axial
interactions of SnCH− with the co-catalyst leading to SnC regeneration and rhodium-hydride complex formation, similar to the reduction pathways
40
proposed for electrocatalysis39 or the reaction of the [Cp*Rh(bpy)H2O]2+ catalyst precursor with HCO2 as the hydride source.
−
the photogenerated NADH with an additionally coupled two-
electron redox process in situ as soon as it is formed. This
coupling competes with possible side reactions and helps to
avoid a loss of the chemical energy stored caused by the limited
long-term stability of the reduced co-factor. It is well-known
that quite similar strategies to preserve the intermediately
stored reduction equivalents carried by the NADH co-factor are
operating in the carbon dioxide reduction cascade (Calvin
cycle) and also in the cyclic electron flow processes occurring in
natural photosynthesis.61,62 It is important to mention here also
some of the control experiments carried out, which showed that
a photolysis under the same conditions as described above
(Figure 11), but without the enzyme and NAD+ present, did
not lead to any observable n-BuOH formation. Nor did a dark
sample containing all functional components result in any
variations of the UV−vis absorption spectra or indicate any
formation of the alcohol product.
While the light-driven reaction cascade discussed here is
certainly not yet performed under optimized conditions, the
present results clearly demonstrate that an efficient regiose-
lective photocatalytic reduction of the nucleotide co-factor
NAD+ required for the coupling to an enzymatic process has
been achieved. Other types of “dark reactions”, which are more
relevant in the context of artificial photosynthesis and solar
energy conversion such as NADH-dependent abiotic CO2-
fixation processes2,5 will now also be tested with this type of
biomimetic photocatalysis.
2.6. Aspects of Multielectron Transfer Sensitization
and Hydride Transfer: A Tentative Reaction Mechanism.
In the study presented here, a systematic variation of the
reaction parameters, including pH, electron donor supply, and
excitation wavelengths, has been performed in order to
elucidate the conditions required for an efficient photocatalytic
recycling of nucleotide co-factors. It turned out that a
complicated sequence of additional reactions coupling the
light-induced primary processes of the photoexcited tin chlorin
SnC with further electron transfer and protonation steps is
necessary to enable the accumulation of NADH. Nevertheless,
an attempt is made here to provide a simplified mechanistic
picture, which, to the best of our knowledge, is fully consistent
with the available literature and all of our experimental and
spectroscopic data.
Upon irradiation with visible light absorbed only by the
sensitizer, the photocatalytic process is initiated by reductive
quenching of photoexcited SnC with an electron donor D. This
photoinduced electron transfer process leads to the formation
of the one-electron reduced tin chlorin π-radical anion SnC•−
as the first reaction intermediate (see eq 1).
hν
SnC + D → D•+ + SnC•−
(1)
In the case of similar sensitizers carrying high-valent main
group central metals such as tin(IV) or antimony(V), it is well-
established that such tetrapyrrole π-radical anions are
metastable species,50,63 which, under steady-state reaction
conditions, are readily generated in degassed alkaline solution
and identified by their weak Soret band at ∼400 nm and a
structured band pattern in the 700−800 nm spectral region
(also see Figures 6 and 9). The chlorin π-radical anion SnC•−
can be reversibly further reduced in a second electron transfer
step to generate the tin chlorin π-dianion species SnC2−
carrying two additional electrons in the aromatic ring system
of the sensitizer. This process, which is considered as one of the
crucial steps for coupling the initial one-electron photo-
reduction process to a net two-electron redox reactivity of
the catalytic system, can proceed in two different ways.
Depending on the sensitizer concentration and the nature of
the electron donor D applied, a second electron transfer step
(eq 2) or a disproportionation of the π-radical anion (eq 3) will
finally lead to the formation of the diamagnetic SnC2− dianion.
SnC•− + D•+ → D2+ + SnC2−
(2)
2SnC•− → SnC + SnC2−
(3)
The π-dianion is able to take up one or two protons at different
positions of the tetrapyrrole ring. These steps are critically
influenced by pH, solvent, central atom, and substitution
pattern of the macrocyclic system. The exact position of such a
ring protonation can be clearly distinguished spectroscopically.
