Peroxynitrite chemistry derived from nitric oxide reaction with a Cu(ii)-OOH species and a copper mediated NO reductive coupling reaction

Article abstract of DOI:10.1039/c3cc47942k

New peroxynitrite-copper chemistry ensues via addition of nitric oxide (NO_(g)) to a Cu^II-hydroperoxo species. In characterizing the system, the ligand-Cu(i) complex was shown to effect a seldom observed NO_(g) reductive cou

Full text of DOI:10.1039/c3cc47942k

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Peroxynitrite chemistry derived from nitric oxide
reaction with a Cu(II)–OOH species and a copper
mediated NO reductive coupling reaction†
Cite this: Chem. Commun., 2014,
50, 2844
Received 15th October 2013,
Accepted 27th November 2013
Sunghee Kim, Maxime A. Siegler and Kenneth D. Karlin*
DOI: 10.1039/c3cc47942k
www.rsc.org/chemcomm
4c,d,5
New peroxynitrite–copper chemistry ensues via addition of nitric oxide ꢀNO(g)
,
and superoxo–iron4h,i or –cobalt4j porphyrinates
(ꢀNO(g)) to a CuII–hydroperoxo species. In characterizing the system, the plus nitric oxide. Recent reports also describe peroxynitrite
ꢀ
ligand–Cu(I) complex was shown to effect a seldom observed ꢀNO(g) intermediates forming from NO(g) reaction with Cr–superoxo
reductive coupling reaction. Biological implications are discussed.
or peroxo species,4k or for aÀCu–superoxo complex.4f CuII–PN
complexes may decay to NO2 plus O2(g) (Scheme 1).4f,g
As indicated, the importance of peroxynitrite generation and
Reactive nitrogen species (RNS) have been of great interest due
to their important role in biomolecule oxidation such as in aging chemistry is in its biological role as a RNS. GivenÀthe chemistry
and disease development.1 It is RNS which induce tyrosine (just mentioned) implicating M–O2 (R Mn+1(O2 )) + ꢀNO(g) or
ꢀ
nitration in proteins, leading to damage and enzyme inactiva- M–NO + O2(g) in peroxynitrite generation and peroxynitrite-like
tion; a signaling role has been also discussed.1 Tyrosine nitration reactivity, we have previously suggested that such reactivity may
occurs in the presence of either peroxynitrite (ÀOONQO or its occur in biological systems, for example at the active sites of the
conjugate acid; here, PN is defined as either) or nitrogen dioxide superoxide dismutases of manganese6 or copper.7 Here, we
(ꢀNO2). PN is presumed to be generated in vivo by the diffusion describe the reaction of a CuII–hydroperoxo species with ꢀNO(g)
,
controlledꢀ combination of nitric oxide (ꢀNO) and superoxide a new but complementary chemical transformation in the
anion (O2 À),2 or from nitrite (NO2À) oxidation by peroxidase generation of PN chemistry. We suggest that this is quite
enzymes (Scheme 1). Nitrogen dioxide forms from the homolytic possibly a biologically relevant reaction.
O–O cleavage of PN, however most aqueous PN isomerizes to
The copper–hydroperoxo complex [(BA)CuII(OOH)]+ (2) (BA,
nitrate (NO3À).1,3 Metal ion PN chemistry3i,4 has developed a tetradentate N4 ligand possessing a pendant –N(H)CH2C6H5
greatly in just the last few years, and Scheme 1 also outlines group), which is stabilized by H-bonding from the ligand side-
recently established M–PN transformations.
arm N–H group to the proximal oxygen of the CuII–OOH moiety
8
ꢀ
Peroxynitrite generation is implicated from studies on (Fig. 1), reacts with excess NO(g) in acetone at À90 1C, result-
the reactions of oxy-heme (formally FeIII–O2 À) proteins with ing in a color change from bright green to a pale greenish blue.
ꢀ
Scheme 1
Department of Chemistry, The Johns Hopkins University, 3400 N. Charles Street,
Baltimore, MD 21218, USA. E-mail: karlin@jhu.edu; Fax: +1 410-516-8420;
Tel: +1 410-516-8027
† Electronic supplementary information (ESI) available: Details of synthesis,
reaction conditions and spectroscopic monitoring, crystallographic data for 3.
CCDC 965635. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3cc47942k
Fig. 1 [(BA)CuII(OOH)]+ (2), formed from precursor 1, reacts with ꢀNO
leading to the nitrato compound [(BA)CuII(NO3À)]+ (3) which can also be
generated by the reaction of (1) with TBANO3. EPR data of (1), (2) and (3)
are shown (X-band, n = 9.186 GHz; acetone at 70 K, also, see ESI†).
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À
ꢀ
O2 + ꢀNO, so if starting with peroxide (in 2) + ꢀNO, there is an
extra electron, resulting in cupric ion reduction to Cu(I) (eqn (1)).
