Alternative photoinduced release of HNO or NO from an acyl nitroso compound, depending on environmental polarity

Article abstract of DOI:10.1039/c001502d

A hydrophilic hetero-Diels-Alder cycloadduct was synthesized as a novel photocontrollable donor of reactive nitrogen species. Production of either nitric oxide (NO) or nitroxyl (HNO) was photoinduced from this compound, depending on the environmental polarity.

Full text of DOI:10.1039/c001502d

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Alternative photoinduced release of HNO or NO from an acyl nitroso
compound, depending on environmental polarityw
Kazuya Matsuo, Hidehiko Nakagawa,* Yusuke Adachi, Eri Kameda, Hiroki Tsumoto,
Takayoshi Suzuki and Naoki Miyata*
Received 22nd January 2010, Accepted 19th March 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/c001502d
A hydrophilic hetero-Diels–Alder cycloadduct was synthesized
as a novel photocontrollable donor of reactive nitrogen species.
Production of either nitric oxide (NO) or nitroxyl (HNO) was
photoinduced from this compound, depending on the environ-
mental polarity.
between the dark condition and photoirradiated condition
(Fig. 2B), although conversion of compound B to the anthra-
cene derivative and p-nitroaniline was very largely increased
upon photoirradiation in photometric analysis (Fig. 2A). In
GC-MS analysis, compound B yielded little N2O compared
to compound A in the same reaction solvent, whereas
compounds A and B showed the accelerated effect on conversion
by photoirradiation in photometric analysis.
Nitric oxide (NO) is one of the most familiar reactive nitrogen
species (RNS). In the 1980s, NO was found to be biologically
synthesized in mammalian systems,1 and since then both the
chemical and biological properties of NO have been extensively
investigated. Recent studies have shown that nitroxyl (HNO),2
a one-electron-reduced form of NO, has many pharmacological
properties, including positive inotropy,3 vasodilation,4 and
cardioprotection.5 HNO exerts these effects via mechanisms
different from those of NO, and is a promising candidate for
treatment of heart failure.6 However, HNO has varied chemical
properties, including dimerization (N2O formation),7 high pKa
value,8 high thiophilicity,9 reductive nitrosylation,10 and unique
reactivity with phosphines.11 So, to elucidate the biological
potential of HNO in detail, novel HNO donors which can
release HNO under precise temporal and spatial control are
needed.
Therefore, we hypothesized that compound B might release
an RNS other than HNO in response to photoirradiation, and
we thus focused on NO.
When compounds A and B were irradiated with UVA for 10
min in 90% DMSO solvent, it was found that compound B
released NO, while compound A hardly did, as determined by
electron paramagnetic resonance (EPR) analysis using a typical
NO-selective trapping reagent, carboxy-2-phenyl-4,4,5,5-tetra-
methylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) (see ESIw)
We next examined the role of water. The NO release from
compound B in response to photoirradiation was measured by
the EPR spin trapping method in DMSO/50 mM Tris buffer
(pH 7.5) with various mixing ratios. Release of NO decreased
as the ratio of buffer was increased, that is, as the polarity
increased, and in 1/9 DMSO/50 mM Tris buffer (pH 7.5), NO
was not observed (Fig. 3) while in 9/1 DMSO/50 mM Tris
buffer (pH 7.5), the amount of NO (36.4 mM) released by
photoirradiation corresponded to about 90% of the conversion
(see ESIw)
King et al. reported that cycloadducts of N-hydroxyurea
derivatives and 9,10-dimethylanthracene (DMA) can act as
spontaneous HNO donors, though DMA itself is toxic.12 We
applied this basic reactivity to design photoinducible HNO
donors such as compound A, which can release HNO in
response to UV-A irradiation, although our donors have poor
hydrophilicity.13 To improve the hydrophilic character, we
designed and synthesized compound B, which consists of a
hydrophilic anthracene derivative14 and N-hydroxyurea
derivative (Fig. 1, Scheme 1). Compound B was found to be
well soluble (>100 mM) in buffer solution containing 0.1%
DMSO. It was expected that not only would the cycloadduct
be hydrophilic, but also the anthracene derivative would be
more easily excreted due to the increased water solubility so
that toxicity via metabolic bioactivation might be decreased.
During the characterization of compound B, we found that
the amounts of N2O formed were almost the same level
These observations suggested the involvement of HNO and
NO releasing pathways in the photoinduced conversion of
compound B (Scheme 2). In the HNO release pathway, it is
considered that an acyl nitroso compound would be released
Graduate School of Pharmaceutical Sciences, Nagoya City University,
3-1 Tanabe-dori, Mizuho-ku, Nagoya, Aichi 467-8603, Japan.
E-mail: deco@phar.nagoya-cu.ac.jp, miyata-n@phar.nagoya-cu.ac.jp;
Fax: +81-52-836-3408; Tel: +81-52-836-3408
w Electronic supplementary information (ESI) available: Complete
ref. 6a. Details of the synthesis of compounds A and B, and details
of their photometric, EPR, GC-MS and NMR analysis. See DOI:
10.1039/c001502d
Scheme 1 Synthesis of compound B.
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Fig. 1 Our photo-inducible HNO donors.
Scheme 2 Proposed mechanisms of HNO and NO release.
N2O in low-polarity solvent did not increase both under dark
condition and under photoirradiated condition (Fig. 2B),
time-dependent release of N2O was observed in this high
polarity solvent. When the amount of N2O under photoirradiated
condition was compared to that under dark condition, apparent
facilitation was observed upon photoirradiation (Fig. 4 and
ESIw). Therefore, compound B in 1/9 DMSO/Tris buffer
(pH 7.5) released HNO upon UVA irradiation.
Moreover, in both cases of compounds A and B, N2O was
