Oxidation of ascorbic acid in the presence of phthalocyanine metal complexes. Chemical aspects of catalytic anticancer therapy. 1. Catalysis of oxidation by cobalt octacarboxyphthalocyanine

Article abstract of DOI:10.1023/A:1020900529609

The kinetics of oxidation of ascorbic acid in the presence of cobalt octa-4,5-carboxyphthalocyanine sodium salt (Teraphthal) was studied. A kinetic equation was obtained and a scheme of the process was proposed. According to the scheme, the first stage is the formation of a complex of the catalyst with dioxygen, and the limiting stage is the reaction of dioxygen with the ascorbate monoanion. The influence of the pH of the medium and the presence of a transport protein (albumin) on the state and catalytic activity of Teraphthal was studied. The involvement of hydrogen peroxide and oxygen-centered radicals in the catalytic oxidation of ascorbic acid was proved.

Full text of DOI:10.1023/A:1020900529609

Russian Chemical Bulletin, International Edition, Vol. 51, No. 7, pp. 1231—1236, July, 2002
1231
Oxidation of ascorbic acid in the presence of phthalocyanine metal
complexes. Chemical aspects of catalytic anticancer therapy.
1. Catalysis of oxidation by cobalt octacarboxyphthalocyanine
b
a
E. G. Girenko,a S. A. Borisenkova, and O. L. Kaliya
ꢀ
аFederal State Unitary Enterprise ´State Research Center
"Research Institute of Organic Intermediate Products and Dyes," ´
Building 4, 1 ul. B. Sadovaya, 101999 Moscow, Russian Federation.
Fax: +7 (095) 254 1200. Еꢀmail: jerrik@cityline.ru
bDepartment of Chemistry, M. V. Lomonosov State University,
Leninskie Gory, 119899 Moscow, Russian Federation.
Fax: +7 (095) 932 8846. Eꢀmail: borisenk@petrol.chem.msu.ru
The kinetics of oxidation of ascorbic acid in the presence of cobalt octaꢀ4,5ꢀcarboxyꢀ
phthalocyanine sodium salt (Teraphthal) was studied. A kinetic equation was obtained and a
scheme of the process was proposed. According to the scheme, the first stage is the formation
of a complex of the catalyst with dioxygen, and the limiting stage is the reaction of dioxygen
with the ascorbate monoanion. The influence of the pH of the medium and the presence of a
transport protein (albumin) on the state and catalytic activity of Teraphthal was studied. The
involvement of hydrogen peroxide and oxygenꢀcentered radicals in the catalytic oxidation of
ascorbic acid was proved.
Key words: cobalt phthalocyanine, ascorbic acid, oxidation, catalysis, anticancer therapy.
The method of catalytic anticancer therapy using free
Experimental
oxygenꢀcentered radicals as cytotoxic agents was proꢀ
posed, tested, and patented in 1995.1 The radicals are
formed in the oxidation of ascorbic acid (АН2) with
dioxygen catalyzed by phthalocyanine metal complexes
(РсМ). The relative catalytic activity of several substiꢀ
tuted PcM in this reaction was tentatively estimated in
1996.2 Further development of the method required a
deeper study of the kinetics and mechanism of the reacꢀ
tion to establish quantitative criteria for the evaluation
of catalysts and recommendations on controlling the efꢀ
ficiency of the catalytic generation of cytotoxic agents.
We have recently3 analyzed the published data on the
oxidation of АН2 with dioxygen both for autooxidation
and in the presence of different catalysts and determined
the main problems to be studied.
In this work, we present the first results obtained with
cobalt octaꢀ4,5ꢀcarboxyphthalocyanine (PcCo) as the
catalyst. The sodium salt of PcCo named Teraphthal
(TP) is most actively being studied in various biological
aspects4 and is already undergoing the secondꢀphase cliniꢀ
cal tests as a component of a catalytic system for antiꢀ
cancer therapy. The kinetic characteristics of АН2 oxiꢀ
dation in an aqueous buffer solution and in a solution of
albumin, which models, to some extent, the blood plasma,
were obtained. The scheme of the reaction was proposed.
