Two decomposition mechanisms of nitrosyl iron complexes [Fe2(μ-SR)(NO)4]

Article abstract of DOI:10.1007/s11172-017-1751-6

Two decomposition mechanisms of nitrosyl iron complexes (NICs) [Fe₂(μ-SR)(NO)₄] in aqueous medium are known. One mechanism (for instance, in the case of complex [Fe₂(μ-SC₄H₃N₂)₂(

Full text of DOI:10.1007/s11172-017-1751-6

Russian Chemical Bulletin, International Edition, Vol. 66, No. 3, pp. 432—438, March, 2017
432
Two decomposition mechanisms of nitrosyl iron complexes [Fe2(μꢀSR)(NO)4]
O. V. Pokidova, N. A. Sanina, L. A. Syrtsova,† B. L. Psikha, N. I. Shkondina, A. I. Kotelnikov, and S. M. Aldoshin
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 522 3507. Eꢀmail: olesia16@mail.ru; shkon_ni@icp.ac.ru
Two decomposition mechanisms of nitrosyl iron complexes (NICs) [Fe2(μꢀSR)(NO)4] in
aqueous medium are known. One mechanism (for instance, in the case of complex
[Fe2(μꢀSC4H3N2)2(NO)4]) involves irreversible and rapid hydrolysis of NIC with the NO
release accompanied with the formation of the products of further NO transformations. In the
other mechanism (for instance, in the case of complexes [Fe2(μꢀS(CH2)2NH3)2(NO)4]SO4•
•2.5H2O and [Fe2(μꢀSC5H11NO2)2(NO)4]SO4•5H2O), no hydrolysis occurs but NICs reversꢀ
ibly dissociate to release both NO and thiolate ligand into the medium. In the present work, the
difference in the mechanisms of the NIC decomposition is explained by the difference in the
NIC redox potentials. The experimental evidences of this fact are given.
Key words: nitrosyl iron complexes, nitric oxide donor, cytochrome C, redox potential.
Numerous recent studies revealed an essential role of
nitric oxide (NO) as the most important physiological
regulator.1,2 This prompted an increase in the interest to
the synthesis and study of new compounds capable of NO
delivering to target tissues under biological pH values and
being the candidates for design of a new generation of
NOꢀreleasing drugs.3 Among these compounds, thiolate
nitrosyl iron complexes (NICs) [Fe2(SR)2(NO)4] hydroꢀ
lyzing in protic media similarly to Sꢀnitrosothiols4 and
diazenium dialates5,6 with the NO release7 are outstandꢀ
ing and comprise a new class of universal NO donors for
pharmacological applications.8—10
NOꢀcyt c3+ complex. The reactions of cyt c3+ with nitric
oxide significantly affect the mitochondrial respiratory
chain.13 For instance, the formation of the NOꢀcyt c3+
complex can inhibit electron transfer. Moreover, the reacꢀ
tion of NO with cyt c3+ can modify the peroxidase activity
of cyt c3+, which is apparently applicable for the apoptosis
control.13
Complexes I and II show vasodilation15 and antiꢀ
tumor16 activities. Complex I induces apoptosis of human
erythroleukemic cells (K562) and human colon cancer
cells (LS174T).17
The studies focused on the mechanism of action of
NICs and their transformations are of special interest.
Earlier,7,11 we have found that sulfurꢀnitrosyl iron comꢀ
plexes spontaneously decompose in protic media to reꢀ
lease NO and the decomposition rate constants depend on
the molecular structure of the complexes. Introduction of
deoxyhemoglobin (Hb) into the medium resulted in the
HbNO adduct formation. This adduct has a role of a nitric
oxide depot and is not only a storage pool of NO (the
lifetime of free NO in the living cell is a few seconds12) but
also determines prolonged action of NICs as NO donors.
