Spontaneous reactions and reduction by iodide of peroxynitrite and peroxynitrate: mechanistic insight from activation parameters

Article abstract of DOI:10.1021/jp971506f

Thermal and pressure activation parameters are reported for the decomposition of peroxynitrate, isomerization of peroxynitrite, and their reduction by iodide in aqueous solutions. The spontaneous decomposition reactions are characterized by activation ent

Full text of DOI:10.1021/jp971506f

7114
J. Phys. Chem. A 1997, 101, 7114-7118
Spontaneous Reactions and Reduction by Iodide of Peroxynitrite and Peroxynitrate:
Mechanistic Insight from Activation Parameters
Sara Goldstein,*,† Dan Meyerstein,‡ Rudi van Eldik,§ and Gidon Czapski†
Department of Physical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, Chemistry
Department, Ben-Gurion UniVersity of the NegeV and The College of Judea and Samaria, Kdumim-Ariel,
Israel, and Institute for Inorganic Chemistry, UniVersity of Erlangen-Nuernberg, 91058 Erlangen, Germany
ReceiVed: May 5, 1997; In Final Form: July 9, 1997X
Thermal and pressure activation parameters are reported for the decomposition of peroxynitrate, isomerization
of peroxynitrite, and their reduction by iodide in aqueous solutions. The spontaneous decomposition reactions
are characterized by activation enthalpies of ca. 16 kcal/mol, activation entropies close to zero, and significantly
positive activation volumes between 7 and 10 cm3/mol. These parameters suggest that the rearrangement of
both peroxo species, leading to intermediates of increased partial molar volume, must include partial bond
cleavage. The iodide-induced reduction reactions are characterized by significantly smaller activation
enthalpies, negative activation entropies between -19 and -23 cal/(mol‚K), and negative activation volumes
between -6 and -11 cm3/mol. These parameters suggest that bond formation between the redox partners
occurs before electron transfer and favors an inner-sphere mechanism. The results are discussed regarding
mechanisms based on ambient kinetic data suggested in the literature.
Introduction
have shown, using the pulse radiolysis technique, that both
suggestions do not agree with the kinetics and the yields of the
indirect oxidation of ferrocyanide and NADH by peroxynitrate.8
We have suggested that the decomposition of peroxynitrate takes
place via the formation of yet unidentified intermediates (O2-
NOOH*, O2NOO-*) through the following mechanism:8
Peroxynitrite (ONOOH/ONOO-) has a pKa of 6.8 and is
stable in alkaline solutions.1 The acid form isomerizes to
nitrate2,3 with k ) 1.3 s-1 at 25 °C.1,4,5 Recently, Goldstein et
al.4 proposed that the decomposition of ONOOH involves a
direct isomerization to nitrate and in parallel the formation of
a yet unidentified intermediate, ONOOH*, which subsequently
isomerizes to nitrate. The mechanism of the decomposition of
ONOOH is described in Scheme 1,
Scheme 2
k5
HNO2 + O2
k4
k6
O2NOOH
O2NOOH*
pKa*
•NO2 + HO2•
Scheme 1
k–4
k–6
NO3– + H+
pKa
k1
k3
k7
k8
O2NOO–
O2NOO–*
NO2– + O2
k2
k–7
ONOOH
ONOOH*
k–2
The goal of this study was to measure the thermal and
pressure activation parameters for the spontaneous decomposi-
tion of peroxynitrite and peroxynitrate in order to obtain further
insight into our earlier suggested mechanisms, which were based
on kinetics and oxidation yields in the presence of various
substrates under ambient conditions.8,11 The activation param-
eters were also determined for the direct oxidation of iodide by
ONOOH and O2NOOH,8,11 to gain more information on the
nature of the oxidation mechanism.
where k1 ) 1.0 s-1, k2 ) 0.65 s-1, k-2 ∼ k3 and 10 e k3 e 2
× 105 s-1 at 25 °C.4 Arguments against the formation of
hydroxyl radicals through the homolysis of ONOOH were given
and discussed in detail elsewhere.4 These arguments include
thermodynamic calculations,5 the effect of viscosity on the rate
of the self-decomposition rate of peroxynitrite,1,6 and the effect
of scavengers on the oxidation yields of the indirect oxidation
of various substrates by peroxynitrite.4
Peroxynitrate (O2NOOH/O2NOO-) has a pKa of 5.9,7,8 and
it is relatively stable in acid solutions (τ1/2 ∼ 30 min).7-10 It
has been suggested that peroxynitrate decomposes mainly
through a unimolecular dissociation of the anion into nitrite and
oxygen,7-10 and that the decomposition of O2NOOH takes place
Experimental Section
Materials. All chemicals were of analytical grade and were
used as received. The pH was adjusted with 1 mM acetate or
phosphate buffers.
