Coordination compounds of cobalt porphyrins with nitrogen monoxide

Article abstract of DOI:10.1007/s11172-007-0111-3

Complex formation of NO and the NO ₂ ^- ion with cobalt porphyrins bearing various substituents in the porphyrin macrocycle, viz., tetraphenylporphine (1), β-octabromo-meso-tetraphenylporphyrin (2), protoporphyrin IX (3), and 5,10,15,

Full text of DOI:10.1007/s11172-007-0111-3

Russian Chemical Bulletin, International Edition, Vol. 56, No. 4, pp. 743—747, April, 2007
743
Coordination compounds of cobalt porphyrins with nitrogen monoxide
S. G. Pukhovskaya,aꢀ L. Zh. Guseva, and O. A. Golubchikov
b
a
aIvanovo State University of Chemical Technology,
7
prosp. F. Engel´sa, 153460 Ivanovo, Russian Federation.
Еꢀmail: golubch@isuct.ru
Institute of Solutions Chemistry, Russian Academy of Sciences,
b
1
ul. Akademicheskaya, 153045 Ivanovo, Russian Federation.
Еꢀmail: lgg@iscꢀras.ru
Complex formation of NO and the NO – ion with cobalt porphyrins bearing various
2
substituents in the porphyrin macrocycle, viz., tetraphenylporphine (1), βꢀoctabromoꢀmesoꢀ
tetraphenylporphyrin (2), protoporphyrin IX (3), and 5,10,15,20ꢀtetra(4Nꢀcarboxymethyleneꢀ
pyridyl)porphyrin tetrabromide (4), was studied. The stability constants of the nitrosyl and
nitrite extracomplexes in water and in ethanol were determined. Porphyrin 4 forms the most
stable extracomplexes.
Key words: coordination compounds, cobalt(II) porphyrins, nitrogen monoxide, nitrite ion,
nitrosyl complexes, nitrite complexes, stability constants.
Nitrogen monoxide is produced by enzymatic systems
in mammals and acts as a versatile regulator of metabolic
CoII porphyrins of different structural types: tetrapheꢀ
nylporphine (1), βꢀoctabromoꢀmesoꢀtetraphenylporꢀ
phyrin (2), protoporphyrin IX (3), and 5,10,15,20ꢀ
tetra(4Nꢀcarboxymethylenepiridyl)porphyrin tetrabroꢀ
processes.1
—3
Nitrogen monoxide is found
4,5
to be an
endogenic vasodilator and neurotransmitter and oppresses
thrombocyte aggregation; the superoxide radical and NO
during oxidative stress act in conjugation.
Nitrogen monoxide is produced in biological systems
by NO synthases from Lꢀarginine, which is classified as an
•
enzyme forming the NO free radical and citrulline as a
6
byꢀproduct. In the organism nitrogen monoxide is rapidly
oxidized to stable end products, NO2– and NO3 ions.
The nitrite ion in vivo oxidizes oxyhemoglobin and cataꢀ
lyzes reactions involving NO. The mechanism of toxic
effect of nitrites is mainly attributed to the oxidation of
hemoglobin to methemoglobin accompanied by the vioꢀ
lation of oxygen transport to tissues due to which hemic
hypoxia is developed. Nitrogen monoxide can coordinate
to thiols, sulfur—iron proteins of plasma, secondary
amines, and hemoglobin and be consumed in reactions
with oxygen, superoxide radicals, and hemoglobin.5
Therefore, nonꢀheme models of ironꢀcontaining proteins
are being developed and their nitrosyl complexes are unꢀ
–
—8
9
der study. However, the single compensatory adjustment
form of NO depositing and utilization is complex formaꢀ
tion with hemoglobin that occurs via axial coordination
on the protoheme.1 Therefore, investigation of binding
0
(
axial coordination) processes of nitrogen monoxide and
its metabolite (nitrite ion) by metal porphyrins in soluꢀ
tions is key for understanding of the essence of biological
processes involving NO.
In the present work, we studied the equilibrium of
–
formation of the axial NO and NO2 complexes with
R = H (1), Br (2)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 715—718, April, 2007.
066ꢀ5285/07/5604ꢀ0743 © 2007 Springer Science+Business Media, Inc.
1

