Light-driven artificial enzymes for selective oxidation of guanosine triphosphate using water-soluble POSS network polymers

Article abstract of DOI:10.1039/c4ob01115e

The light-driven artificial enzymes were constructed to realize unnatural reactions concerning bio-significant molecules. In this manuscript, the guanosine triphosphate (GTP)-selective oxidation is reported using the network polymers composed of polyhedral oligomeric silsesquioxane (POSS). We synthesized the water-soluble POSS network polymer containing the naphthyridine ligands to capture GTP inside the networks and the ruthenium complexes to oxidize the captured GTP under light irradiation. Initially, the binding affinities of the guanosine nucleosides to the naphthyridine ligand inside the POSS network polymer were evaluated from the emission quenching experiments. Accordingly, it was observed that the naphthyridine ligand can form the stable complex only with GTP (K_a = 5.5 × 10⁶ M^-1). These results indicate that only GTP can be captured by the network polymer. Next, the photo-catalytic activity of the ruthenium complex-modified POSS network polymer was investigated. Finally, it was revealed that the network polymer can decompose GTP efficiently under light irradiation. This is the first example, to the best of our knowledge, to offer not only the GTP-selective host polymers but also the light-driven artificial enzyme for GTP oxidation. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob01115e

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Biomolecular Chemistry
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Light-driven artificial enzymes for selective
oxidation of guanosine triphosphate using
water-soluble POSS network polymers†
Cite this: Org. Biomol. Chem., 2014,
12, 6500
Jong-Hwan Jeon, Kazuo Tanaka and Yoshiki Chujo*
The light-driven artificial enzymes were constructed to realize unnatural reactions concerning bio-signifi-
cant molecules. In this manuscript, the guanosine triphosphate (GTP)-selective oxidation is reported
using the network polymers composed of polyhedral oligomeric silsesquioxane (POSS). We synthesized
the water-soluble POSS network polymer containing the naphthyridine ligands to capture GTP inside the
networks and the ruthenium complexes to oxidize the captured GTP under light irradiation. Initially, the
binding affinities of the guanosine nucleosides to the naphthyridine ligand inside the POSS network
polymer were evaluated from the emission quenching experiments. Accordingly, it was observed that the
naphthyridine ligand can form the stable complex only with GTP (Ka = 5.5 × 106 M−1). These results indi-
cate that only GTP can be captured by the network polymer. Next, the photo-catalytic activity of the
ruthenium complex-modified POSS network polymer was investigated. Finally, it was revealed that the
network polymer can decompose GTP efficiently under light irradiation. This is the first example, to
the best of our knowledge, to offer not only the GTP-selective host polymers but also the light-driven
artificial enzyme for GTP oxidation.
Received 30th May 2014,
Accepted 25th June 2014
DOI: 10.1039/c4ob01115e
www.rsc.org/obc
Guanosine nucleosides, particularly guanosine triphos-
phate (GTP), concern various significant biological events such
Introduction
The development of artificial enzymes is the topic of high rele- as signal transduction, metabolism, and enzymatic reactions.
vance in biotechnology. Many researchers have aimed to estab- The recognition of guanine derivatives is applicable for devel-
lish the systems not only for emulating the biological systems oping biosensors, drugs, and biotechnological tools for moni-
but also for realizing unnatural reactions concerning bio- toring these significant biological events.3 In particular, the
molecules and biological events.1 These sophisticated systems modulation of the local concentration of guanosine nucleo-
are applied in various fields such as energy generation, sides is a key technology for the regulation of biological
sensors, and molecular machines.1 Based on the bottom-up events. For example, since GTP is one of the substrates of RNA
approach, the functional units are organized according to the polymerases and DNA primases, the suppression of the GTP
preprogrammed design, resulting in the cooperative functions concentration can be applied for the inhibition of nucleic acid
with each part. Therefore, the development of the parts and productions.4 Such a system can be used for obtaining anti-
the assembling protocol is of significance. In addition, by cancer or -virus drugs. In vital bodies, the concentrations of
incorporating the light-driven mechanism into the systems,2 GTP are regulated restrictedly by the series of GTP-related
we can have several advantages: the time- and site-specificity enzymes.5 There are classes of enzymes that can cause both
can be readily received by modulating the timing and spot of GTP degradation and production. Moreover, most of the GTP
light irradiation. Moreover, photo-activation can be achieved degradation enzymes can catalyze the hydrolysis at the tripho-
without further artifacts. Thus, light-driven artificial enzymes sphate moiety to convert to nucleoside, monophosphate, or
are promised to be an efficient tool to control the biological other phosphate ester species.5 Thereby, even if we can over-
events more precisely.