Cobalt octaꢀ4,5ꢀcarboxyphthalocyanine sodium salt
СоРс(СООNa)8, viz., Teraphthal (TP), was synthesized at the
Federal State Unitary Enterprise ´State Research Center "Reꢀ
search Institute of Intermediate Products and Dyes" ´ and its
quality corresponds to the Pharmacopeial Enterprise Clause
(PEC No. 42ꢀ0047ꢀ1680) advocated by the Pharmacopeial
Committee of the Russian Federation. The electronic absorpꢀ
tion spectrum (EAS) of TP in an aqueous 0.025 М phosꢀ
phate buffer, рН 6.9, has maxima at λ = 674 nm (ε =
1.2•105 mol –1 L cm–1) and 333 nm.
Ascorbic acid АН2 (pharmaceutics grade, Russia) was used.
Its EAS in an 0.025 М buffer solution with рН 6.9 has a maxiꢀ
mum at λ = 267 nm, ε = 1.4•104 mol–1 L cm–1
.
Oxidation of ascorbic acid in the presence of TP was studꢀ
ied using a Specord UV VIS spectrophotometer in 1ꢀcm quartz
cells. All experiments were carried out in buffer solutions with
рН 5.5—7.4. The concentration of ascorbic acid was deterꢀ
mined from the absorption at λ = 267 nm with a correction for
the absorption of TP and products of the catalyst destruction in
this spectral region. The concentration of TP was calculated
from the absorption in the visible region. The concentration
intervals for the reactants were 3.5•10–5—5.0•10–3 mol L–1
and 0.35•10–5—0.35•10–4 mol L–1 for АН2 and TP, respecꢀ
tively, and corresponded to those of therapeutic doses of these
substances. Dioxygen—dinitrogen mixtures were prepared for
studying the dependence of the reaction rate on the parꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1137—1142, July, 2002.
1066ꢀ5285/02/5107ꢀ1231 $27.00 © 2002 Plenum Publishing Corporation

1232
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 7, July, 2002
Girenko et al.
tial dioxygen pressure. The temperature in experiments was
18—37 °C. The initial rate of ascorbic acid oxidation (W 0
W 0 •106/mol (L min)–1
W 0 2 •106/mol (L min)–1
AH
2
AH
)
AH
2
60
14
was determined from the tangents to the initial regions of the
kinetic curves. The nonꢀcatalytic component of the rate of
ascorbic acid oxidation is negligible in the concentration interꢀ
val studied of the reactants and the catalyst, which follows, in
particular, from the rather accurate extrapolation to zero of the
dependence of the oxidation rate on the catalyst concentration.
Effective activation energy was determined in an 0.025 М
phosphate buffer, рН 6.9 (j = 0.1) in air in the 18—37 °C
temperature range.
Superoxide dismutase (SOD) (Serva), catalase (Olaine Plant
of Chemical Reagents, Latvia), and bovine serum albumin
(BSA) (Sigma) were used as received. The specific trap of
hydroxyl radicals, viz., pꢀnitrosoꢀN,Nꢀdimethylaniline (NDMA,
reagent grade), was purified before use by double crystallizaꢀ
tion from benzene.
a
1
2
12
50
40
30
20
10
10
8
6
4
2
0
0.2 0.4 0.6 0.8 1.0
С 0
•103/mol L–1
AH
2
W 0 •106/mol (L min)–1
AH
2
10
b
Results and Discussion
8
6
4
2
3
1. Kinetics and mechanism of the reaction
The oxidation of ascorbic acid is the firstꢀorder reacꢀ
tion with respect to both the catalyst and ascorbic
acid (Fig. 1).
0
1
2
3
4
С 0 •105/mol L–1
TP
Fig. 1. Initial rate of ascorbic acid oxidation (W 0 ) as a
AH
2
function of the initial concentrations of the substrate (a) and
catalyst (b); СTP = 0.35•10–4 (1), 0.8•10–5 (2), and СAH
7•10–5 mol L–1 (3), 18 °С, рН 6.86.
=
2
sary to evaluate whether a combination of hyperthermism
and catalytic anticancer therapy is reasonable or not.
The overall experimental kinetic equation takes the
form (2), and the corresponding simplest scheme of the
reaction is presented by reactions (3a) and (3b) if we take
into account that the total concentration of Teraphthal
is analytical.