•
•
9,10
either through its dissociation into HO2 and NO2
or into
HNO2 and O2.7 The half-life of the anion is ca. 1 s.7,8 We
Methods. Pulse radiolysis experiments were carried out with
the Varian 7715 linear accelerator with 5 MeV electrons pulses
of 0.4-1.5 µs and 200 mA current. The dose per pulse was
10-29 Gy, respectively, and was determined with the hexacy-
anoferrate(II) dosimeter (5 mM K4Fe(CN)6 in N2O-saturated
water) using Gꢀ(Fe(CN)63-) ) 6.7 × 103 M-1 cm-1 at 420 nm.12
In some of the systems, repetitive pulsing was used to increase
* To whom all correspondence should be addressed. Tel., 972-2-6586478;
fax, 972-2-6586925; e-mail, SARAG@HUJI.VMS.AC.IL.
† The Hebrew University of Jerusalem.
‡ Ben-Gurion University of the Negev and The College of Judea and
Samaria.
§ University of Erlangen-Nuernberg.
X Abstract published in AdVance ACS Abstracts, September 1, 1997.
S1089-5639(97)01506-5 CCC: $14.00 © 1997 American Chemical Society

Reactions and Reduction of Peroxo Species
J. Phys. Chem. A, Vol. 101, No. 38, 1997 7115
the yield of the radicals. A 150 W Xe lamp produced the
analyzing light, and appropriate filters were used to minimize
photochemistry. Irradiations at ambient pressure were carried
out in a 4-cm long Spectrosil cell using three light passes.
The high-pressure setup, consisting of a 1.5 cm pillbox optical
cell, made of Suprasil, was placed near the thin stainless steel
window13 of the modified high-pressure cell so that the high-
energy electrons would have a minimal path through the
surrounding water-pressurizing medium. Repetitive pulsing was
used for the pressure experiments, which were performed up to
1500 atm at room temperature (20 °C).
Formation of Peroxynitrite and Peroxynitrate. Peroxyni-
trite and peroxynitrate were produced by irradiation of air-
saturated solutions containing formate and nitrite or nitrate,
respectively. At pH g 3, the following reactions take place:
Figure 1. Kinetic trace obtained at 280 nm when 15 pulses of 1.5 µs
are delivered into the solution at 1500 atm and ambient temperature.
Air-saturated solution contained 0.15 M formate and 3 mM nitrite at
pH 4.1 (Io ) -1060 mV, ∆V1 ) 57 mV, ∆V2 ) 48 mV).
H2O f e-aq (2.6), •OH (2.7), H• (0.6), H2 (0.45),
TABLE 1: kobs of the Decomposition of O2NOO- and of the
Reactions of ONOOH and O2NOOH with Iodide at Various
Temperatures
H2O2 (0.7), H3O+ (2.6) (9)
kobs, s-1
kobs, s-1
kobs, s-1
(The numbers in parentheses are G-values, which represent the
number of molecules formed per 100 eV of energy absorbed
by pure water).