express these GTP degradation enzymes via hydrolysis in the
cells, it is difficult to induce irreversible suppression of the
GTP level at the local sites because of the homeostatic regu-
lation by the GTP synthetic enzymes. We proposed the strat-
egies for reducing the GTP concentrations at the local site via
the direct decomposition into the guanine base irreversibly.
It has been reported that a lot of biomolecules such as
proteins, fatty acids, and nucleic acids produce the crowding
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: chujo@chujo.synchem.kyoto-u.ac.jp; Fax: (+81) 75-383-2605;
Tel: (+81) 75-383-2604
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob01115e
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circumstances in the cell.6 In such environments called mole- guanosine diphosphate (GDP) can form the complex with the
cular crowding, various significant differences in the mole- naphthyridine ligand, and the binding affinity with GTP was
cular recognition are observed from the buffer solutions; 104 times larger than those with GDP. Next, the photo-
the binding affinity of the complexes via the hydrogen bonds triggered guanine oxidation was performed in the presence of
is enhanced under the relatively hydrophobic circumstances the POSS network polymer containing the Ru complex.11 From
created by the crowding of proteins or lipids comparing to that the evaluation of the consumption rates of guanosine deriva-
in the buffer.6 By mimicking molecular crowding with arti- tives by the photo irradiation, we found that only GTP can be
ficial systems and by applying to the molecular recognition efficiently decomposed by the POSS network polymer. This is
system, we have recently established the selective encapsula- the first example, to the best of our knowledge, not only
tion of GTP into the polyhedral oligomeric silsesquioxane to offer the polymeric host materials for GTP-selective recog-
(POSS)-core dendrimer via the complex formation with the nition but also to construct the light-driven artificial enzyme
naphthyridine derivatives.7 At the surface of the hydrophobic for guanine oxidation.
POSS core8 inside the dendrimer, the complex stability
via hydrogen bonds9 between the guanine moiety of GTP and
the naphthyridine derivative was significantly improved.7 In
addition, the negatively charged compounds such as tripho-
Experimental
General
sphates made a strong interaction with the ammonium groups
1H NMR and 13C NMR spectra were measured with a JEOL
at the surface dendrimer. As a result, it was observed that only
EX-400 (400 MHz for 1H and 10 MHz for 13C) spectrometer.
GTP can form the stable complex via hydrogen bonds with the
29Si NMR spectra were measured with a JEOL JNM-A400
naphthyridine derivative10 inside the POSS-core dendrimer.
(80 MHz) spectrometer. Coupling constants (J value) are
Finally, we observed the selective encapsulation of GTP by the
naphthyridine–POSS-core dendrimer complex.
We were inspired by the application of the selective encap-
sulation with the POSS-based polymeric materials as a recog-
reported in hertz. The chemical shifts are expressed in ppm
downfield from tetramethylsilane, using residual chloroform
1
(δ = 7.24 in H NMR, δ = 77.0 in 13C NMR) or residual DMSO
1
(δ = 2.49 in H NMR, δ = 39.5 in 13C NMR) as an internal stan-
nition unit for the artificial enzyme. Herein, we report the
dard. MASS spectra were obtained on a JEOL JMS-SX102A.
light-driven artificial enzyme for GTP oxidation utilizing the
Emission from the samples was monitored using a Perkin
water-soluble POSS network polymer (Fig. 1). The POSS-based
Elmer LS50B at 25 °C using 1 cm path length cell. MASS
water-soluble network polymer involving the naphthyridine
spectra were obtained on a JEOL JMS-SX102A. The detailed
ligand and the Ru complex was synthesized. Initially, from the
procedures and characterization data of the materials are
titration experiments with the series of guanosine nucleosides,
shown in ESI.†
the selectivity in the encapsulation for each guanosine nucleo-
Encapsulation of nucleoside derivatives7
side by the POSS network polymer in the buffer solution was
evaluated from the magnitude of binding affinity to the
The general procedure for the complexation of guanosine
naphthyridine ligand. Accordingly, it was shown that GTP and
nucleosides by the POSS network polymers is described here.