Apart from Hb, ferricytochrome C (cyt c3+) can also
form relatively stable nitrosyl complexes despite the fact
that the heme iron of cyt c3+ is completely coordinated to
four N atoms of porphyrin core and amino acids (histiꢀ
dineꢀ18 and methionineꢀ80).13 We have recently found14
that introduction of NIC [Fe2(μꢀSR)(NO)4] bearing cysꢀ
teamine thiol ligands (I) into the cyt c3+ solution gives the
R =
, n= 2.5 (I);
, n = 5 (II);
(III)
Earlier, we have described two mechanisms of the deꢀ
composition of NICs [Fe2(μꢀSR)(NO)4]. One mechanism
involves hydrolysis of NIC III resulting in its complete
decomposition;18,19 the other mechanism is a reversible
dissociation of cationic NICs [Fe2(μꢀSR)(NO)4] (comꢀ
plexes I and II) to release both NO and thiolate ligand
into the medium.20,21 Due to this, complexes I and II
undergo ligand exchange in the presence of excess glutaꢀ
† Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0432—0438, March, 2017.
1066ꢀ5285/17/6603ꢀ0432 © 2017 Springer Science+Business Media, Inc.

Decomposition of nitrosyl iron complexes
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
433
Working concentration of NIC III was 2•10–4 mol L–1, concenꢀ
tration of DMSO was 3.3%. The absorption spectra were recordꢀ
ed at selected time intervals at the 450—650 nm range at 25 °C.
Hydrolysis of NIC III in the presence of K3[Fe(CN)6]. A freshly
prepared 6•10–3 M solution of NIC III in DMSO was used. An
aliquot of 0.1 mL of the complex solution was introduced into an
anaerobic experimental cuvette containing 2.6 mL of anaerobic
0.05 M phosphate buffer (pH 7.0). A reference cuvette conꢀ
tained 2.7 mL of 0.05 M phosphate buffer (pH 7.0). The reaction
thione to give more stable complexes with glutathione
ligand.17 The detected exchange between thiolate and
glutathione ligands may result in the regeneration of the
glutathionylated essential enzymes and the recovery of
their antitumor activity. The discovered ligand exchange
reaction occurring in NICs is also of interest for predictꢀ
ing antitumor activity of NICs. The aim of the present
work is to find the reasons for the difference in decompoꢀ
sition mechanisms of NIC III and NICs I and II.
was initiated by addition of 0.3 mL of an anaerobic 4•10–2
M
solution of K3[Fe(CN)6] in 0.05 M phosphate buffer (pH 7.0).
Working concentration of NIC III was 2•10–4 mol L–1, concenꢀ
tration of DMSO was 3.3%. Absorption spectrum was recorded
immediately after addition of K3[Fe(CN)6].
Experimental
Commercially available 2ꢀaminoꢀ2ꢀhydroxymethylpropaneꢀ
1,3ꢀdiol (Tris) (Serva, Germany), Na2HPO4•6H2O, NaH2PO4•
•H2O, safranin T, methylene blue (MP Biomedicals, Germany),
methyl viologen, iron(II) sulfate, Dꢀpenicillamine, and cytoꢀ
chrome C from equine heart (SIGMA, USA) were used. Water
was purified by distillation using a Bi/Duplex apparatus
(Germany).
Complexes I (CCDC 663194), II (CCDC 680286), and III
(CCDC 681094) were synthesized following the known proceꢀ
dures.16,18,22
Elemental analyses of the crystalline samples were carried
out in the Analytical Centre of Collective Use of the Institute of
Problems of Chemical Physics of the Russian Academy of
Sciences.
IR spectra were recorded with Spectrum BXꢀII IRꢀFourierꢀ
transform spectrophotometer in the KBr pellets (1 mg of the
sample for 300 mg of KBr).
Decomposition of NIC II. The experiments were carried out
with the same stock solution of NIC II. To a weighted sample of
NIC II placed in a vial filled with nitrogen, 0.05 M TrisꢀHCl
(pH 7.0) was added in the amount to obtain 6•10–4 mol L–1
stock solution. The mixture was stirred for 15 min until complete
dissolution of the complex. The solution was collected by an
anaerobic syringe and frozen dropwise into liquid nitrogen. The
stock solutions of NIC II were thawed under a nitrogen flow for
20 min prior to use. Then an aliquot of the solution of 0.75 mL
was added into a cuvette (V = 4 mL, optical path length of 1 cm)
containing 2.25 mL of 0.05 M anaerobic buffer (pH 7.0) to obꢀ
tain working concentration of NIC II of 1.5•10–4 mol L–1
.
