Coordination of nitrogen(II) oxide by metal prophyrins

Article abstract of DOI:10.1134/S0036023607020283

The equilibrium constants have been determined for the extra coordination of nitrogen(II) oxide and its metabolite formed in biochemical reactions (the nitrite ion) by cobalt complexes of porphyrins with various structures in ethanolic and aqueous solutions with near-physiological pHs. The far higher values of the thermodynamic stability constants of (NO)CoP nitrosyl complexes compared to those for nitrite complexes argue for a considerable covalence of the Co-NO bond. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607020283

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 2, pp. 293–296. © Pleiades Publishing, Inc., 2007.
Original Russian Text © S.G. Pukhovskaya, L.Zh. Guseva, A.S. Semeikin, O.A. Golubchikov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 2,
pp. 337−340.
PHYSICAL CHEMISTRY OF SOLUTIONS
Coordination of Nitrogen(II) Oxide by Metal Porphyrins
S. G. Pukhovskayaa,b, L. Zh. Gusevaa, A. S. Semeikinb, and O. A. Golubchikovb,a
a Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya ul. 1, Ivanovo, 153045 Russia
b Ivanovo State University of Chemical Technology, pr. Engelsa 7, Ivanovo, 153460 Russia
Received April 18, 2006
Abstract—The equilibrium constants have been determined for the extra coordination of nitrogen(II) oxide and
its metabolite formed in biochemical reactions (the nitrite ion) by cobalt complexes of porphyrins with various
structures in ethanolic and aqueous solutions with near-physiological pHs. The far higher values of the thermo-
dynamic stability constants of (NO)CoP nitrosyl complexes compared to those for nitrite complexes argue for
a considerable covalence of the Co–NO bond.
DOI: 10.1134/S0036023607020283
In the last decade, there has been considerable sci-
R
R'
R'
entific interest in nitrogen oxide NO, the simplest inor-
ganic compound that is continuously produced by the
enzymatic systems in humans and animals and func-
tions as a universal regulator of metabolism. Nitro-
gen(II) oxide is an endogenic vasodilator and neu-
rotransmitter; it inhibits thrombocyte aggregation. The
superoxide radical and nitrogen(II) monoxide were
found to have a conjugate effect in the release of oxida-
tive stress [1–3]. Nitrogen(II) oxide hyperproduction is
of paramount importance in the pathogenesis of inflam-
matory reactions, autoimmune perturbations, and
infectious toxic shock [4]. Nitrogen(II) oxide in biosys-
tems is produced by L-arginine, which is an NO-syn-
thetase (NOS), an enzyme producing NO free radicals
and citrulline as a byproduct. Nitrogen(II) oxide has a
very short lifetime; it rapidly oxidizes to stable final
products (nitrite and nitrate ions). The nitrite ion is
known to oxidize oxyhemoglobin in vivo and to cata-
lyze NO-involving reactions. The toxic effect of nitrites
is mainly related to the oxidation of hemoglobin to
methemoglobin. The oxidation perturbs oxygen supply
to tissues, and hemic hypoxia is developed as a result.
Embryonic hemoglobin is, in particular, sensitive to
this reaction [5]. The bioeffects of NO and NO2 are not
quite understood. In this context, investigations of the
binding (extracoordination) of nitrogen oxide and its
metabolite (nitrite ion) by metal porphyrins in model
blood solutions give the key to understanding the bio-
logical processes in which NO participates.
R'
R
R'
R
N
N
N
Co
N
R'
R'
R'
R
R'
CoPI: R = C6H6; R' = H
CoPII: R =
N+
· Br , R' = H
–
CH COOH
2
CoPIII: R = C6H6; R' = Br
HC CH2
CH3
H3C
C
H
CH2
N
N
Co Cl
N
N
H3C
CH3
CH2
CH2
CH2
CH2
COOH
COOH
CoPIV
Here, we determine the stability constants for cobalt
complexes of synthetic porphyrins with nitrogen(II)
oxide and nitrate ion.
293