The plots of W 0
by the equation
vs. PO (Fig. 2, а) are described
2
AH
2
W 0
= K´PO /(1 + K″PO )
2
(1)
АН
2
2
(K´ and K″ are the constants in the Michaelis—Menten
equation), which is confirmed by the corresponding linꢀ
ear anamorphoses (Fig. 2, b). The data obtained allow
the estimation of the rate of АН2 oxidation in the living
organism (points corresponding to the dioxygen content
in the venous and arterial blood are marked in curve 2
(Fig. 2)) and prediction that it can substantially be inꢀ
creased by an increase in the dioxygen concentration in
the blood using, e.g., hyperbaric oxygenation.
The effective activation energy of ascorbic acid oxidaꢀ
tion in the presence of Teraphthal was determined in the
18—37 °C temperature interval in air as 31 1 kJ mol–1
The effective activation energy needs to be estimated beꢀ
cause the catalytic system under study should be optiꢀ
mized as applied to biological conditions. It is also necesꢀ
W 0 2 = KΣ[AH2][Ct]PO /(1 + K″PO ),
(2)
AH
2
2
Ct + O2
CtO2,
(intermediate stages)
(3a)
CtO2 + AH2
Ct + products.
(3b)
According to this scheme, a complex of dioxygen
with the catalyst (CtO2) is formed in the first stage as it
has been assumed in Ref. 5 for nonꢀsubstituted РсСо.
The reaction of this complex with АН2, more precisely,
with the АН– ascorbate anion (because under our conꢀ
ditions (рН 6.1—7.4) ascorbic acid (рКА 4.25) almost
entirely exists as the monoanion), limit1s the process.
.

Ascorbic acid oxidation
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 7, July, 2002
1233
W 0 •106/mol (L min)–1
I. Formation of primary radicals
AH
2
1
2
3
a
•–
АН– + О2(TP) → Н+ + А•– + О2 (TP).
(4.1)
2
50
40
30
20
10
II. Reactions of primary radicals
О2•– + АН– + Н+ → А•– + Н2О2,
(4.2)7
(4.3)7
(4.4)8
О2•– + А•– + Н2О → А + НО2– + ОН–,
1
•–
О2 + А•–→ А + О2
,
О2•– + О2•– + 2 H+ → H2O2 + О2,
(4.5)9
0
20
40
60
80 PO /kPa
2
А•– + А•– + Н+ → А + НА–.
III. Subsequent reactions
(4.6)10
1/W 0 2•10–5/L min mol–1
1/W 0 •10–6/L min mol–1
AH
AH
2
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
8
b
2
7
6
5
4
3
2
1
1. Decomposition of hydrogen peroxide
1
H2O2 + РсСоII → PcCoIII (HO–) + •OH,
H2O2 + О2•– + H+ → O2 + H2O + •OH,
H2O2 + •OH → НО2• + Н2О,
(4.7)
(4.8)9
(4.9)11
(4.10)
PcCoIII(OH) + H2O2 → destruction of PcCo.
2. Reactions of ОН• radicals
0
0.05
0.10
0.15 (1/PO )/kPa–1
2
Fig. 2. Initial rate of АН2 oxidation as a function of the dioxygen
concentration (a) and linear anamorphoses of the curves (b);
•OH + РсСоII → РсСоIII(ОН),
•OH + •ОН → Н2О2,
(4.11)
С 0
= 0.35•10–5, С 0
= 7.00•10–5 mol L–1. Temperaꢀ
(АН )
2
(TP)
ture: 18 °С (1) and 31 °С (2), 3, content of dioxygen in the
venous and arterial blood.
(4.12)9,12
(4.13)9
(4.14)13
•–
•OH + О2 → OH– + O2,
The stages following the limiting stage were not earlier
considered in the works devoted to catalysis of АН2 oxiꢀ
dation in the presence of PcM. We believe that in the
catalytic system under study the same processes as in the
nonꢀcatalytic variant mainly occur in these stages.6
•OH + AH– → А•– + H2O.