A reference cuvette contained 3 mL of the same buffer. Absorpꢀ
tion spectra were recorded at selected time intervals at the
250—650 nm range at 25 °C.
Decomposition of NIC II in the presence of K3[Fe(CN)6] was
carried out as described for NIC III using 0.05 M Trisꢀbuffer
(pH 7.0).
All procedures were performed under the inert gas atmoꢀ
sphere as earlier described.7
Kinetics of the reactions of NICs II and III with c3+ in the
presence of K3[Fe(CN)6]. The experiments were carried out with
the same 2•10–4 M stock solution of cytochrome C (commercial
cytochrome C contained 10% of ferricytochrome). A weighted
sample of cytochrome C was dissolved in 0.05 M phosphate
buffer (pH 7.0) to obtain a stock solution with concentration of
2•10–4 mol L–1. After stirring for 40 min, the obtained solution
was frozen dropwise into liquid nitrogen. The stock solution of
cytochrome C was thawed for 20 min in the 5ꢀmL vials under an
Complex I, [Fe2(μ ꢀSC2H7N)2(NO)4]SO4•2.5H2O. Found (%):
2
C, 8.53; H, 2.77; N, 15.70; S, 17.71. C4H19Fe2N6O10.5S3. Calꢀ
culated (%): C, 9.10; H, 3.60; Fe, 21.25; N, 15.93; O, 31.87;
S, 18.27. IR, ν/cm–1: 3454 (m), 3003 (w), 2927 (w), 1769 (s),
1727 (s), 1461 (w), 1385 (m), 1340 (m), 1266 (m), 1120 (s),
770 (w), 620 (s); 1769, 1727 (νNO).
Complex II, [Fe2(μ ꢀSC5H11NO2)2(NO)4]SO4•5H2O.
2
Found (%): Fe, 15.60; C, 16.72; H, 4.50; N, 11.75; O, 38.02;
S, 13.40. C10H32Fe2N6O17S3. Calculated (%): Fe, 15.64; C, 16.76;
H, 4.47; N, 11.73; O, 37.99; S, 13.41. IR, ν/cm–1: 1771 (s),
1723 (s), 1626 (m), 1375 (m), 1337 (m), 1269 (m), 1189 (m),
1114 (m), 1089 (m), 746 (m); 1771, 1723 (νNO).
Complex III, [Fe2(SC4H3N2)2(NO)4]. Found (%): C, 2.00;
H, 1.17; Fe, 24.47; N, 23.83; S, 13.96. C8H6Fe2N8O4S2. Calcuꢀ
lated (%): C, 2.12; H, 1.34; Fe, 24.59; N, 24.68; S, 14.15. IR,
ν/cm–1: 3472 (w), 1797 (b.s), 1746 (v.s), 1551 (m), 1425 (w),
1376 (m), 1191 (w), 1151 (m), 1070 (w), 811 (w), 774 (w),
739 (w), 629 (w), 550 (w); 480 (w); 1797, 1746 (νNO).
argon flow immediately prior to use. To 2.6 mL of 2•10–4
M
cytochrome C solution placed into an anaerobic cuvette, 0.3 mL
of 4•10–2 mol L–1 solution of K3[Fe(CN)6] was added. A referꢀ
ence cuvette contained 2.6 mL of 0.05 M phosphate buffer
(pH 7.0) and 0.3 mL of K3[Fe(CN)6] solution. The absorption
spectrum was recorded with the aim to confirm the obtaining of
cyt c3+. The reaction was initiated by simultaneous addition of
0.1 mL of 6•10–4 mol L–1 solution of NIC III in DMSO into
both the experimental and reference cuvettes. The volumes of
both reaction mixtures were 3 mL. The working concentration
of NIC III in the experimental cuvette was 2•10–5 mol L–1. The
difference absorption spectra were recorded at selected time inꢀ
tervals at the 450—650 nm range at 25 °C.