II
+
I À
+
ꢀ
[Cu –OOH] + NO - [Cu ( OONQO)] + H
RCuI + H+ + ÀOONQO
(1)
(2)
À
CuI + H+ + ÀOONQO - CuI + NO3 + H+
Peroxynitrite could then isomerize to nitrate as observed
(eqn (2)). Precedent for this metal ion reduction in the formation
of peroxynitrite comes from the work of Mondal and coworkers.11
In an alternative approach but yet overall isoelectronic with our
present chemistry, they preformed a Cu(II)–nitrosyl (derived from
Fig. 2 UV-vis spectra of the reaction of [(BA)CuII–OOH]+ (2) with ꢀNO at
À90 1C in acetone under Ar.
The accompanying UV-vis bands attributed to 2 (393, 672 nm)
decrease in intensity, while a new feature at lmax = 824 nm
(e = 315) develops (Fig. 2).
ꢀ
a ligand Cu(II) compound plus NO(g)) and added H2O2, giving a
Cu(I)–nitrate complex, which was structurally characterized.
To provide further evidence that a PN moiety must be present
during the reaction, we carried out a reactivity study employing
2,4-di-tert-butylphenol (2,4-DTBP) as a potential substrate and
tyrosine mimic, see discussion and references above. When
2,4-DTBP (1 equiv.) is added to a À90 1C solution of [(BA)CuII–
OOH]+ (2), prior to ꢀNO(g) addition, and it is allowed to warm to RT,
workup leads to a 75–80% yield of nitrated phenol (2,4-di-tert-butyl-
6-nitrophenol) and a copper(II) complex (Scheme 2) (ESI†). Phenol
(e.g., tyrosine) nitration is normally considered to occur via peroxy-
nitrite homolytic O–O cleavage to give ꢀOH + ꢀNO2; the former (as a
Cu ion adduct?) would abstract a phenolic hydrogen-atom and the
latter would then add to the resulting phenoxyl radical to give the
nitrated product. A control reaction indicates that the combination
From this solution a complex 3 was isolated (60% yield),
identified as a nitrato complex: (i) electrospray ionization mass
spectrometry (ESI-MS) reveals a prominent ion peak at m/z =
520.11, consistent with the chemical formula [(BA)CuII(NO3)]+
(ESI†). (ii) Single crystals of 3 as a perchlorate salt could be
isolated, [(BA)Cu(NO3)](ClO4) (3); X-ray structure determination
(Fig. 3) confirms the formulation and reveals for this complex a
slightly distorted square pyramidal (SP) coordination (t = 0.29;
t = 0 for idealized SP geometries).9
This supports a change in geometry, from the trigonal
bipyramidal (TBP) coordination geometry observed for the
precursors 1 and 2 (EPR criteria).8 The coordination changes
result in a weaker H-bond between the ligand side-arm N–H to
of [(BA)CuII(CH3COCH3)]2+ (1) with 2,4-DTBP plus NO(g) leads to
ꢀ
the Cu(II) bound Onitrito atom, H–Onitrito = 2.23(2), N–Onitrito
=
2.963(3) Å, as compared to that in [(BA)CuII(CH3COCH3)]2+ (1),
H–Oacetone = 2.10(2)/2.15(3), N–Oacetone = 2.886(3)/2.981(3) Å
(also see the ESI†).
less than a 20% yield of 2,4-di-tert-butyl-6-nitrophenol (ESI†).
As mentioned, in the original reaction of [(BA)CuII(CH3COCH3)]2+
(1) + ꢀNO(g) where PN isomerization to nitrate occurred, Cu(I) should be
formed. This is also the case for the phenol nitration chemistry,
Scheme 2. Yet, in both cases, we instead observed that the final solution
mixture contained Cu(II), the nitrate adduct for the former case and 1 in
the phenol nitration chemistry. To explain this, we further interrogated
(iii) The EPR spectrum of [(BA)CuII(NO3)]+ (3) is that of an
axial type.10 (iv) Further proof that the product of NO(g) reaction
with hydroperoxo complex [(BA)CuII(OOH)]+ (2) is the nitrato
compound 3 derives from the fact that addition of tetrabutyl-
ammonium nitrate (TBANO3) to a solution of [(BA)CuII-
(CH3COCH3)]2+ (1) gives a product with identical spectroscopic
(UV-vis, EPR) features (Fig. 1).
The isolation of Cu(II)–nitrate complex 3 supports the inter-
mediacy of a ÀOONQO species in the 2 + NO(g) reaction (Fig. 1),
which has isomerized (Scheme 1). The peroxynitrite inter-
mediate would develop from the nucleophilic attack of the
hydroperoxo ligand within [(BA)CuII(OOH)]+ (2) on nitric oxide.
