Conversion of nitrite to nitric oxide at zinc via S-nitrosothiols

Article abstract of DOI:10.1039/c3cc46102e

Nitrite is an important reservoir of nitric oxide activity in the plasma and cells. Using a biomimetic model, we demonstrate the conversion of zinc-bound nitrite in the tris(pyrazolyl)borate complex ^iPr2TpZn(NO ₂) to the corresponding S-nitrosothiol RSNO and zinc thiolate ^iPr2TpZn-SR via reaction with thiols H-SR. Decomposition of the S-nitrosothiol formed releases nitric oxide gas.

Full text of DOI:10.1039/c3cc46102e

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Conversion of nitrite to nitric oxide at zinc
via S-nitrosothiols†
Cite this: Chem. Commun., 2014,
0, 168
5
Allan Jay P. Cardenas, Rebecca Abelman and Timothy H. Warren*
Received 9th August 2013,
Accepted 27th October 2013
DOI: 10.1039/c3cc46102e
www.rsc.org/chemcomm
Nitrite is an important reservoir of nitric oxide activity in the plasma and
cells. Using a biomimetic model, we demonstrate the conversion of
iPr2
zinc-bound nitrite in the tris(pyrazolyl)borate complex
TpZn(NO
2
) to
Scheme 1
iPr2
the corresponding S-nitrosothiol RSNO and zinc thiolate
TpZn–SR
via reaction with thiols H–SR. Decomposition of the S-nitrosothiol
2
1
formed releases nitric oxide gas.
of disulfides. In general, copper ions are particularly effective in
22,23
promoting release of NO from RSNOs (Scheme 1).
In 2009, Fago showed that that carbonic anhydrase (CA) can
Nitric oxide (NO) is an endogenous molecule connected to many
24
important biological processes such as vasodilation, neurotransmission generate nitric oxide from nitrite. Carbonic anhydrase is one of the
1
25
and cytoprotection. Although NO is ubiquitous, its lifetime in fastest enzymes known, responsible for maintaining the blood pH
2+
biological media is very short due to its ready oxidative meta- by converting carbon dioxide to bicarbonate at a His
3
Zn active
2
25
bolism to nitrite and nitrate. Thus, reservoirs of NO activity such site. Due to the rough structural similarity between bicarbonate
2
–5
6–10
as nitrite and S-nitrosothiols
in physiological processes connected to nitric oxide.
Nitrite serves as an important, air-stable reservoir for nitric oxide role in NO production from nitrite via dehydration of acidified
are of significant importance and nitrite and the ability of CA to bind nitrite which inhibits CO
2
24
hydration, Fago suggested that CA could also play an important
2
–5,11,12
24
3
26
activity.
Its concentration in human plasma and red blood nitrite (HONO) to N
2
O
2
which readily homolyzes to NO and NO .
cells is tightly regulated in the ranges of 0.121 Æ 0.009 mM and In the absence of such a dehydration–disproportionation sequence,
1
3
0
.288 Æ 0.047 mM, respectively. Conversion of nitrite to NO however, reduction of nitrite to nitric oxide at redox inactive zinc
requires 1-electron reduction that can be accomplished by several requires use of an external reductant.
metalloenzymes such as globins, xanthine oxidoreductase and Herein we employ zinc tris(pyrazolyl)borate complexes that
2,11,14–16
2+
cytochrome c oxidase under hypoxic conditions.
other hand, nitrite can be converted to S-nitrosothiols RSNO with centers in many zinc enzymes to examine the reaction of thiols
3
On the possess a similar coordination environment as found at His Zn
2
7
thiols H–SR under conditions of high acidity in a redox neutral at zinc-bound nitrite. We have previously studied the reactivity of
1
7
+
reaction. Such interconversion is thought to be particularly RSNOs, NO, and NO at the zinc thiolate linkage employing the
16
iPr2
28
important in the gut. Moreover, formation of RSNOs from nitrite thiolate derivatives
TpZn–SR.
We found that RSNOs
represents a pathway for ultimate NO generation since RSNOs are undergo rapid transnitrosation reactions, formally transferring
+
+
prone to NO loss due to their modest RS–NO bond strengths NO between zinc and NO -bound thiolate residues. Moreover,
À1 18–20
+
ranging from 20–32 kcal mol
.
While this process may take reaction with NO resulted in Zn–SR cleavage to form RSNO
place thermally, enzymes such as CuZn superoxide dismutase while NOgas showed no reactivity with the Zn–SR unit.
2
8
iPr2
28
catalyze the release of NO from RSNOs with concomitant formation
Reaction of
Ar = p-MeC
the corresponding zinc thiolate
TpZn(NO (1) with the aromatic thiol H–SAr
2
)
(
6
H
4
) at RT in CH Cl results in rapid conversion of 1 to
2 2
iPr2
TpZn–SAr (2) (Scheme 2). Com-
Department of Chemistry, Georgetown University, Box 571227-1227, Washington,