Hydrolysis of NIC III. A freshly prepared 6•10–3 M solution
of NIC III in DMSO was used. To a weighted sample of NIC III
place in a vial filled with argon, anaerobic DMSO was added
in the amount to obtain a stock solution with concentration of
6•10–3 mol L–1. The mixture was stirred under an argon flow
until complete dissolution of the complex (3—5 min). Then
0.1 mL of the resulting solution was placed into an anaerobic
experimental cuvette (V = 4 mL, optical path length of 1 cm)
containing 2.9 mL of anaerobic 0.05 M phosphate buffer (pH 7.0).
A reference cuvette contained 0.05 M phosphate buffer (pH 7.0).
The reactions of cyt c3+ with NIC II in the presence of
K3[Fe(CN)6] were carried out similarly.
Reaction of NIC II with cytochrome C without K3[Fe(CN)6].
Into an anaerobic cuvette containing 1 mL of 0.05 M phosphate
buffer (pH 7.0), 1 mL of 1.8•10–4 mol L–1 solution of commerꢀ
cial cytochrome C in 0.05 M phosphate buffer (pH 7.0) was
added. A reference cuvette contained 2 mL of 0.05 M phosphate

434
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
Pokidova et al.
buffer (pH 7.0). The reaction was initiated by simultaneous adꢀ
dition of 1 mL of 6•10–4 mol L–1 solution of NIC II in the same
buffer into both cuvettes. Working concentration of NIC II was
2•10–4 mol L–1 in both cuvettes. The difference absorption specꢀ
tra were recorded at selected time intervals at the 450—650 nm
range at 25 °C.
is a spinꢀforbidden reaction; therefore, the most probable
elementary step producing nitrous oxide is the reaction
between NO and NO–. In both cases, reduction of the NO
molecules is the most probable initial stage of the reaction
resulting in the nitrous oxide formation: NO + e– = NO–.
To produce NO– from NO, the reducing agent is required;
the standard redox potential of NO is –0.8( 0.2) V.26
Complex III can play a role of the reducing agent after the
Fe—NO bond dissociation and coordination of a solvent
(water) molecule at an open coordination site. The authors
rationalized the enhanced reducing ability of the resulting
species in terms of the increased electron density near the
Fe atom due to the loss of the partially negative NO ligand
and coordination of the donor water molecule. Therefore,
two following reactions consuming NO can be suggested.
The first reaction (reaction (3)) is the electron transfer
oxidation of iron complex
Reactions of NICs I and II with redox indicators. Into an
anaerobic cuvette (V = 4 mL, optical path length of 1 cm) conꢀ
taining 2 mL of a redox indicator solution with stock concentration
of either 3•10–5 or 1.5•10–5 mol L–1, 1 mL of 6•10–4 mol L–1
solution of NICs I and II was added. A reference cuvette contained
2 mL of water and 1 mL of a solution of NIC I or II. The folloꢀ
wing working concentrations were obtained: NICs of
2•10–4 mol L–1, redox indicator of either 1•10–5 or 2•10–5 mol L–1
.
Absorption spectra were recorded at the 450—750 nm range
at 25 °C.
Absorption spectra were recorded at 25 °C with a Specord
Mꢀ40 spectrophotometer equipped with an interface for comꢀ
puterꢀaided registration of spectra and a thermostatic cuvette
holder.
Measurements of amount of cyt c3+ and NOꢀcyt c3+. Amounts
of cyt c3+ and NOꢀcyt c3+ were estimated using absorption spectra.
To find the concentration of NOꢀcyt c3+, the absorption spectra
of the system containing cyt c3+ and NOꢀcyt c3+ were resolved
into individual components (cyt c3+ and NOꢀcyt c3+) using
MathCad software as previously described.7
LFe + NO
LFeox + NO–.
(3)
The second reaction (reaction (4)) is the deep transꢀ
formation occurring only when the relatively large amounts
of the reduced NO– species are released via reaction (3)
and accumulated in the medium.