In our case (Figures 9 and 10), the absence of a typical Soret
band and the apparent bleaching of the entire spectrum of the
sensitizer, leaving only broad, unstructured absorption bands in
the 400−500 nm and 600−800 nm regions indicates the uptake
of one additional proton in the meso-position of the macrocycle,
thus forming a new compound with typical phlorin-anion
characteristics.50,64 A quite similar air-sensitive reduction
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hν
product of a metallochlorin has been described and
characterized before by Stolzenberg and co-workers.65
SnC + 2e− + H+ → SnCH−
(7)
(8)
cat
SnCH− + NAD+ → SnC + NADH
SnC2− + H+ → SnCH−
(4)
The resulting tin(IV) chlorin-phlorin anion complex SnCH−
containing a 2,3,10-saturated porphyrin ring chromophore (eq
4) is thought to be the mechanistic key intermediate acting as
the photochemically produced primary hydride source for the
further catalytic steps involved in NADH generation (see
Figure 12).
3. CONCLUSION
We have presented and characterized a new photocatalytic
system for the regioselective two-electron reduction of
nucleotide co-factors driven by light. For the first time, it
could be demonstrated in an abiotic system that photons in the
long-wavelength region of the visible spectrum (>610 nm) can
be exploited to power the accumulation of NADH. Red-light
energy could also be successfully converted using the synthetic
co-factor analogue BNADH as a storage medium of reduction
equivalents. The light-harvesting and photocatalytic product
formation process was achieved with a tin(IV) chlorin complex
(SnC), which showed an excellent water solubility and long-
term stability. The artificial photosynthetic reaction sequence
described here in detail, involving a primary electron donor, a
multielectron transfer photosensitizer with a chlorophyll-type
spectrum, and an additional redox-mediator catalyzing
(B)NADH formation can be regarded as the first true
functional model system for the overall light reactions occurring
in natural photosystem I (PSI). It could be shown that various
sacrificial donors, even in a very low excess, can be applied to
maintain the energy-storing photoinduced electron transfer and
chemical bond formation chain. More remarkably, the simple
biomimetic process in an unprecedented way displays a very
promising performance in terms of relevant efficiency criteria,
including the limiting threshold energy of actinic light and the
absolute quantum yield of permanent photoproduct formation.
Both values are indeed approaching the orders of magnitude
reported for natural photosynthesis. As part of our ongoing
interest in bioinspired reaction cascades driven by sunlight and
enzyme models,2,13,67,68 we have also coupled an enzymatic
dark reaction to the artificial photosynthesis of NADH. It
turned out that, similar to the steady-state situation in natural
photoautotrophic metabolism, such a consecutive reaction
cascade helps to avoid back-reactions and other limitations
caused by NADH instability, which, in our case, led to an
approximately 10-fold improvement of the permanent product
formation efficiency. Further mechanistic studies regarding the
components of the photostationary reaction mixture and
variations of the coupled substrate conversion routes based
on photogenerated (B)NADH are currently underway.
Oxidation of SnCH− to completely regenerate the initial tin
chlorin species may occur in the presence of dioxygen (Figure
10) or, much more importantly, by transferring two electrons
and a proton to the rhodium co-catalyst to drive the reaction
sequence given below (eqs 5 and 6). The net hydride transfer
process described in eq 5 continuously recycles the photo-
reduced SnC sensitizer for further irradiation with red light, as
already indicated in the simplified picture given in Scheme 2.
SnCH− + [Cp*Rh(bpy)H2O]2+
→ H2O + [Cp*Rh(bpy)H]+ + SnC
(5)
NAD+ + [Cp*Rh(bpy)H]+ + H2O
→ [Cp*Rh(bpy)H2O]2+ + NADH
(6)
We have also shown, in different control experiments, that the
chemically generated rhodium hydride complex [Cp*Rh(bpy)-
H]+ is able to reduce tin porphyrins to phlorin-type species in a
proton-coupled two-electron transfer process related to the
back reaction of eq 5, which additionally supports the
possibility of a hydride exchange reaction sequence as proposed
here (see the Supporting Information). It is also interesting to
note that, during the course of all of the steady-state irradiation
experiments reported here, an enhanced increase of NADH is
always observed after a certain lag-phase, which is finished
when the absorption spectrum of the chlorin-phlorin anion
SnCH− is already dominating the photostationary mixture.