D.C. 20057, USA. E-mail: thw@georgetown.edu; Fax: +1 202 687 6209;
Tel: +1 202 687 6362
pound 2 can be isolated in 70% yield from pentane as colorless
crystals. The X-ray structure of 2 (Fig. 1) is similar to that of several
28–30
related TpZn-thiolates
with a Zn–S distance of 2.2310(6) Å and
†
Electronic supplementary information (ESI) available: Experimental details,
Zn–N distances that span 2.0332(19)–2.0487(19) Å. These distances
are very similar to those reported by Lippard for a family of tris-
characterization of new compounds, and crystallographic data for complexes 2
and 4 CCDC 953113 and 953114. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc46102e
Ph,Me
30
(3-phenyl-5-methylpyrazolyl)borate zinc arylthiolates
TpZn–SAr.
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Scheme 3
Scheme 2
iPr2
acid–base reaction between
2
TpZn(NO ) (1) and H–SR could give
iPr2
TpZn–SR and HONO (Scheme 3a). The HONO thus generated
could react with a second equivalent of H–SR to give RSNO and
H O. Alternatively, H–SR could directly react with the zinc-bound
17
2
iPr2
iPr2
nitrite anion in
TpZn(NO ) to give
TpZn–OH and RSNO.
TpZn–OH would be expected
2
TpZn–SR and H O under these conditions
2
32 iPr2
The highly reactive zinc-hydroxide
iPr2
to rapidly give
2
9,30
(Scheme 3b).
While we do not have unambiguous data that unequivocally
suggest one pathway over the other, at this stage we believe that
RSNO and zinc thiolate formation may occur via an acid–base
reaction (Scheme 3a). Since the pK ’s of HONO and HOC(O)Me are
a
33,34
similar (3.3 and 4.7 in water)
the zinc acetate complex TpZn(OAc) (4). This zinc acetate complex
we examined the thiol reactivity of
iPr2
iPr2
4
2H
2
may be isolated in 73% yield by addition of
O in methanol (Scheme 4). The X-ray structure of 4 shows highly
unsymmetrical binding of the acetate ligand with Zn–O distances of
.9385(17) and 2.578(2) Å (Fig. 2), similar to other TpZn(OAc) com-
TpK to Zn(OAc)
2
Á
iPr2
Fig. 1 X-ray structure of
TpZn–SAr (2).
1
A green color also develops upon reaction of 1 with the
35
iPr2
plexes. Interestingly, this species bears semblance to
2
TpZn(NO )
aromatic thiol H–SAr which indicates formation of the corresponding
S-nitrosothiol ArSNO. The intensity of this green color decreases
31
(1) which also exhibits highly unsymmetrical binding of the nitrite
28
ligand with Zn–O distances of 1.981(2) and 2.430(2) Å.
over time due to the decomposition of the thermally unstable
iPr2
36
18
Reaction of
a
TpZn(OAc) (4) with H–SAr (pK = 6.8) and
aromatic S-nitrosothiol to the corresponding disulfide ArS–SAr.
3
7
iPr2
H–SCH Ph (pK = 9.4) in CDCl3 (10.8 and 15.4 in DMSO,
2
a
After 10 min at RT, addition of 2.3 eq. H–SAr to
2
TpZn(NO ) (1)
in CDCl3 resulted in a 45% yield of ArSNO (d 2.47 ppm for
iPr2
p-Me signal) and a 79% yield of the zinc thiolate
(
TpZn–SAr (2)
1
d 2.23 ppm for p-Me signal) as monitored by H NMR spectroscopy.
The disulfide ArS–SAr was also observed (d 2.31 ppm for p-Me signal),
representing the thermal decomposition product of the corres-
ponding S-nitrosothiol ArSNO. After 24 h, no ArSNO was present as
it was completely converted to the disulfide ArS–SAr observed in 86%
overall yield (Scheme 5).
Scheme 4
The qualitative rate of reaction of 1 with thiols appears highly
dependent on the nature of the thiol. Addition of the primary alkyl
2 3
thiol H–SCH Ph (2.3 eq.) to 1 under similar conditions in CDCl
results in a slow conversion of 1 to the corresponding zinc-thiolate
iPr2
28
TpZn–SCH
2
Ph (3) (Scheme 2). After 1 h, the reaction afforded a
iPr2
3
2% yield of 3 as judged by observation of the
TpZn–SCH
benzylic H NMR resonance at d 4.03 ppm. Stirring for 24 h gave
an 86% yield of 3 after which time the S-nitrosothiol PhCH SNO
d 4.68 ppm) and disulfide PhCH S–SCH Ph (d 3.60 ppm) were
2
Ph
1
2
(
2
2
present in 50% and 12% yields, respectively. Water was also observed
in this transformation (64% yield). The especially bulky thiol
iPr2
3 2
H–SCPh did not react with TpZn(NO ) (1) under these conditions.
We considered two possible mechanisms in the conversion
of zinc-bound nitrite to S-nitrosothiol (Scheme 3). In the first, Fig. 2 X-ray structure of iPr2TpZn(OAc) (4).
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À1 20
(
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THW is grateful for financial support of this work from the
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170 | Chem. Commun., 2014, 50, 168--170
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