NO + NO–
ONNO–
N2O
(4)
Results and Discussion
Studying the reactions of NIC I with cyt c3+ 14
two
,
Earlier,18,19 the mechanism of decomposition of NIC
III with pyrimidine thiolate ligands, (tetranitrosyl)[μꢀSꢀ
bis(pyrimidinꢀ2ꢀylthio)diiron], has been described. The
kinetic dependences of the amount of the NO released by
NIC III into the media at different pH values and the
same other conditions show maxima, which appearance
was attributed to the further NO transformations.18 It was
suggested that these transformations proceed in two
following steps:
following important findings were made. First, the numꢀ
ber of the NO groups released by NIC I can be determined
by monitoring the NOꢀcyt c3+ adduct formation. Second,
K3[Fe(CN)6] oxidizes complex I resulting in the release of
all NO groups of NIC I immediately after addition of
ferricyanide. Therefore, we suggested that reaction (3) is
a key step causing irreversible decomposition of NICs.
First of all, we studied the kinetics of the decomposition
of NIC III (Fig. 1, a) and its reaction with K3[Fe(CN)6]
(see Fig. 1, b). Immediately after mixing the reagents,
intensity of absorption noticeably decreased indicating the
decomposition of NIC III.
LFe(NO)
where L = [Fe(μꢀSC4H3N2)2(NO)3],
NO Products.
LFe + NO,
(1)
To determine the number of the released NO groups,
cyt c3+ was used. The absorption spectra shown in Fig. 2
visualize the reaction of NIC III with cyt c3+. Due to
reversibility of the formation of the NOꢀcyt c3+ adduct via
(2)
Since nitrous oxide is formed in the solutions of diꢀ
nitrosyl iron complexes, it was assumed23 that the final
product of the NO transformations is N2O. Formation of
nitrous oxide is indicative of the N—N bond formation.
Two molecules of NO cannot be strongly bound to each
other. For the N—N bond formation to occur, the reduced
form of NO molecule is required. It is known that24 NO
and NO– react with very high rate to give hyponitrite
radical anion ONNO– exhibiting strong oxidative properꢀ
ties. Further transformations of this species gives nitrous
oxide.25 Two nitroxyls (protonated form of NO–) dimerize
affording unstable molecule HONNOH, which decomꢀ
poses into N2O and H2O. However, protonation of NO–
→
reaction cyt c3+ + NO NOꢀcyt c3+, the excess of cyt c3+
←
is required.14 The experiment (see Fig. 2) was conducted
under the most optimal conditions. The absorption of the
cyt c3+ solution with the used starting concentration is
nearly 2. To avoid the distortion of the spectra, the cyt c3+
concentration cannot be increased. The NIC concentraꢀ
tion cannot exceed 2•10–5 mol L–1 without loss of accuꢀ
racy due to small sample volume. The reaction should be
performed in the presence of K3[Fe(CN)6] since in the
absence of potassium ferricyanide cyt c3+ is reduced to
ferrocytochrome; the latter does not react with NO at
pH 7.27 Figure 2 shows that the reaction results in a rapid

Decomposition of nitrosyl iron complexes
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
435
A
A
a
2.5
0.20
2.0
1
0.15
0.10
0.05
15
1
1.5
1.0
2
0.5
500
550
600
650 λ/nm
500
550
600
650 λ/nm
Fig. 2. Changes in the difference absorption spectra for the reꢀ
action of NIC III (2•10–5 mol L–1) with K3[Fe(CN)6]
(4•10–3 mol L–1) and cyt c3+ (1.5•10–4 mol L–1): 1, spectrum
of a solution containing cyt c3+ and K3[Fe(CN)6]; 2, spectrum
recorded immediately after addition of NIC III.