Stock solutions of the POSS network polymers (×10) and each
guanosine nucleoside (×10) were mixed at room temperature,
and then 500 μL of the samples were prepared by adding the
PBS buffer (pH 7.4) solution. The complexation with POSS-Ru
was also prepared with the same procedure in PBS buffer solu-
tion containing potassium ferricyanide (K3[Fe(CN)6]) as an
oxidizer for the Ru complex.11
Fluorescence measurements of the complexes
The fluorescence emission of the naphthyridine ligand (20 μM
with POSS-N and 10 μM with POSS-Ru) in the presence and
absence of the guanosine nucleosides with the excitation light
at 424 nm was monitored using a Perkin Elmer LS50B at 25 °C
using 1 cm path length cell. The excitation and the emission
bandwidth were 1 nm. The quantum yields were determined
as an absolute value with an integral sphere.
Fig. 1 Schematic illustration of the photo-triggered oxidation to the
Stern–Volmer plots7
guanine derivative in the POSS network polymer. Initially, GTP is cap-
tured by the POSS network polymer (POSS-Ru), and the complex for-
The data were analyzed with the Stern–Volmer equation:
mation between the naphthyridine ligand and the guanine base
occurred via hydrogen bonds. By the photo irradiation, the guanine
moiety can be decomposed by the photo-oxidation with the Ru
complex inside the network polymer.
I0
¼ 1 þ KSV½Qꢀ
ð1Þ
I
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guanine moiety in GTP.7,10 The ammonium groups are intro-
duced to maintain the water-solubility and discriminate the
triphosphate unit via the electrostatic interaction. The POSS
core is expected to provide hydrophobic spaces inside each
POSS unit in the networks, leading to the enhancement of
binding affinity of the naphthyridine moiety to the guanosine
nucleosides via hydrogen bonding in aqueous solution.8 The
Ru complex was used as a photo-catalyst in the guanine oxi-
dation under light irradiation.2 To induce the recognition and
the oxidation tandemly, the naphthyridine ligands and the Ru
complexes were introduced into the same POSS core. In pre-
vious reports, the increase of the crosslinking ratios which can
be modulated by the feed ratio of the cross linker caused the
decrease of the water-solubility and the increase of the encap-
sulation ability.8 Thus, we maximized the cross-linking rate
without loss of water-solubility.
Synthesis of the POSS network polymers is presented in
Scheme 1. The naphthyridine ligand and the bipyridine ligand
which are scaffolds for the Ru complexation were introduced
into octa-amino POSS to gather the recognition and the
decomposition units. The number of the tethered molecules
were determined from the integration ratios at the propyl
chains of octa-amino POSS in 1H NMR spectra (POSS core–
bipyridine–naphthyridine = 1 : 1.2 : 0.2). In the presence of the
modified POSS, octa-ammonium POSS, the condensation
reagent and the cross-linker, the network formation proceeded
(octa-amino POSS–modified POSS = 5.2 : 1). It was estimated
that the 2.3 of amino groups in 8 vertices of a single POSS core
was cross-linked by the linkers in the network. The products
were obtained as an orange powder from the reprecipitation in
acetonitrile containing 0.1% hydrochloric acid. To compare
the encapsulation ability of the POSS network polymers, we
also prepared the polymer without the Ru complex. All pro-
ducts showed good solubility in the PBS buffer as well as in
water. Precipitation or color changes were hardly observed
after two weeks at room temperature in the dark. Hence, we
conclude that the POSS network polymers used in this study
can have enough stability for further measurements.
Fig. 2 (a) Emission changes of 20 μM naphthyridine ligands in POSS-N
by adding GTP (0 to 400 μM) in PBS buffer solution (pH 7.4) at 25 °C.
Excitation wavelength was 364 nm. (b) Stern–Volmer plots with the
solution containing 20 μM naphthyridine ligand in POSS-N by adding
various kinds of guanosine nucleosides.
The emission intensities were plotted according to a Stern–
Volmer equation, reporting I0/I versus the concentrations of
nucleosides [Q], where I0 is the intensity in the absence of
nucleosides and I is the intensity in the presence of a nucleo-
side concentration. KSV is the Stern–Volmer quenching con-
stant. A plot of I0/I versus [Q] yields an intercept of one and a
slope equals KSV.