This would result in release of a proton along with formation of
Cu(I) (eqn (1)). Note that PN normally is said to derive from
ꢀ
the product solutions. Since small excesses of NO(g) were normally
employed, we surmised that Cu(I) which formed then mediated a
ꢀNO(g) disproportionation reaction (eqn (3)). This well-known reaction
would lead to nitrous oxide (N2O(g)) formation along with a CuII–NO2À
complex, for our case (eqn (3)).12 In fact, for the [(BA)CuII(OOH)]+ (2) +
ꢀNO(g) reaction, we detected significant amounts of N2O(g) (GC analyses).
However, only a trace of nitrite could be detected, and as described
above, NO3À was formed in high yield (eqn (2)).
CuI + 3ꢀNO - CuII–NO2 + N2O (RCuI + NO2 + N2O)
(3)
À
We can now explain these observations as follows. Under
acidic conditions, where a proton derives from the hydroperoxo
moiety in 2 (eqn (2)), classic disproportionation (eqn (3)) does not
Fig. 3 Displacement ellipsoid plot (50% probability level) of [(BA)CuII(NO3À)]-
(ClO4), (3). The perchlorate counterion and the H atoms (except for that
attached to N5) were removed for clarity.
Scheme 2
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occur, but rather ꢀNO reductive coupling takes place (eqn (4)). Notes and references
This reaction is rare in copper nitrogen-oxide (bio)chemistry.13
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2CuI + 2H+ + 2ꢀNO - 2CuII + H2O + N2O
(4)
We further proved that this chemistry occurs by demonstrat-
ing that a Cu(I) derivative of the BA ligand, [(BA)CuI]+, reacts
ꢀ
with NO(g) in the presence of 1 equiv. HClO4 to give the Cu(II)
complex 1 and N2O(g). However, in the absence of the proton
source only a nitrite complex [(BA)CuII(NO2)]+ is generated i.e.,
the classic disproportionation reaction (eqn (3)) (see ESI†).
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site in bacterial ꢀNO Reductases (NOR’s), for flavo-non-heme
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NOR activity in the presence of acid (eqn (4)) explains the finding
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product in the reactions described herein. Thus, the overall
chemistry that occurs in the [(BA)CuII(OOH)]+ (2) + ꢀNO(g) reaction
is described by eqn (5); two processes take place, the generation of
peroxynitrite via CuII(OOH) + NO(g), and the Cu(I) therein produced
effects NOR chemistry with the excess nitric oxide.
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2[CuII–OOH]+ + 4ꢀNO(g) - 2CuII–NO3 + H2O + N2O(g)
(5)
À
To summarize, the importance of peroxynitrite chemistry in
biological systems, in terms of PN as a destructive oxidant or
nitrating agent, calls for further investigation of relevant metal
ion (M) chemistry. What reactivity promotes PN formation and/or
reactivity and/or its own destruction, the latter in terms of health
related metal-based PN decomposition catalysis?15 PN or PN asso-
À
ꢀ
ciated chemistry can occur from M–O2 + NO(g), M–NO2 + H2O2,
M–NO + O2, or M–NO + H2O2 chemistry. Here, we have described a
II
ꢀ
new reaction of a discrete Cu –hydroperoxo complex with NO(g).
Evidence strongly supporting the intermediacy of a Cu–PN complex
includes the finding the (i) nitrate ion forms in high yield and (ii) if a
phenol as substrate is added, efficient nitration occurs. We also
document a rare case of Cu mediated ꢀNO(g) reductive coupling, not
unlike what occurs for NOR’s. The nature of intermediates formed
and mechanism of that reaction remain to be elucidated.
We believe that the specific reaction type studied here,
[M–OOH]+ + ꢀNO(g), is relevant to biological chemistry. How is it
that phenol tyrosine nitration occurs at a position close to the active
site inside Mn–superoxide dismutase (MnSOD)?6b,c In vitro exposure
of the protein to an excess of PN effects the indicated reaction.6c Is it
not more likely that in the natural environment, a Mn–OOH
intermediate, that known to form in the enzyme reaction, reacts
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¨
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¨
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ꢀ
with endogenous NO(g), and as described here, leads to tyrosine
nitration? CuZnSOD is also known to promote Tyr nitration7c,16 and
based on the chemistry presented herein, we suggest that a protein
CuII–hydroperoxo intermediate may react with ꢀNO(g) leading to PN
mediated chemistry. (Note, for either MnSOD or CuZnSOD, a
M–superoxo enzyme intermediate could react with ꢀNO(g) to give
PN.) Further consideration of metal ion mediated PN generation
and reactivity, like that outlined in Scheme 1, is required.
We are grateful to the USA National Institutes of Health
(GM-28962) for support of this research.
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and J. S. Beckman, Arch. Biochem. Biophys., 1992, 298, 431.
2846 | Chem. Commun., 2014, 50, 2844--2846
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