A
b
0.20
0.15
0.10
0.05
1
of NIC III. To confirm the fast release of the NO groups
during this process, we performed the reaction with cyt c3+
(Fig. 4). Mixing the reagents causes the rapid formation of
adduct NOꢀcyt c3+. Concentration of this adduct was
6.7•10–5 mol L–1, which corresponds to the release of
3.3 NO groups per 1 molecule of NIC II.
At the same time, we found that in other cases (in
the absence of oxidizing agents) no hydrolysis of comꢀ
plexes20,21 [Fe2(μꢀS(CH2)2NH3)2(NO)4]SO4•2.5H2O and
[Fe2(μꢀSC5H11NO2)2(NO)4]SO4•5H2O occurs but
NICs reversibly dissociate to release both NO and thiolate
ligand into the medium. Assuming that ligands (L) are
released from the NIC II and further from product P1
formed by the nitric oxide release from NIC II, a plausiꢀ
ble mechanism of the transformation can be presented
as follows:21
2
500
550
600
650 λ/nm
Fig. 1. a, Changes in the absorption spectra of NIC III
(2•10–4 mol L–1). Spectra were registered after 1 (1) and 5 min
(2) after preparation of the NIC solution. Spectra (3)—(15) were
recorded at 15 min time intervals. b, Absorption spectra for
the reaction of NIC III (2•10–4 mol L–1) with K3[Fe(CN)6]
(4•10–3 mol L–1): 1, spectrum of the starting NIC III; 2, specꢀ
trum recorded immediately after addition of K3[Fe(CN)6].
Reaction conditions (here and on Figs 2 and 4): 25 °C, 0.05 M
phosphate buffer (pH 7.0).
II
P1 + NO,
P2 + L,
P1
II
formation of 5•10–5 mol L–1 of NOꢀcyt c3+, which indiꢀ
cates the release and binding with cyt c3+ of 2.5 NO groups.
Under the same conditions, NIC I decomposes similarly
producing 3.08 NO groups.14
In the present work, we also studied the reaction of
K3[Fe(CN)6] with another cationic complex, namely,
NIC II. First, we studied the kinetics of the decomposition
of NIC II (Fig. 3, a). Figure 3, a shows the time depenꢀ
dence of the absorption of NIC II. Similarly to NIC III,
addition of K3[Fe(CN)6] results in a rapid decomposition
of the oxidized NIC II, which is evidenced by the noticeꢀ
able decreased in the VIS absorption intensity (see Fig. 3, b).
It is clear that complex II decomposes immediately
after addition of K3[Fe(CN)6] as it was found in the case
P3 + L.
Earlier,17 the measurements of the k1 and k–1 rate conꢀ
stants by a sensor electrode gave the following values: k1 =
= (4.6 0.1)•10–3 s–1 and k–1 = (9.7 0.2)•103 L mol–1 s–1.
To describe the system completely, the k2 (the reaction
rate constant of the penicillamine ligand releasing from
NIC II), k–2, and k3 values should be determined. We
suggest that NIC II and product P1 cannot be distinguished
spectrophotometrically. Consequently, we can measure
experimentally only x(t) = [II] + [P1].
The reaction scheme is described by the following
kinetic equations:

436
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
a
Pokidova et al.
A
A
2.0
1.5
1.0
0.5
2.0
1
25
1.5
1.0
0.5
1
25
2
1
300
400
500
600
λ/nm
500
550
600
650 λ/nm
Fig. 4. Changes in the difference absorption spectra for the reacꢀ
tion of NIC II (2•10–5 mol L–1) with cyt c3+ (1.5•10–4 mol L–1
)
A
b
in the presence of K3[Fe(CN)6] (4•10–3 mol L–1): 1, spectrum
of cyt c3+ in the presence of K3[Fe(CN)6]; 2, spectrum recorded
immediately after addition of NIC II.
0.06
0.05
0.04
0.03
0.02
0.01
A
1
2
1.4
1.2
1.0
0.8
0.6
500
550
600
650 λ/nm
Fig. 3. a, Changes in absorption spectra of NIC II
(1.5•10–4 mol L–1). Spectra were recorded after 1 (1), 5 (2), and
10 min (3) after preparation of the NIC solution; spectra 4 and 5
were recorded at 10 min intervals, spectra 6—8, at 15 min time
intervals, spectra 9—25, at 30 min time interval. b, Changes in
0.4
0.2
1
the absorption spectra for the reaction of NIC II (2•10–5 mol L–1
)
with K3[Fe(CN)6] (4•10–3 mol L–1): 1, spectrum of the starting
NIC II; 2, spectrum recorded immediately after addition of
K3[Fe(CN)6]. Reaction conditions: 25 °C, trisꢀНCl (pH 7.0).