294
PUKHOVSKAYA et al.
ity of Co(OH)2 is about 2 × 10–4 wt %) [9] in the 1 : 100
ratio and magnetically stirring for 8 h at 50°ë. Metal
porphyrin formation was monitored spectrophotometri-
cally. The solution was twice filtered. Then, the com-
plex was precipitated by benzene. The resulting precip-
itate was washed with acetone and dried at 75°ë.Yield:
96%. The cobalt complex of protoporphyrin-IX (IV)
was synthesized by refluxing protoporphyrin-IX dime-
thyl ester with excess cobalt acetate in acetic acid, fol-
lowed by alkaline hydrolysis and precipitation with
dilute hydrochloric acid. Yield: 90%. The electronic
absorption spectra of the cobalt porphyrins in ethanol
and water are displayed in Table 1.
Table 1. Electronic absorption spectra of cobalt porphyrin
complexes (CoP)
λ, nm ( logε ) in ethanol
CoP
band I
band II
Soret band
CoPI
–
542 (4.61)
550 (3.75)
574 (4.17)
426 (5.68)
429 (5.33)
459 (5.03)
CoPII
CoPIII
608 (3.63)
614 (4.06)
λ, nm ( logε ) in water
595 (sh)
The electronic absorption spectra were recorded on
a Specord M400 spectrophotometer in 50-ml cells (the
absorbing layer thickness was 100 mm) at 293.0 0.1 K.
When extracoordination was studied, an exact volume
of an NO-saturated solution was injected into the cell
with a microsyringe through the rubber plug in order to
avoid atmospheric oxygen.
CoPII
(pH 7.4)
540 (3.78)
427 (5.02)
417 (5.41)
CoPIV
(pH 9)
564 (3.98)
534 (4.01)
Table 2. Stability constants for cobalt porphyrin complexes
with nitrogen bases at 298 K
Ethanol rectificate was purified using a routine pro-
cedure [9]. The remnant water as determined by Fischer
titration was 0.05%. The ethanol and water solutions of
metal porphyrins were purged with argon in order to
free them from oxygen. The NO-saturated solutions
were prepared by passing nitrogen(II) oxide in nitrogen
(0.49% NO) through the solvent. The NO concentration
was determined by a special procedure [10]. Chemi-
cally pure NaNO2 was doubly crystallized [11].
Kst, mol–1 in ethanol
Porphyrin
Py
NO
NO–2
CoPII
CoPI
38.6 0.5 24200 800
810 200
30
17
1
2
10400 300
2690 200
47
3
CoPIII
CoPIV
–
RESULTS AND DISCUSSION
160 20
Spectrophotometric titration was used to determine
the thermodynamic constants Kst of nitrogen(II) extra-
coordination to cobalt complexes of porphyrins I–III.
The measurements were carried out in ethanolic solu-
tions because of the solubility of the test compounds.
The results are compiled in Table 2.
Ks, mol–1 in water
CoPII (pH 7.4)
CoPIV (pH 9)
–
–
–
–
26
4
3.9 0.5
The electronic absorption spectra of the cobalt com-
plexes of porphyrins I–III in ethanol, and of porphyrin
II in water (the solvents were first deoxygenated), were
studied while nitrogen(II) oxide was passed through
the solution. (NO)CoP complexes were formed almost
instantaneously; the Soret band shifted to the longer
wavelengths from 429 to 446 nm for II. In a free
excess to oxygen, the spectrum changed over time: the
maximum of the Soret band shifted hypsochromically
to λ = 435 nm. In the IR spectra of the ethanolic solu-
tions of cobalt porphyrin complexes, three types of
coordinated ligands were discovered: NO, NO+, and
NO–. The major compounds were nitrosyl complexes
(NO)CoP [12].
EXPERIMENTAL
The porphyrins used (tetraphenylporphyrin (I),
5,10,15,20-tetra(4N-carboxymethylenepyridyl)porphy-
rin tetrabromide (II), and β-octabromo-meso-tetraphe-
nylporphyrin (III)) were prepared by procedures
known from [6–8]. The cobalt complexes of porphyrins
I and III were prepared by reacting the porphyrins with
an excess of cobalt acetate (chemically pure grade) in
boiling dimethylformamide and chromatographed on
grade III alumina with chloroform as an eluent. The
purity of the complexes was monitored by thin-layer
chromatography on silufol plates (chloroform + ben-
zene (1 : 1) was the developer) and by electron absorp-
tion spectra. The cobalt complex of porphyrin II was
prepared by adding freshly prepared cobalt hydroxide
to an aqueous solution of porphyrin (the water solubil-
The coordination of the extraligand by metal por-
phyrins occurs according to
åê + nL
LnMê,
n = 1 or 2.
(1)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 2 2007