744
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 4, April, 2007
Pukhovskaya et al.
spectrophotometrically using a special procedure.15 Purity of
porphyrins 1—4 was checked by TLC on aluminum plates with
the 0.5ꢀmm fixed layer of silica gel F254 (Merck) with a
mide (4). The complex of protoporphyrin IX (3) was choꢀ
sen as the closest analog of natural protoheme.
CHCl —EtOH eluate (ratio of solvents was chosen depending
3
on individual properties of porphyrins) and by the electronic
1
6—18
absorption spectra, which corresponded to literature data.
Results and Discussion
The electronic spectra of Co porphyrins 1—4 in water
and in EtOH were studied with purging a gaseous mixture
of nitrogen and nitrogen monoxide (NO content in nitroꢀ
gen is 0.49%) through the solution and showed the immeꢀ
diate formation of the axial (NO)CoР complexes (P is
porphyrin). This is indicated by a considerable bathoꢀ
chromic shift of the Soret band and a slight shift of the
visible spectral band. The spectral characteristics are given
in Table 1. The synthesized coordination compounds are
rather stable, and their spectra are also invariable for at
least 1 day.
Complex 4 is soluble in aqueous solutions in a wider
pH range than complex 3; this allows one to study its
interaction with NO at physiological acidity values of the
medium. Although protoheme is a waterꢀsoluble comꢀ
plex, in the hemoglobin composition its nearest environꢀ
ment is hydrophobic. Therefore, it seems reasonable to
study complexes 1 and 2 that are soluble in organic solꢀ
vents. These compounds have different sets of peripheral
substituents with different donorꢀacceptor properties and
different structures of the central macrocyclic ring: planar
for complex 1 and distorted for complex 2.
On contact with air oxygen the electronic absorption
spectrum changes in time: a hypsochromic shift is obꢀ
served (Soret band for porphyrin 4 is shown in Fig. 1).
This is related, most likely, to the oxidation of NO to
–
NO2 and is confirmed by the appearance of a new abꢀ
sorption band at 352 nm (see Fig. 1), which is characterꢀ
istic of a solution of the nitrite ion in EtOH.
Upon the addition of a solution of sodium nitrite to
solutions of the cobalt porphyrins, the character of the
visible part of the electronic absorption spectrum remains
unchanged and typical of the cobalt(II) complexes. In this
case, the Soret band shifts hypochromically by 4—9 nm,
depending on the nature of the porphyrin ligand (see
Table 1).
Experimental
Electronic absorption spectra were recorded on a Specord
Mꢀ400 spectrophotometer in 50ꢀmL cells (optical path 100 mm)
at 293.0±0.1 ꢁ. Complex formation was studied by spectrophoꢀ
tometry. A solution of porphyrin with a concentration of
–
6
–5
–1
1
0 —10 mol L in ethanol or water was deaerated with an
Ar flow. The exact volume of an NOꢀsaturated solution was
added to the cell with a microsyringe through a rubber septa to
prevent contact with air oxygen.11
Table 1. Electronic absorption spectra of the cobalt porphyrin
complexes (CoP)
IR spectra were recorded on an Avatar 360 FT—IR ESP
spectrometer in the 400—4000 cm– interval in anhydrous EtOH
and in ꢁBr pellets.
CoP
λ/nm (logε)
1
Q band I
—
Q band II
Soret band
Ethanol used as solvent was purified according to a known
procedure.12 The residual water content determined by titration
with the Fischer reagent13 was 0.05%. Sodium nitrite (reagent
grade) was twice recrystallized.14 Saturated solutions of NO in
water and EtOH (solvent was deaerated with an Ar flow) were
prepared purging the gas containing 0.46% NO and 99.54% N2
from the cylinder through the solvent. Nitrogen monoxide was
obtained according to an earlier described procedure.14 For this
purpose, copper chips were placed into a twoꢀneck flask and
HNO3 (density 1.10—1.15 g cm–3) was slowly added from a
dropping funnel. The reaction mixture was cooled to avoid the
formation of other nitrogen oxides. To remove higher nitrogen
oxides, the gas formed was passed through a flask containing a
In ethanol
540(4.61)
545
542
562(4.20)
1
426(5.68)
432
428
445(5.21)
462
NOꢀ1
NO2–ꢀ1
2
NOꢀ2
4
608(3.63)
550(3.75)
555
429(5.33)
446
NOꢀ4
NO2–ꢀ4
437
In water
534 (4.01)
3
(pH 9)
564 (3.98)
595 (sh)
417 (5.41)
420
432 (5.02)
440
–
NO2 ꢀ3
(pH 7.4)
4
540 (3.78)
545
5
% solution of NaOH and collected above water. The gas was
NOꢀ4
NO2–ꢀ4
dried by passing through a tube packed with solid ꢁOH. The
concentration of nitrogen monoxide in solutions was determined
437