2—6
500
550
600
650 λ/nm
d[II]/dt = –k1[II] + k–1[P1][NO] – k3[II],
d[NO]/dt = k1[II] – k–1[P1][NO],
Fig. 5. Changes in the difference absorption spectra for the reacꢀ
tion of NIC II (2•10–4 mol L–1) with Cyt (6•10–5 mol L–1) in
the absence of K3[Fe(CN)6]. Spectra were recorded after 1 (2),
9 (3), 24 (4), 54 (5), and 69 min (6) after the reaction onset.
1, Spectrum of the commercial cytochrome C (a mixture of
ferrocytochrome and ferricytochrome). Reaction conditions:
25 °C, 0.05 M phosphate buffer (pH 7.0).
d[P1]/dt = k1[II] – k–1[P1][NO] – k2[P1] +
+ k–2[P2][L],
(5)
Table 1. Redox potentials of the reagents
Redox
Redox potential/V
Color
indicator
(at pH 7.0)
reduced form
oxidized form
Methyl viologen
Safranin T
Methylene blue
Cytochrome C
K3[Fe(CN)6]
–0.4
–0.29
+0.01
+0.24
+0.35
Blue
Colorless
Colorless
—
Colorless
Redꢀviolet
Blue
—
—
—

Decomposition of nitrosyl iron complexes
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
437
d[P2]/dt = k2[P1] – k–2[P2][L],
All experiments were carried out under argon since
oxygen accelerates the decomposition of given NICs.18 It
has been found14 that NICs I and II reduce cyt c3+, whose
redox potential E0 is +0.254 V28 (Fig. 5).
At the same time, NICs I and II do not reduce methyl
viologen and safranin T (Figs 6 and 7) and only NIC II
reduces methylene blue (see Fig. 7).
d[L]/dt = k2[P1] – k–2[P2][L] + k3[II].
Solving the differential equations by kinetic modꢀ
eling technique gives k2 = (1.8
k–2 ≈ 0.1—0.2 L mol–1 s–1, and k3 ≈ 1.0•10–3 s–1
0.2)•10–3 s–1
,
.
Thus, if NIC is able to reduce NO (E0 < –0.8 V), it will
inevitably decompose on going into the oxidized state;
this fact is confirmed by Figs 1, b and 3, b as well as by
earlier published data18 (for NIC III). At the same time,
we assume that NICs with E0 > –0.8 V and therefore
unable to reduce NO do not transform into the oxidized
state and, consequently, undergo reversible dissociation.
To confirm this conclusion, we estimate the E0 values
for NICs I and II from their reactions with dyes with the
known E0 values (Table 1).
Based on the obtained results, it can be concluded that
the E0 values of NICs I and II fall in the following ranges:
+0.01 V < E0 (I) < +0.24 V,
–0. 29 V < E0 (II) < +0.01 V.
Consequently, NICs I and II cannot reduce nitric
oxide, therefore these complexes do not undergo oxidaꢀ
tion and irreversible hydrolysis in the aqueous medium.
a
b
A
A
2—4
1.2
0.9
0.6
0.3
0.4
0.3
0.2
0.1
1, 2
1
500
600
700
λ/nm
500
550
600 λ/nm
Fig. 6. Changes in absorption spectra for the reaction of NIC I (2•10–4 mol L–1) with methylene blue (2•10–5 mol L–1) (a) and
safranin T (1•10–5 mol L–1) (b). Spectra were recorded after 1 (2), 5 (3), and 10 min (4) after the reaction onset. 1, Spectrum of
methylene blue (a) or safranin T (b). Reaction conditions: 25 °C, water.
A
A
a
b
0.9
0.6
0.3
2, 3
1
1.2
0.8
0.4
1
2
3
500
550
600
650
700
750 λ/nm
500
550
600
λ/nm
Fig. 7. Changes in absorption spectra for the reaction of NIC II (2•10–4 mol L–1) with methylene blue (2•10–5 mol L–1) (a) or safranin
T (1•10–5 mol L–1) (b). Spectra were recorded 1 (2) and 5 min (3) after the reaction onset. 1, Spectrum of methylene blue (a) or
safranin T (b). Reaction conditions: 25 °C, water.