Reaction
4.2
k/mol L–1 s–1
5.75•104
2.6•108
5•102
Reaction
4.8
4.9
k/mol L–1 s–1
6.4
4.3
4.4
4.5
4.6
3.7•107
5•109
1•1010
1•1010
The limiting stage affords primary radicals, namely,
4.12
•–
superoxide radical anion О2
and ascorbate radical
<100
1•106
4.13
4.14
А•– (4.1). The transformations of these radicals in the
nonꢀcatalytic variant (4.2)—(4.6) were described and
characterized by the corresponding rate constants.7—13
All of them form hydrogen peroxide except for the reꢀ
combination of the ascorbate radicals (А•–) and their
reaction with dioxygen. Subsequent reactions are variꢀ
ants of transformations of Н2О2 (4.7)—(4.10) and chain
termination stages (4.12)—(4.15). Thus, the scheme of
ascorbic acid oxidation in the presence of Teraphthal
(PcCo)
When reactions occur in biological objects, one has
to consider three additional termination stages (4.15),
which, actually, form the basis of the therapeutic effect
of the TP—АН2—О2 catalytic system. These stages are
considered as termination stages because radicals which
can be formed are more stable than the initial radicals
and cannot propagate the kinetic chain.
•OH
•–
O2
+ cell organelles → cell damage
(4.15)
AH– + O2(TP)
A + H2O2 + TP,
(4)
A•–
A is dehydroascorbic acid, can be presented by the folꢀ
lowing series of stages beginning from the limiting one:
Stages (4.7) and (4.10) reflect the participation of TP
in hydrogen peroxide decomposition. The catalytic acꢀ

1234
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 7, July, 2002
Girenko et al.
W 0 •106/mol (L min)–1
(Сat) decreases the reaction rate (Fig. 4). The limiting
magnitude of the effect indicates that more than 80%
ascorbic acid are consumed in the route of its reaction
with hydrogen peroxide or its decomposition products,
i.e., •ОН radicals. However, it cannot be excluded that
the effect of catalase is associated with the protein, which
is the catalase apoenzyme, rather than with the main
function of catalase. The catalase apoenzyme, like albuꢀ
min (see below), can sharply decrease the activity of
Teraphthal in catalysis of ascorbic acid oxidation.
The substantial role of the •ОН radicals in the proꢀ
cess under study was confirmed by the reactions in the
presence of pꢀnitrosoꢀN,Nꢀdimethylaniline (NDMA),
which is the known trap for these radicals.17,18 The inꢀ
troduction of NDMA in the equimolar (with respect to
the catalyst) concentration (3.5•10–5 mol L–1) results in
both a decrease in the rate of catalytic oxidation of ascorꢀ
bic acid (Fig. 5) and consumption of NDMA. Further
increase in the NDMA concentration produces more
complicated effects due to the interaction of TP with
the trap.
AH
2
60
50
40
30
20
10
1
2
0
2
4
6
8
12
С 0 •104/mol L–1
AH
2
Fig. 3. Influence of superoxide dismutase (SOD) on the rate of
АН2 oxidation in the presence of Teraphthal: in the absence
(1) and presence of SOD (2), СSOD = 0.05 g L–1, СTP
=
0.35•10–4 mol L–1
.
tivity of cobalt phthalocyanine in this reaction is well
known.14—16
We confirmed that the О2•– radical anion is involved
in the oxidation of АН– by a series of experiments in
which superoxide dismutase (SOD) catalyzing О2•– disꢀ
proportionation was introduced into the reaction mixꢀ
ture. The reaction rate decreases in the presence of SOD,
and the magnitude of the effect depends on the ratio of
the concentrations of ascorbic acid to SOD (Fig. 3).
Comparison of the rate constants for different stages
suggests that the stage of the reaction of the monoanion
with the •ОН radical can substantially contribute to the
rate of ascorbic acid oxidation. According to the scheme
proposed, the •ОН radicals necessary for the reaction
are generated upon the decomposition of hydrogen perꢀ
oxide, viz., intermediate reaction product. If this asꢀ
sumption is valid, a decrease in the steadyꢀstate concenꢀ
tration of Н2О2 should decrease the rate of ascorbic acid
oxidation. We found that the introduction of catalase
The participation of the highly reactive •ОН radicals
in the process is additionally argued by the fact that,
according to the EAS, Teraphthal is destroyed during
the oxidation of АН2.
2. рНꢀDependence of the rate of oxidation
of ascorbic acid
The TP+АН2 system in aqueous solutions is characꢀ
terized by phase stability only at рН ≥ 6.4. At рН < 6.0,
TP is transformed into the corresponding acid insoluble
in water. At рН 6.0—6.3, TP in a solution is strongly
aggregated and precipitates when АН2 is introduced (рН
of the solution remains unchanged). The maximum rate
of the catalytic reaction is observed at рН ∼6.5 (Fig. 6).