The suggested axial coordination sphere interaction between
the reduced sensitizer SnCH− and the rhodium co-catalyst
(Figure 12) is fully compatible with the well-established
mechanism of regioselective 1,4-NADH formation based on
mixtures of [Cp*Rh(bpy)H2O]2+ and sacrificial hydride
sources, which intermediately bind through an O-atom to the
rhodium center.40 Furthermore, the close proximity and face-to
face alignment of the sensitizer and co-catalyst π-electron
systems, as depicted in Figure 12, are expected to provide a very
favorable situation66 for effective electron transfer from the
tetrapyrrole complex donor to the rhodium site acting as the
two-electron acceptor.
It should be kept in mind, however, that only a detailed
deuterium labeling study could provide direct evidence for this
plausible but, at the moment, still tentative suggestion. It is also
not yet clear whether a stepwise electron transfer sequence
followed by protonation or a more or less concerted process is
operating, although we assume that cleavage of the Rh−O bond
and protonation of the Rh(I)-intermediate formed after two-
electron reduction39 is more probable.
In summary, the artificial photosynthetic NADH production
described in the present work is thought to involve the
photocatalytic formation of an intermediate hydride source (eq
7) coupled to a subsequent regioselective hydride transfer
reaction (eq 8).
4. EXPERIMENTAL SECTION
4.1. Materials and Instrumentation. All chemicals, if not
otherwise stated, were used as supplied. Water was purified
with a Milli-Q system (Millipore, Bedford, MA, USA). Absolute
acetonitrile was added as an internal standard for the enzymatic
reactions. Synthesis and characterization of the water-soluble
tin-chlorin photosensitizer (SnC) and the nucleotide co-factor
analogues BNAD+ and BNADH are described in detail in the
Supporting Information. The hydride transfer mediator
precursor compound Cp*Rh(bpy)Cl]Cl was prepared follow-
ing literature procedures.69 Disodium ethylene diaminoacetate
(Titriplex III) dihydrate salt p.a. (EDTA) was purchased from
Merck and triethanolamine (98%) from ABCR Chemicals.
Alcohol dehydrogenase (ADH) from Saccharomyces cerevisiae
(≥300 units/mg protein) was purchased from Aldrich, as well
as β-nicotine amide adenine dinucleotide hydrate from yeast
(NAD+).
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For UV-vis-NIR absorption measurements up to 1100 nm, a
Varian Cary 50 spectrophotometer was used. Luminescence
spectra were recorded with a Horiba Jobin Yvon FluoroLog 3
modular spectrofluorometer equipped with two double-grating
monochromators. Irradiation experiments were carried out on
an optical bench in rectangular 1-cm quartz cuvettes. As the
light source for polychromatic irradiation experiments, a 150 W
xenon lamp (Osram, Model XBO 150W/1) was used with an
Oriel Newport universal arc lamp housing equipped with an
Aspherab UV-grade fused silica condenser lens and an AMKO
IR liquid filter filled with water (80 mm light pass). The power
supply was set to 100 W (5.6 A; 17.7 V) for all experiments. To
cut off short-wavelength light, suitable Schott long-pass glass
color filters were used: OG 530, OG 590 and RG 610,
respectively. For the determination of quantum yields and for
carrying out wavelength-selective irradiation experiments,
different 3 W light-emitting diodes (LEDs) were used with
λmax = 525, 592, or 623 nm. All quantum yields were measured
with a home-built setup calibrated against the ferrioxalate
actinometer, as described in detail in the Supporting
Information.
ments, and quantum yield determination procedures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Guenther.Knoer@jku.at.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to thank Evren Aricanli for additional
measurements. This manuscript is based on research supported
by the Austrian Science Fund (FWF Project No. P21045, “Bio-
inspired Multielectron Transfer Photosensitizers”). NMR
facility funding from the European Union through the EFRE
INTERREG IV ETC-AT-CZ program (Project No. M00146,
“RERI-uasb”) is gratefully acknowledged. G.K. also thanks the
German Research Foundation (DFG No. GRK1626, “Chemical
Photocatalysis”) for partial support of this work.
Room-temperature 1H and 119Sn NMR spectra were
recorded on a Bruker DRX 500 spectrometer operating at
500.13 MHz (1H) or at 186.4 MHz (119Sn). Chemical shifts are
either given in ppm, relative to residual solvent (H2O 4.7 ppm)
for 1H, or were measured using Sn(Ph)4 as a secondary
standard for 119Sn with all values reported relative to the
external reference Sn(Me)4 (δ = 0 ppm).