438
Russ.Chem.Bull., Int.Ed., Vol. 66, No. 3, March, 2017
Pokidova et al.
12. A. L. Lehninger, D. L. Nelson, M. M. Cox, in Principles of
Biochemistry, Ed. V. Neal, Worth Publishers, New York,
1993, p. 769.
13. A. N. Osipov, G. G. Borisenko, Yu. A. Vladimirov, Bioꢀ
chemistry (Moscow), Special Issue, Biol. Chem. Rev., 2007,
72, 1491
14. N. A. Sanina, L. A. Syrtsova, N. I. Shkondina, T. N. Rudꢀ
neva, A. I. Kotel´nikov, S. M. Aldoshin, Russ. Chem. Bull.,
2010, 59, 1565.
15. Pat. RF 2460531 C1; Byul. Izobret. [Invention Bull.], 2012,
No. 25 (in Russian).
Decomposition of these complexes in the aqueous mediꢀ
um to release NO and thiolate ligand is reversible.20,21
In summary, we developed simple method for predicꢀ
tion of decomposition pathway of NICs. This method is
based on the reaction of NIC with methyl viologen. The
absence of reduction of the dye with NIC is evidenced in
favor of the reversible decomposition mechanism of the
complexes, the possibility of the ligandꢀglutathione exꢀ
change, and, in turn, the potential antitumor activity
of NIC.
16. Pat. US 8067628 B2; 2011.
The authors are grateful to A. V. Chudinov for writing
a computer program for computer processing the absorpꢀ
tion spectra by the least square method using Mathcad
software.
17. N. A. Sanina, O. S. Zhukova, Z. S. Smirnova, L. M. Borisoꢀ
va, M. P. Kiseleva, S. M. Aldoshin, Ros. Bioterapevt. Zh.
[Russ. Bioter. J.], 2008, 7, No. 1, 52 (in Russian).
18. N. A. Sanina, G. V. Shilov, S. M. Aldoshin, A. F. Shestakov,
L. A. Syrtsova, N. S. Ovanesyan, E. S. Chudinova, N. I.
Shkondina, N. S. Emel´yanova, A. I. Kotel´nikov, Russ.
Chem. Bull., 2009, 58, 572.
19. L. A. Syrtsova, N. A. Sanina, A. F. Shestakov, N. I. Shkonꢀ
dina, T. N. Rudneva, N. S. Emel´yanova, A. I. Kotel´nikov,
S. M. Aldoshin, Russ. Chem. Bull., 2010, 59, 2203.
20. L. A. Syrtsova, N. A. Sanina, E. N. Kabachkov, N. I. Shkonꢀ
dina, A. I. Kotel´nikov, S. M. Aldoshin, RSC Adv., 2014,
4, 24560.
21. L. A. Syrtsova, N. A. Sanina, K. A. Lyssenko, E. N. Kabachꢀ
kov, B. L. Psikha, N. I. Shkondina, O. V. Pokidova, A. I.
Kotel´nikov, S. M. Aldoshin, Bioinorg. Chem. Appl., 2014, 9;
http://dx.doi.org/10.1155/2014/641407.
22. Pat. RF 2441873 C2, Byul. Izobret. [Invention Bull.], 2012,
No. 4 (in Russian).
23. A. F. Vanin, Biokhimiya, 1998, 63, 924 [Biochemistry
(Moscow) (Engl. Transl.), 1998, 63].
24. S. V. Lymar, V. Shafirovich, G. A. Poskrebyshev, Inorg.
Chem., 2005, 44, 5212.
25. G. A. Poskrebyshev, V. Shafirovich, S. V. Lymar, J. Am.
Chem. Soc., 2004, 126, 891.
26. M. D. Bartberger,W. Liu, E. Ford, K. M. Miranda, C. Switꢀ
zer, J. M. Fukuto, P. J. Farmer, D. A. Wink, K. N. Houk,
Proc. Natl. Acad. Sci. USA, 2002, 99, 10958.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 13ꢀ03ꢀ00549).
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Received June 14, 2016;
in revised form October 18, 2016