W 0 •106/mol (L min)–1
AH
2
W 0 •106/mol (L min)–1
AH
2
35
30
25
20
15
10
5
60
50
40
30
20
10
1
2
0
5
10
С 0 •104/mol L–1
CCat/g L–1
AH
2
0
0.1
0.2
0.3
0
Fig. 4. Initial rate of ascorbic acid oxidation (W
)
=
Fig. 5. Initial rate of АН2 oxidation (W 0 ) as a function of
AH
2
AH
2
as
a
function of the catalase concentration; СTP
С 0
in the presence of TP (1) and TP+NDMA (2); СTP =
(AH )
2
0.35•10–4 mol L–1, СAH = 8•10–4 mol L–1
.
3.5•10–5 mol L–1, СNDMA = 3.5•10–5 mol L–1
.
2

Ascorbic acid oxidation
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 7, July, 2002
1235
W 0 •106/mol (L min)–1
by analysis of the electronic absorption spectra of the
individual components, pairs, and the whole system.
Comparison of the EAS of solutions of TP and a
TP—BSA mixture shows that the spectra are identical
within the experimental error. This implies that either
no interactions of TP with the protein occur or such
interactions are not manifested in the EAS. In our opinꢀ
ion, the latter is more probable because albumin is known
to interact with a broad array of compounds, including
those containing charged fragments (anions, cations)19
and developed aromatic structures, for example, anꢀ
thraquinone.20
AH
2
40
30
20
6.4
6.6
6.8
7.0
7.2
7.4
pH
Fig. 6. Initial rate of ascorbic acid oxidation (W 0 ) as a
AH
2
function of the pH: j = 0.1; СTP = 0.35•10–4 mol L–1, СAH
=
2
7.0•10–4 mol L–1
.
When solutions of BSA and АН2 are mixed, the inꢀ
tensity of the band with λ = 279 nm increases sharply,
which, according to published data,21,22 indicates a
change in the conformation of the protein molecule. In
our case, this is a consequence of the interaction of
ascorbic acid with albumin. According to the EAS data,
this interaction is disturbed when Teraphthal is introꢀ
duced in concentrations above some value minimum for
this ratio of the acid to protein. In other words, TP
prevents the formation of adducts of BSA with АН2 beꢀ
cause it likely binds to the protein globule.
Table 1. Rate of АН2 oxidation in the presence of TP at differꢀ
ent ionic strengths of the solution
С 0
•104
W 0
•106
AH
AH
2
/mol L–1
/mol (L 2min)–1
II
I
III
3.5
7.0
10.5
8
18
25
20
33
40
18
30
38
3.2. Catalytic activity of TP in the oxidation
of ascorbic acid in the presence of albumin
Note. Conditions: рН 6.9, СTP = 0.35•10–4 mol L–1; I — j = 0.1
(phosphate buffer), II — j = 0.2 (NaCl), III — j = 0.2 (NaClO4).
We carried out three series of experiments in which
the TP concentration differed by an order of magnitude
(0.35•10–5 and 0.35•10–4 mol L–1) and the СTP/СBSA
The dependence of the reaction rate on the ionic
strength of the solution (Table 1) is in complete accorꢀ
dance with the assumption about the interaction of the
similarly charged ions (monoanion АН– and polyanion,
which is a complex of TP with dioxygen) in the limiting
stage. The effect of the ionic strength is independent of
the nature of the salt, which creates this ionic strength.
This fact rules out alternative explanations, such as an
axial coordination of the anion of the salt.
ratio and the interval of САН change corresponded to
2
the region in which, according to the above data, no
changes in the albumin conformation occur.
The experiments showed (Fig. 7) that for СBSA/СTP ≥ 1
the rate of АН2 oxidation decreases sharply. The influꢀ
ence of albumin on the reaction rate is much less proꢀ
nounced in the systems where СTP substantially exꢀ
ceeds СBSA
.
3. Influence of albumin on the rate of oxidation
of ascorbic acid in the presence of Teraphthal
W 0 •106/mol (L min)–1
AH
2
1
50
40
30
20
10
2
The challenges of using TP in therapy of oncological
diseases caused urgency of the study of its catalytic propꢀ
erties under conditions simulating biological media. This
concerns, first, the problem of the influence of albumin,
which is the main transport protein, whose concentraꢀ
tion in the blood is 35—50 g L–1 or (5.2—7.5)•10–4
3
mol L–1
.