Complexes of cobalt porphyrins with NO
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 4, April, 2007
745
A
compared to those in the NOꢀ1 complex is associated
with the partial positive charge on the nitrogen atoms of
the pyridinium substituents resulting in electron density
pulling through the conjugated system of the porphyrin and
metal atom from the N—O bond in nitrogen monoxide.
The stability constants (К) of the axial complexes of
3
2.0
1.5
1.0
0.5
–
NO and NO2 with complexes 1—4 were determined by
2
spectrophotometric titration. The characteristic example
for spectral changes during titration of the cobalt porphyꢀ
rin complexes with a solution of nitrogen monoxide is
presented in Fig. 2.
In the general case, the extraligands coordinate with
the Co porphyrins according to the equation
1
CoP + nL
n = 1 or 2
L CoP,
(1)
n
3
60
400
440
λ/nm
Fig. 1. Electronic absorption spectra of complexes 4 (1),
NOꢀ4 (2), and NO2–ꢀ4 (3).
II
where CoР is the porphyrin complex with Co , L CoР is
n
its axial complex, and L is the axial ligand (in this case,
NO or NO2 ). For the systems studied, n is 1. The stabilꢀ
ity constants of the axial complexes (К ) were calculated
–
Additional bands at 2256, 1925, and 1273 cm–1 appear
in the IR spectrum of the solution upon the saturation of
by the equation
ethanol with nitrogen monoxide. Based on the theoretical
calculations and experimental data,1
9—20
these bands
K = [LCoP]/([CoP]•[L]).
(2)
+
should be attributed to stretching vibrations of NO , NO,
and NO , respectively. The study of the IR spectra of
–
The absorbances at the analytical wavelengths were
used for the calculation of the equilibrium concentrations
of the extraꢀcomplex
complexes 1 and 2 showed that after saturation with niꢀ
trogen monoxide all the three types of ligands appeared in
+
–
their alcoholic solutions: NO , NO, and NO (bands at
–
1
2
256, 1925, and 1273 cm ). In addition, an additional
cLCoP = c0CoP(A – A )/(A – A).
(3)
0
к
–
1
band at 1659 cm appears in the spectra. After argon
–
1
purging, the band at 1925 cm disappears and the abꢀ
Here c
is the equilibrium concentration of LCoР,
is the initial concentration of CoР, and A , A,
and A are the initial, equilibrium, and final absorbances,
respectively. Experiments were carried out under the
conditions where c0 >> c0CoР; the number of added
ligands n was determined from the slope of the plots of
LCoР
sorption spectrum returns to the initial spectrum of СоР.
0
c
CoP
0
–
1
After air purging, the both bands (1925 and 1659 cm
)
f
disappear and the electronic absorption spectrum correꢀ
–
sponds to that of the СоР(NO2 ) nitrite complex. These
L
data make it possible to assign the absorption bands at
–
1
1
925 and 1659 cm to the free and coordinated nitrogen
monoxide, respectively.
A
The position of the band of stretching vibrations νNO
of the pentacoordinate nitrosyl Co porphyrins changes
–
1
–1
from 1693 cm (ꢁBr) for complex NOꢀ1 (1683 cm in
1
–
1
CH Cl ) to 1655 cm (Nujol) for the octaethylisoꢀ
2
2
1.0
–
1
bacteriochlorin with NO (see Refs 20—23) and 1660 cm
Nujol) for the octaethylporphyrin complex with NO, deꢀ
pending on the nature of substituents in the porphyrin
(
macrocycle. The Xꢀray diffraction data2
0,21
indicate that
0.5
the cobalt(II) complexes coordinate nitrogen monoxide
through the nitrogen atom to form nitrosyl complexes
with the nonlinear Со—N—O bonds, the angle between
which being ~120°. The axial ligand interacts with the
coordination center due to the electron density transfer
1
2
4
30
450
470
λ/nm
2
from the πꢀorbital of NO to the d ꢀorbital of the cobalt
0
z
Fig. 2. Electronic absorption spectra of complex 2 (c
1.2•10 mol L ) with additives of an NO solution (cL
10–4—10–3 mol L–1) in ethanol at Т = 298 ꢁ.
=
=
CoР
–
6
–1
cation. The decrease in the frequency of stretching vibraꢀ
tions of nitrogen monoxide in the axial complex NOꢀ4

746
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 4, April, 2007
Pukhovskaya et al.
3
.4
3.0
2.8 –logсL
the content of biologically active compounds in the aniꢀ
mal organism can be developed and produced.
1
0
0
.0
log[(A – A )/(A – A)]
0
c
0
.5
The authors are deeply grateful to A. S. Semeikin for
kindly presented porphyrins 1—4.
.5
2
1
This work was financially supported by the Ministry of
Education and Science of the Russian Federation (Anaꢀ
lytical Departmental Target Program "Development of
Scientific Potential of the Higher School (2006—2008),"
Grant RNP 2.1.1.1180).
0
–
–
0.5
1.0
–
0.5
log[(A – A )/(A – A)]
0
c
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.2
3.8
3.4
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2690±200
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H O
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4
429, 446
437
24200±500 810±200
26±4
1
2
(
pH 7.4)

Complexes of cobalt porphyrins with NO
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