The solute concentrations were such that OH is scavenged
by formate to produce superoxide via reactions 10-12
temp, (self-decay of O2NOO-) (ONOOH + I-) (OONOH + I-)
°C
(pH 8.3)
(pH 4.4)
(pH 3.3)
0.8
1.1
6.2
0.34
•
0.12
0.20
6.6
0.46
0.64
•-
•OH + HCO2- f H2O + CO2
11.9
14.1
18.3
20.5
23.8
28.9
30.5
36.2
37.3
47.9
0.33
k10 ) 3.5 × 109 M-1 s-1 (14) (10)
19.8
0.75
1.71
1.08
1.96
•-
CO2•- + O2 f CO2 + O2
27.6
47.8
k11 ) 3.5 × 109 M-1 s-1 (14) (11)
3.93
•
H+ + O2•- h HO2
pKa ) 4.8(15)
(12)
which reacts fast with superoxide to produce peroxynitrate:
and the solvated electrons are scavenged by O2 and nitrite to
produce peroxynitrite:
•NO2 + O2•- f O2NOO-
k20 ) 4.5 × 109 M-1 s-1 (7)
•-
eaq- + O2 f O2
k13 ) 2 × 1010 M-1 s-1 (14)
(13)
(20)
•
•NO2 + HO2 f O2NOOH
2-
eaq- + NO2- f NO2
k21 ) 1.8 × 109 M-1 s-1 (7)
k14 ) 4.1 × 109 M-1 s-1 (14) (14)
(21)
NO22- + H2O f •NO + 2OH-
Results
k15 ) 4.3 × 104 M-1 s-1 (14) (15)
The volume of activation of the spontaneous decomposition
of ONOOH was determined upon repetitive pulsing of air-
saturated solutions containing 0.15 M formate and 3 mM nitrite
at pH 4.1. Under these conditions, ONOOH is formed via
reactions 12-17, and its decomposition was followed at 280
nm. When the pressure was increased from 1 to 1500 atm, the
observed first-order decomposition rate constant decreased from
0.74 ( 0.07 to 0.41 ( 0.08 s-1, respectively, which corresponds
to ∆V* ) 9.6 ( 1.0 cm3/mol. A typical trace for 1500 atm is
given in Figure 1. Thermal activation parameters for the
spontaneous decomposition of ONOOH were reported to be
∆H* ) 171 kcal/mol and ∆S* ) 32 cal/(mol‚K).5
O2•- + •NO f ONOO-
k16 ) 4.3 × 109 M-1 s-1 (16) (16)
•
•NO + HO2 f ONOOH
k17 ) 3.2 × 109 M-1 s-1 (15) (17)
In the presence of high concentrations of nitrate, the solvated
electrons are scavenged by nitrate
When the same solutions were irradiated in the presence of
-
2-
eaq- + NO3- f NO3
k18 ) 9.7 × 109 M-1 s-1 (14)
1 mM iodide, the formation of I3 was followed at 380 nm in
the temperature range 1.2-47.9 °C. The kinetic results are
summarized in Table 1. The observed first-order rate constant
in the presence of 1.3 mM iodide increased with increasing
pressure (Table 2). The resulting activation parameters are given
in Table 3.
(18)
NO32- + H2O f •NO2 + 2OH-
k19 ) 5.5 × 104 s-1 (14) (19)

7116 J. Phys. Chem. A, Vol. 101, No. 38, 1997
Goldstein et al.
TABLE 2: kobs (s-1) as a Function of Pressure in the
Absence and Presence of Iodide at 20 °C
given by eq 22:
kobs ) k1 +
k2k3
O2NOOH +
∼ k1 + k2/2
(22)
ONOOH +
3I- + H+
I3- + NO3
H2Od
f
+
k-2 + k3
-
-
P,
ONOOH f
3I- f I3
+
O2NOO-
f
c
atm NO3- + H+ a NO2- + OH- b NO2- + O2
Thus, both reactions 1 and 2 contribute to kobs, and therefore
they also contribute to the observed volume of activation. The
direct isomerization of cis-ONOOH to nitrate (reaction 1) is
clearly complex and requires more than one step. If this reaction
would occur in one step, a significantly negative ∆V* is
expected due to charge creation (ONOOH f NO3- + H+).20,21
Since we measured a significantly positive ∆V* of 9.6 cm3/
mol, it is suggested that the direct isomerization of cis-ONOOH
takes place through the formation of an intermediate, which is
accompanied by a volume increase. This intermediate decom-
poses rapidly to nitrate, and the whole process is described by
eq 23:
1
750
0.74 ( 0.07
28.7 ( 3.1
42.0 ( 3.8
1.24 ( 0.08
0.82 ( 0.06
0.97 ( 0.1
1.40 ( 0.13
2.00 ( 0.09
1500 0.41 ( 0.08
a pH 4.1. b pH 4.4 and 1.3 mM iodide. c pH 8.3. d pH 3.5 and 1.05
mM iodide.
O
O
slow
fast
NO3– + H+
N
O
O
H
N
(23)
H
.......