Binding constant calculation7
Initially, the encapsulation behaviors of the POSS network
polymer with nucleosides were investigated according to the
Cywinski’s method.10 We measured the changes of fluore-
scence emission from the naphthyridine ligand in POSS-N
by adding the series of guanosine nucleosides (Fig. 2). In pre-
vious reports, it was demonstrated that the fluorescence emis-
sion of the naphthyridine ligand can be quenched by the
complexation with the nucleobases via the hydrogen bonds.7,10
The emission spectra were monitored in the PBS buffer solu-
tion (pH 7.4) containing 20 μM of the naphthyridine ligand in
POSS-N and various concentrations of guanosine nucleosides
(0 to 400 μM) with the excitation light at 364 nm (Fig. 2a).
Correspondingly, the strong emission with the peak around
According to the non-emissive complex formation between the
naphthyridine ligand and guanosine nucleosides, the binding
constant (KA) can be calculated with the number of the
binding molecules (n) from the following equation:
I0 ꢁ I
log
¼ log KA þ n log ½Qꢀ
ð2Þ
I
Fig. 2 represents the plots for evaluating the KA values of
the naphthyridine ligands to GDP and GTP.
Results and discussion
The schematic illustration of the chemical structure of the 424 nm was observed from the naphthyridine ligand in
POSS network polymers and the photo-triggered GTP POSS-N in the absence of guanine nucleosides. The addition
decomposition is shown in Fig. 1. The network polymer is of GTP induced the significant quenching of the emission
composed of the combination of the four components: The from the naphthyridine ligand. In addition, the emission of
naphthyridine ligand is employed for the recognition of the the naphthyridine ligand gradually decreased by the addition
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Scheme 1 Synthesis of the POSS network polymers. Reagents and conditions: (a) bipyridine ligand, DMT-MM, triethylamine, MeOH, r.t., 1 d, 79%;
(b) Npt ligand, DMT-MM, triethylamine, DMSO, r.t., 1 d, 43%; (c) octaammonium-POSS, oxalic acid, MeOH, r.t., 1 d, 57%; (d) RuII(bpy)2Cl2, water,
reflux, 12 h, 93%.
Table 1 Optical properties and the binding constants of the ligand 1a
KSV × 103 c
of up to 400 μM GTP. Finally, the emission was mostly
quenched to 15% by adding 20 eq. of GTP. These data rep-
resent that the naphthyridine ligand can recognize GTP by the
complexation via the hydrogen bonds.
Φb
[mol L–1
]
nd
KA d [M–1
]
TONf [h−1
]
None
rG
GMP
cGMP
GDP
GTP
0.35
0.35
0.35
0.34
0.29
0.22
n.d.e
n.d.e
n.d.e
n.d.e
1.33 0.18
12.05 1.07
n.d.e
n.d.e
3.2 1.3
1.0 0.6
2.7 1.0
2.3 0.7
2.2 0.9
7.6 0.7
Next, to investigate the selectivity of the complexation, the
titration was performed with other nucleosides. It was found
that the addition of guanosine (rG), guanosine monophos-
phate (GMP), and cyclic guanosine monophosphate (cGMP)
can slightly influence the emission properties of the naphthyr-
idine ligand.11 Even in the presence of excess amounts of
these nucleosides (20 eq.), less significant changes were
observed in the emission spectra. In contrast, the addition of
GDP (20 eq.) induced the quenching of the emission by 41%.
These results indicate that only GDP can form the complex
with the naphthyridine ligand in POSS-N. In other words, GTP
and GDP can be encapsulated into the POSS network polymer.
Furthermore, it also indicated that the number of the phos-
n.d.e
n.d.e
n.d.e
n.d.e
n.d.e
n.d.e
0.91 0.22
1.83 0.19
580
5.5 × 106
a Procedures and conditions are described in the Experimental section.
b Determined as an absolute value. c Quenching constants were
determined with the slopes of the fitting line in the Stern–Volmer
plots. The errors mean a standard error. d Calculated according to ref.
7. e n.d.