not detected in the presence of 2-mercaptoethanol in the
reaction solvent (see ESIw) This indicates that HNO was
trapped by 2-mercaptoethanol, thus precluding N2O formation.
From these results, it was considered that compound B is
relatively stable, and that HNO release depends at least in part
on the concentration of surrounding nucleophiles, such as
H2O. When the concentration of nucleophile is sufficiently
high that nucleophilic attack on the acyl nitroso compound is
rapid, hydrolysis occurs before photolysis, resulting in HNO
release. On the other hand, in low-polarity solvent, the acyl
nitroso compound derived from compound B is homolytically
cleaved by photoirradiation to produce NO before it can be
attacked by a nucleophile. Another possible way of releasing
NO from compound B is by homolytically cleavage followed
by retro-Diels–Alder reaction.
Fig.
2
(A) Conversion in photometric analysis and (B) N2O
formation in GC-MS analysis of compounds A and B in 90% DMSO
solvent.
Fig. 3 Detection of NO release from compound B by EPR with
carboxy-PTIO in (a) 9/1 DMSO/50 mM Tris buffer (pH 7.5), and (b)
1/9 DMSO/50 mM Tris buffer (pH 7.5).
Importantly, since this characteristic was not observed in
compound A, there must be an additional factor, other than
by photo-triggered retro-Diels–Alder reaction, and attacked
by water with release of HNO, in the same manner as in
thermal decomposition. This pathway was found to occur in
high-polarity solvent in our experiment.
The other pathways are concerned with NO release. One
possible pathway is that an acyl nitroso compound would be
released by photo-inducible retro-Diels–Alder reaction, and
homolytic cleavage of the C–N bond adjacent to the carbonyl
group would occur by photoirradiation before the attack of
nucleophile, resulting in NO release. Another possible route is
that photoirradiation induces N–C(O) bond homolysis to
produce the acyl radical plus the Diels–Alder adduct of NO
with the anthracene derivative. These pathways were observed
in low-polarity solvent.
Next, we measured N2O release in 1/9 DMSO/50 mM Tris
buffer by means of GC-MS analysis. While the amounts of
Fig. 4 GC-MS analysis of N2O formation from compound B in
1/9 DMSO/Tris buffer (pH 7.5).
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The release of HNO/NO from compound B is controllable
by varying the UVA exposure and solvent polarity. The
present findings should also be helpful to improve HNO donor
design.
This work was supported in part by Grants-in-Aid for
Scientific Research on Innovative Areas (Research in a
Proposed Research Area) (No. 21117514 to H. N.) from
the Ministry of Education, Culture, Sports Science, and
Technology, Japan, and
Foundation (H. N.).
a grant from Takeda Science
Notes and references
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Fig. 5 Evidence of intramolecular hydrogen bonding in NMR spec-
tra of compound B in 9/1 DMSO/Tris buffer at various pH values.
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nucleophile concentration, which further slows the nucleo-
philic attack of H2O in low-polarity solvent.
1H NMR analysis indicated the possibility of intramolecular
hydrogen bond formation15 involving the NH group in
compound B (Fig. 5). The NH signal in 9/1 DMSO/buffer
was broadened and there was a downfield chemical shift at
neutral to basic pH. In this pH range, the stable structure of
compound B should be the dianionic form, and the existence
of a hydrogen bond between the carboxylate and NH group
was also reasonable as ascertained from the result of DFT
calculation of a stable conformation with Spartan ’08 (see
ESIw). Furthermore, hydrogen bond formation was also
strongly suggested from the dramatic chemical shift change
of the NH signal in an NMR titration using tetrabutylammonium
acetate (TBAOAc)16 (see ESIw).
7 D. A. Bazylinski and T. C. Hollocher, Inorg. Chem., 1985, 24,
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anthracene moiety are associated through a hydrogen bond,
which might serve to increase the Diels–Alder cycloadduct,
contributing to the unexpected stability of compound B in 9/1
DMSO/Tris buffer solvent.
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It is assumed that the acyl nitroso moiety would be in close
proximity to the anthracene moiety after retro-Diels–Alder
reaction, and this would sterically inhibit the nucleophilic attack
of water, and facilitate the hetero-Diels–Alder cycloaddition.
Such a proximity effect would not occur in compound A.
Thus, an intramolecular hydrogen bond is suggested to
configure the molecular properties for photoinduced conversion,
and this hydrogen bond would contribute to both the stability
of compound B and NO production in response to UVA
irradiation. A high content ratio of Tris buffer would favor
solvation of the carboxylate anion, weakening the intra-
molecular hydrogen bond. For this reason, HNO is dominantly
photo-released from compound B in 1/9 DMSO/Tris buffer.
Many NO donors17 and several HNO donors2 have been
developed, but this is the first report of a photoinducible donor
that can release either HNO or NO, depending on the polarity
of the solvent. Further, this is the first report that an acyl
nitroso compound can release NO by photodecomposition,
though N-hydroxyurea was reported to release NO via an
oxidation reaction.18
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17 Many NO donors have reported; some examples of developed NO
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Z. Wen, T. Cai and A. J. Janczuk, Chem. Rev., 2002, 102, 1091;
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18 S. B. King, Curr. Top. Med. Chem., 2005, 5, 665.
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This journal is The Royal Society of Chemistry 2010
3790 | Chem. Commun., 2010, 46, 3788–3790