O
O
cis-ONOOH
Figure 2. Kinetic trace obtained at 352 nm when air-saturated solution
containing 0.1 M formate, 30 mM nitrate, and 1.05 mM iodide at pH
3.5 was pulse irradiated at 1500 atm and ambient temperature (Io )
-967 mV, ∆V1 ) 16 mV, ∆V2 ) 88 mV).
In addition, the conversion of cis-ONOOH to ONOOH* may
also contribute to the positive value of ∆V* if this process
involves a volume increase due to bond lengthening or partial
desolvation.
The activation parameters for the spontaneous decomposition
of O2NOO- and for the reaction of O2NOOH with 1.05 mM
iodide were determined upon irradiation of air-saturated solu-
tions containing 20-100 mM formate, 20-30 mM nitrate at
pH 8.3 and 3.5, respectively. Under these conditions, perox-
ynitrate is formed via reactions 18-21. The spontaneous
decomposition of O2NOO- was followed at 310 nm upon
repetitive pulsing, and the formation of I3- was followed at 352
nm by delivering one pulse into the solution. A typical kinetic
trace at 1500 atm is given in Figure 2. The results are
summarized in Tables 1-3.
O
O
O
O
H
O•
or
•N
O•
(24)
N
O
O
H
N
.......
.......
H
O•
cis-ONOOH
I
II
ONOOH*
The transient complex II is that proposed recently by Houk et
al.18 in the gas phase and could be formed while the hydrogen
bond existing in the cis conformation is retained. It is, however,
difficult to envisage that either I or II will be long-lived enough
to enable reactions with a variety of reducing agents.
Bond breakage could in principle account for the observed
∆V* value, but not when it is accompanied by charge creation
since this is expected to lead to a drastic increase in electro-
striction and an overall volume collapse. The thermal activation
parameters indicate a relatively high activation enthalpy that
would favor a bond breakage process. However, the almost
zero activation entropy indicates that no substantial increase in
electrostriction can accompany this process. It follows that both
the thermal and pressure parameters are in line with a mecha-
nism that involves rearrangement and partial bond cleavage
without a significant charge creation component.
Discussion
The ONOOH System. The unusual stability of peroxynitrite
in alkaline solutions is due to folding into the cis conformation,
which cannot directly isomerize to nitrate. When protonated,
peroxynitrite isomerizes rapidly to nitrate. It was suggested
previously that peroxynitrite iomerizes to the trans conformation,
which is less stable by 3-5 kcal/mol,17,18 and is also capable
of rearranging to nitrate and also forms an excited state, which
can react like the hydroxyl radical and nitrogen dioxide.19
However, most of the experimental results for the indirect
oxidation of various substrates do suggest a direct isomerization
of cis-ONOOH (∼60%) and in parallel its conversion into a
highly reactive intermediate (∼40%) (Scheme 1).4
We cannot exclude the possibility of the homolysis of
ONOOH on the basis of the activation volume data (ONOOH*
•
≡ OH + •NO2) as it is difficult to distinguish between partial
bond cleavage and homolysis in terms of volume change. Many
reactions that involve bond breakage or homolysis have volumes
of activation between 5 and 10 cm3/mol.22
According to our suggested mechanism (Scheme 1), the
observed rate constant for the decomposition of ONOOH is
TABLE 3: Summary of Rate Constants and Activation Parameters Calculated for the Various Processes
k (20 °C)
∆H*, kcal/mol
∆S*, cal/(mol‚K)
∆V*, cm3/mol
ONOOH f NO3- + H+
0.74 ( 0.07 s-1
17 ( 1.0(5)
4.6 ( 0.4
15.6 ( 0.3
7.7 ( 0.3
3 ( 2(5)
-23 ( 1a
-6 ( 1
9.6 ( 1.0
-6.2 ( 0.7
6.7 ( 0.7
ONOOH + 3I- f I3- + NO2- + OH-
O2NOO- f NO2- + O2
(2.3 ( 0.1) × 104 M-1 s-1(11)
1.24 ( 0.08 s-1
O2NOOH + 3I- + H+ f I3- + NO3- + H2O
840 ( 50 M-1 s-1(8)
-19 ( 1a
-11 ( 1.0
a Calculated on the basis of a second-order rate constant for this process.