= not determined because of too weak interaction.
f Determined from the slopes of the consumption rates. The errors
mean a standard error.
phate groups can significantly affect the complex formation of calculated.7 The optical properties, quenching constants, and
POSS-N with guanosine nucleosides. Since the ammonium the binding constants are listed in Table 1. All plots obtained
groups in POSS-N can make an electrostatic interaction with from the titration of GDP and GTP were fitted on the line, and
the phosphate groups, GDP and GTP can be retained in the the binding constants can be determined.7 It was found that
networks. We carried out the same experiment with adenosine POSS-N can enhance the binding affinity with GTP approxi-
triphosphate (ATP), and
a
slight emission change was mately 104 times larger than that with GDP. According to our
observed.12 It was confirmed that the hydrogen bonding is previous results, these results clearly indicate that the POSS
essential for the encapsulation into the POSS network network polymer can greatly enhance not only the binding
polymer.
affinity but also the selectivity for the GTP recognition.7 The
We prepared the Stern–Volmer plots with guanosine multiple POSS cores in the networks should generate the stron-
nucleosides as a quencher (Fig. 2b). Based on the formation of ger hydrophobic spaces,8 leading to the larger enhancement
a non-emissive complex model, the binding constants were effects of the encapsulation than the POSS-core dendrimer.
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Fig. 3 UV-vis spectral changes of 10 µM naphthyridine ligand in
POSS-Ru by adding 200 µM GTP with 100 µM K3[Fe(CN)6] in PBS buffer
solution (pH 7.4) at 25 °C. Photolysis was performed for 120 min at
25 °C with a 4 W UV-lamp (365 nm).
Fig. 4 The rates of the GTP oxidation with POSS-Ru. The amount of
the oxidized GTP was calculated from the normalized values determined
with the HPLC analyses with the sample containing RuII(bpy)3 and GTP
by monitoring the absorption change at 250 nm.
Photoreactions for the oxidation with the guanosine nucleo-
sides were performed with POSS-Ru. The sample solution con-
taining the POSS-Ru network (10 μM naphthyridine) and
100 μM potassium ferricyanide (K3[Fe(CN)6]) in PBS buffer
solution (pH 7.4) were added to 200 μM each guanosine
nucleoside, and the UV light (365 nm) from the transillumina-
tor was irradiated to the samples at room temperature. The
relatively-weak emission of POSS-Ru than that of POSS-N was
observed around 424 nm assigned as the fluorescence from
the naphthyridine ligand in the absence of GTP (Fig. 3). The
addition of 200 μM GTP immediately induced the quenching
of the emission. Corresponding to the above results with
POSS-N, the POSS-Ru network polymer should encapsulate
GTP. Since the emission decreased to the similar extent to
25% with POSS-N after adding GTP, it can be said that the Ru
complexes should cause less significant influence on the com-
plexation of the naphthyridine ligand. These data mean that
the naphthyridine ligand can recruit GTP near the Ru com-
plexes inside POSS-Ru.
To determine the reaction rate, the consumptions of guano-
sine nucleosides were monitored by UV-vis absorption
measurements. Before testing with the POSS network poly-
mers, for the evaluation of the amount of the oxidized GTP
quantitatively, we prepared the sample containing GTP and
RuII(bpy)3 in the absence of POSS-Ru, and the reactions were
monitored by the HPLC analysis. From the ratios of the peak
areas in the HPLC profiles, the reaction yields were calculated.
Accordingly, 55% of GTP in the mixture with free RuII(bpy)3
was oxidized by UV irradiation for 2 h in the absence of
POSS-Ru. This sample showed the significant changes in the
absorption spectra.13 The absorption bands around 250 nm
continuously increased by UV irradiation. These changes were
used as a standard to estimate the oxidative decomposition of
guanosine nucleosides. Fig. 4 shows the changes of absor-
bance obtained from the complexes of POSS-Ru with GTP by
the UV irradiation. Similarly to the RuII(bpy)3-containing
samples, the absorption bands were enhanced after the photo-
reaction. These results indicate that GTP should be decom-
posed by POSS-Ru. The oxidation of GTP gradually proceeded
corresponding with the increase of the irradiation time, and
finally it was found that 78% of the GTP was consumed after
irradiation for 2 h. These data clearly indicate that the
Ru-complexes in POSS-Ru should cause the oxidation of GTP.