4.2. General Procedure for the Photoreduction of
Nucleotide Co-factors. All photocatalytic experiments were
performed at 298 K in 1-cm quartz cuvettes with screw caps
and a septum. The samples were purged with nitrogen or
argon, to remove most of the dioxygen from air, and were
constantly stirred with a magnetic stirrer. Unless otherwise
stated, a typical sample for an experiment had a volume of 3.2
mL, which consisted of an aqueous EDTA or TEOA solution
(10 mM−1 M) with 0.1 M sodium phosphate buffer (pH 8.8 or
7.4). The catalyst concentrations were 7.5 × 10−6 M SnC and
7.7 × 10−5 M [Cp*Rh(bpy)H2O]2+ (at pH 7.4) or [Cp*Rh-
(bpy)OH]+ (at pH 8.8), respectively. (B)NAD+ (1 mM) was
added to the solution, and the reaction progress was followed
spectroscopically.
ABBREVIATIONS
■
SnP = water-soluble tin(IV)-meso-tetrakis(N-methylpyridi-
nium)-porphyrin complexes of the type [(TMPyP)Sn(X)2]4+
with X = Cl or OH, derived at different pH from the starting
compound dichloro-(5,10,15,20-tetrakis(N-methylpyridi-
nium-4-yl)-porphyrinato)tin(IV)-tetrachloride
SnC = water-soluble tin(IV)-meso-tetrakis(N-methylpyridi-
nium)-chlorin complexes of the type [(TMPyC)Sn(X)2]4+
with X = Cl, or OH derived at different pH from dichloro-
(2,3-dihydro-5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-
porphyrinato)tin(IV)-tetrachloride
SnCH− = Tin(IV)-meso-tetrakis(N-methylpyridinium)-chlor-
in-phlorin-anion complex of the type [(TMPyCH)Sn(X)2]3+
with X = Cl or OH, derived by monodeprotonation of
dichloro-(2,3,10,23-tetrahydro-5,10,15,20-tetrakis(N-methyl-
pyridinium-4-yl)-porphyrinato)tin(IV)-tetrachloride
NADH = 1,4-dihydro nicotine amide adenine dinucleotide
NAD+ = nicotine amide adenine dinucleotide
BNADH = N-benzyl-1,4-dihydro nicotine amide
BNAD+ = N-benzyl-3-carbamoyl-pyridinium cation
Chl = chlorophyll
4.3. Catalytic Dark Reactions with NADH-Dependent
Enzymes. Samples for coupling artificial photosynthetic
NADH production to additional biochemical processes were
prepared in 1-cm quartz cuvettes sealed with a septum and
consisted of a 0.1 M EDTA solution in 0.1 M aqueous sodium
phosphate buffer (pH 8.8). The photosensitizer and co-catalyst
D = sacrificial electron donor
[Cp*Rh(bpy)Cl]Cl = chloro-[η5-(pentamethyl)-
cyclopentadienyl]-(2,2′-bipyridyl)-rhodium(III) chloride
[Cp*Rh(bpy)H]+ = Hydrido-[η5-(pentamethyl)-cyclopenta-
dienyl]-(2,2′-bipyridyl)-rhodium(III) cation
concentrations were 7.5 μM SnC and 7.7 × 10−5
M
[Cp*Rh(bpy)OH]+, respectively. To the argon-purged solution
(V = 3.2 mL), 2 mg of the NAD+ co-factor (1 mM), 10 μL each
of acetonitrile and butyraldehyde (35 mM), and 0.7 mg of
lyophilized yeast alcohol dehydrogenase (ADH, Aldrich) were
added. The sample was then irradiated under steady-state
conditions with orange-red light (590 nm cutoff filter; 298 K).
The reaction progress was followed and quantified by UV−vis
spectroscopy and GC measurements.
ADH = alcohol dehydrogenase from Saccharomyces cerevisiae
EDTA = ethylenediamine-tetraacetic-acetate sodium salts
TEOA = triethanolamine
HPLC = high-performance liquid chromatography
HRMS = high-resolution mass spectrometry
ESI = electrospray ionization
GC = gas chromatography
NIR = near infrared
ASSOCIATED CONTENT
■
S
* Supporting Information
Q-TOF = quadrupole time-of-flight mass spectrometer
TON = turnover number
Details on the synthesis of the compounds used, as well as
further spectroscopic and experimental data including more
information on HPLC/HRMS analytics, photolysis experi-
TOF = turnover frequency
U = unit of 1 μmol min−1 enzymatic activity
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Inorganic Chemistry
Article
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Inorganic Chemistry
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