Reactions and Reduction of Peroxo Species
J. Phys. Chem. A, Vol. 101, No. 38, 1997 7117
In the presence of iodide, ∆V* ) -6.2 cm3/mol, which
indicates that this process involves bond association. Therefore,
our earlier suggested mechanism,11 where cis-ONOOH oxidizes
iodide directly through one- or two-electron transfer mecha-
nisms, is probably wrong, and the following possible mecha-
nisms are suggested for this process:
importance of bond formation during the electron-transfer
process. The electron-transfer process itself does not involve
any charge creation that could via an increase in electrostriction
cause a volume collapse.25,26 The activation enthalpy is slightly
larger than that reported for the reduction of peroxynitrite and
results in a decrease in the second-order rate constant.
I
Conclusions
–
O
N–O–O–H + I–
O
N–O–O–H–
HOI + NO3
(25)
The thermal and pressure activation parameters reported in
this study (Table 3) contribute toward a further clarification of
the intimate mechanism of the spontaneous decomposition and
iodide-induced reduction reactions of peroxynitrite and perox-
ynitrate. The spontaneous decomposition must involve rate-
determining rearrangement and partial bond cleavage that can
cause an overall increase in volume in the transition state, before
product formation that will involve significant charge creation
in the case of peroxynitrite decomposition. The more positive
∆V* value in this case clearly demonstrates that charge creation
cannot be involved in the rate-determining step, since the
spontaneous dimerization of peroxynitrate is not accompanied
by charge creation and exhibits an even less positive volume
of activation. The thermal activation parameters agree with this
conclusion.
According to the reported activation parameters, the iodide-
induced reduction of peroxynitrite and peroxynitrate must
involve significant bond formation and proceed according to
an inner-sphere electron-transfer mechanism. These reactions
do not involve any net change in electrostriction and are not
expected to exhibit major changes in electrostriction. Thus, the
significantly negative ∆V* and ∆S* values strongly suggest
direct bond formation between the redox partners and support
the operation of an inner-sphere mechanism.
HOI + I– + H+
I2 + I–
K = 5 x 10–13 M2 (23)
K = 710 M–1 (23)
(26)
(27)
I2 + H2O
–
I3
The significantly negative volume of activation is ac-
companied by an entropy of activation that is also substantially
negative and is in line with a bond formation process. The high
rate constant exhibits a low activation enthalpy, which demon-
strates the low barrier associated with the bond formation
process in the inner-sphere electron-transfer process.
The Peroxynitrate System. Previously, we have suggested
that the decomposition of O2NOO- to nitrite and oxygen takes
place through O2NOO-* as a reactive intermediate (Scheme
2).8 The lower pKa of peroxynitrate as compared to peroxyni-
trite must be related to the higher oxidation state of N in
peroxynitrate. In such a case, the negative charge on the
peroxide arm interacts with the positive charge on the nitrogen-
forming intermediate with a three-membered ring. This interac-
tion weakens the N-O bond so that the formation of this
intermediate leads to bond cleavage and to the formation of
NO2- and O2. This bond cleavage involves the reduction of N
2-
from +5 to +3 and the oxidation of O2 to O2, which will
cause some intrinsic volume change, and could account for the
less positive volume of activation as compared to that found
for peroxynitrite.
The results of this study have again demonstrated how an
analysis of thermal and pressure activation parameters can assist
the elucidation of intimate reaction mechanisms.
O
O
O
O
slow
N
O
NO2– + O2
(28)
O
N
Acknowledgment. This research was supported by Grant
4129 from The Council For Tobacco Research and by The Israel
Science Foundation. R. van Eldik gratefully acknowledges
financial support from the Deutsche Forschungsgemeinschaft
and the Volkswagen Foundation.
O
O
O2NOO–
O2NOO–*
The slightly smaller value of ∆V* as compared to that for
peroxynitrite is also reflected in the more negative value of ∆S*.
The latter value, however, is still close to zero and does not
reflect any significant charge creation in the transition state.
The activation enthalpy is very close to that found for perox-
ynitrite and reflects the importance of bond rearrangement and
bond cleavage.
The negative activation volume in the presence of iodide,
∆V* ) -11 cm3/mol, is in favor of a bond formation process.
Therefore, we suggest that the oxidation of iodide by perox-
ynitrate takes place through the formation of a transient complex:
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