COORDINATION OF NITROGEN(II) OXIDE BY METAL PORPHYRINS
295
log[(A – A )/(A –A )]
log[(Ai – A0)/(Af –Ai)]
i
0
f
0
–2.8
–3.0
–3.2
–3.4
–3.6
–3.8
–4.0
–4.2
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
–2.2
–3.8 –3.6 –3.4 –3.2 –3.0 –2.8 –2.6 –2.4 –2.2 –2.0
–1.0 –0.8 –0.6 –0.4 –0.2
0
0.2 0.4 0.6 0.8
logcL
logcL
Fig. 2. log[(Ai – A0)/(A f – Ai)] vs. logcL plot for the
Fig. 1. log[(Ai – A0)/(A f – Ai)] vs. logcL plot for the
CoPII–NO systems in ethanol at í = 298 K.
ëÓêI–NO system in ethanol.
2
Here, MP is metal porphyrin, LnMê is its extra com- the electron-acceptor effect of bromine, but it also
appreciably distorts the planar configuration of the aro-
matic polyanion. These effects deteriorate the stability
of the extra complexes compared to their planar ana-
logues (Table 2) both for strong organic bases (pyri-
dine) and for nitrogen(II) oxide. The thermodynamic
stability constant for (NO)CoPII is about two times that
for (NO)CoPI. This is evidently because of the exist-
ence of the positive charge on the pyridyl nitrogen
atoms of zwitterion cobalt porphyrin (PII): the effective
positive charge on the cobalt cation increases and, in
association, the extra complex is stabilized. This is true
for both the nitrosyl and nitrite complexes. The lower
Kst values for the nitrite complexes compared to the
nitrosyl complexes imply that the covalence of the
Co−NO bond is significant. The experiments carried
out in aqueous solutions with near-physiological pHs
showed that the stability constants of the nitrite com-
plexes for synthetic porphyrin CoPII are about one
order of magnitude higher than for cobalt complex of
protoporphyrin IX. This result indicates that further
investigations in this field are promising. The biochem-
ical investigations fulfilled by the team headed by Pro-
fessor S.B. Nazarov at the Ivanovo State MedicalAcad-
emy showed that the cobalt complex of water-soluble
porphyrin P(II) has sorption properties to nitrate ions in
the blood and brains of test animals.
plex, and L is the extra ligand. The stability constants
for the extra complexes Kst were calculated from
äst = [LnMê]/([MP][L]n).
(2)
The optical densities at the analytical wavelengths were
used to calculate equilibrium concentration of the extra
complex:
c
LMê = c0MP (A0 – Aeq)/(A0 – Af).
(3)
Here, cLMê is the equilibrium LMP concentration; c0MP
is the starting MP concentration; and A0, Aeq, and Af are,
respectively, the starting, equilibrium, and final optical
densities of the solution. The extra ligand concentration
was determined from
cL = c0L – cM0 P [1 – (A0 – Aeq)/(A0 – Af)].
(4)
The number of the ligands attached to metal porphyrin
(provided that c0L ꢀ cM0 P ) was derived from the slope
of the log[(A0 – Aeq)/(Aeq – Af ) versus logcL plots.
For the cobalt porphyrins studied, the plots were linear
with their slopes being unity within the determination
error (Figs. 1, 2).
The coordination properties of metal porphyrins
toward donor species (in the case at hand, nitrogen(II)
oxide and nitrite ion) are in direct proportion to the
nature of the substituents at the porphyrin core. Not
only does the incorporation of eight bromine atoms in
the β position of the pyrrol rings and phenyl radicals in
the meso position of the porphyrin macrocycle (CoPIII)
change the effective charge in the reaction site due to
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