Same photoreactions were performed with other nucleosides
such as rG, GMP, cGMP, and GDP. The oxidation was observed
with lower ratios than that with GTP.14 These results also
support that the Ru-complexes can selectively oxidize GTP in
POSS-Ru. Complexation between the naphthyridine ligand and
GTP contributes to the increase of the local concentration of
the substrate around the Ru complex, resulting in the efficient
oxidation.
It is proposed that the fluorescence emission of the
naphthyridine ligand should be recovered by the decompo-
sition of the guanine base in the complex. Based on this idea,
the emission properties of the naphthyridine ligands in
POSS-Ru with the peak at 424 nm were investigated. Fig. 5
shows the change of emission intensity observed from the
naphthyridine ligands at 424 nm by different intervals of time.
The emission intensity gradually increased by increasing the
UV irradiation time. This result suggests two issues: firstly, the
guanine moiety should be decomposed by the photoreactions.
Secondly, the decomposed products can no longer form the
complex with the naphthyridine ligand. The amount of the
decomposed GTP was calculated from the change of the emis-
sion intensity before and after the UV irradiation. Accordingly,
55% of GTP was oxidized by POSS-Ru after UV irradiation
for 2 h.
Turnover numbers per hour (TON) in the photoreactions
were calculated from the consumption rates of each nucleoside
(Table 1). The TON with GTP in the presence of POSS-Ru
showed the largest value (7.6 h−1) of the other nucleosides. In
particular, the TON can be improved by adding POSS-N to the
sample. Even with GDP, which can be encapsulated into
POSS-N, the TON value was similar to other guanosine nucleo-
sides. These results clearly indicate that POSS-Ru can be
a light-driven catalyst for GTP oxidation. Interestingly, the
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with the naphthyridine ligands to capture GTP inside the net-
works and the ruthenium complexes to oxidize the captured
GTP under light irradiation were prepared. Initially, the
naphthyridine-modified POSS network polymer showed highly
selective binding to GTP in the series of guanosine derivatives.
Accordingly, the complex with GTP showed significantly large
binding affinity (Ka = 5.5 × 106 M−1) with the naphthyridine
ligands in the network polymers. Next, the photo-catalytic
activity of the complexes with GTP and the Ru complex-modi-
fied POSS network polymers was investigated. Consequently, it
was observed that the modified network polymers can decom-
pose GTP efficiently under light irradiation. By applying the
special spaces created by the POSS core in water for the mole-
cular recognition, we can offer not only the host polymers for
the selective capturing to the significant biomolecule with
high affinity but also the light-driven artificial enzymes for
unnatural bioreaction, GTP oxidation.
Acknowledgements
This work was partially supported by the Kato Memorial Bio-
science Foundation (for K.T.) and a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (no. 2401)” (25102521) of The Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
Fig. 5 (a) Emission changes of 10 µM naphthyridine ligands in POSS-Ru
by adding 200 µM GTP with 100 µM K3[Fe(CN)6] in PBS buffer solution
(pH 7.4) at 25 °C. Excitation wavelength was 364 nm. Photoreaction was
performed for 120 min at 25 °C with a 4 W UV-lamp (365 nm). (b) The
profile was obtained in PBS buffer solution for GTP in the POSS-Ru/GTP
complex under UV irradiation.
Notes and references
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decomposed products should be released during the reaction
because the turnover can be observed. Moreover, compared to
the TON value in the absence of POSS-Ru (3.2 h−1), the other
values except with GTP can be suppressed. This fact represents
that the Ru complexes can induce the reaction mainly inside
the network polymers. According to these data, we conclude
that POSS-Ru should act similarly as an enzyme: the target can
be recognized with high selectivity and can react only inside
the POSS network polymer. Finally, the products should be
released after the reaction. Furthermore, these tandem reac-
tions can proceed only under the light irradiation. To quanti-
tatively monitor the reaction with UV-vis absorption spectra,
we performed the decomposition reaction with the minimum
amount of K3[Fe(CN)6] because of the intrinsic light absorp-
tion. By increasing the concentration of K3[Fe(CN)6], TON
could be enhanced.
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Conclusions
We demonstrate the GTP-selective oxidation using the network
polymers containing the POSS units which can create the
hydrophobic spaces inside polymers like the molecular crowd-
ing state. The water-soluble POSS network polymers modified
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Organic & Biomolecular Chemistry
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