0
5
10
С 0 •104/mol L–1
AH
2
3.1. Interaction of Teraphthal, albumin,
and ascorbic acid
Fig. 7. Initial rate of АН2 oxidation (W 0 ) in the presence of
AH
2
TP (С 0 = 0.35•10–4 mol L–1) as a function of the initial
TP
Some information on the interactions of the compoꢀ
nents of the catalytic system with albumin was obtained
concentration of AH2 in the absence (1) and presence of
BSA (2, 3): 2.3•10–6 (2) and 1.08•10–5 mol L–1 (3).

1236
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 7, July, 2002
Girenko et al.
6. N. Miyaka, M. Kim, and T. Kurata, Biosci., Biotech.,
Biochem., 1997, 61, 1693.
7. (a) A. D. Nadezhhdin and H. B. Dunford, Can. J. Chem.,
1979, 57, 3017; (b) D. E. Cabelli and B. H. J. Bielskii,
J. Phys. Chem., 1983, 87, 1809.
Thus, the results of catalytic studies evidence the
interaction of TP and albumin in solutions and also show
that TP in a complex with the protein looses its catalytic
activity. These results are of great significance for antiꢀ
cancer therapy: when TP is injected into the blood, it
does not catalyze the oxidation of АН2, i.e., the reaction
does not occur in the blood and can begin only after TP
was transported to the cell and its complex with albumin
dissociated.
These results along with the corresponding published
data23—26 suggest the most probable types of effects of
intermediates of the catalytic system on the cell. These
effects are responsible for its high cytotoxicity with reꢀ
spect to malignant cells.
The data of our studies leave virtually no doubts that
the ascorbate radical, superoxide radical anion, hydroꢀ
gen peroxide, and hydroxyl radicals are formed in the
system. According to the published data,23,24 the former
is cytotoxic and can play an independent role. Three
others form a successive chain of intermediates, which
in a normal cell could be destroyed or rendered harmless
by special protective systems designed for oxidative stress
control. However, the content of superoxide dismutase,
catalase, vitamin А, and other antioxidants in tumor
cells is lower than in normal cells.24—26 Therefore, under
conditions of fast local ejection of strong cytotoxic agents,
this apparatus unlikely perform its functions to an entire
extent, due to which the cytotoxic properties of the disꢀ
cussed catalytic system can appear. In addition, the oxiꢀ
dation of ascorbic acid in a tumor decreases the oxygen
level in the cell, serving as yet another factor for the
decay of cancerous cells.24
8. O. S. Fedorova, Ph. D. (Chem.) Thesis, Novosibirsk, 1979,
20 pp. (in Russian).
9. A. Ya. Sychev and V. G. Isak, Gomogennyi kataliz soediꢀ
neniyami zheleza [Homogeneous Catalysis by Iron Comꢀ
pounds], Shtiintsa, Kishinev, 1988, 216 pp. (in Russian).
10. S. Lagercrantz, Acta Chem. Scand., 1964, 18, 562.
11. A. Ya. Sychev, S. O. Travin, G. G. Duka, and Yu. I.
Skurlatov, Kataliticheskie reaktsii i okhrana okruzhayushchei
sredy [Catalytic Reactions and Environmental Protection],
Shtiintsa, Kishinev, 1983, 272 pp. (in Russian).
12. T. Shiga, J. Phys. Chem., 1965, 69, 3805.
13. D. I. Metelitsa, Modelirovanie okislitel´noꢀvosstanovitel´nykh
fermentov [Modeling of Redox Enzymes], Nauka i Tekhnika,
Minsk, 1984, 293 pp. (in Russian).
14. M. R. Tarasevich and K. A. Radyushkina, Kataliz i elektroꢀ
kataliz metalloporfirinami [Catalysis and Electrocatalysis by
Metal Porphyrins], Nauka, Moscow, 1982 (in Russian).
15. Н. Shirai, S. Higaki, and K. Hanabusa, J. of Polymer Sci.:
Polym. Chem. Ed., 1984, 22, 1309.
16. H. Shirai, A. Maruyama, and J. Takano, Macromol. Chem.,
1980, 181, 565.
17. G. P. Pirumyan, A. N. Martiryan, and Yu. I. Skurlatov,
Armyanskii Khim. Zh. [Armenian Chemical J.], 1989, 42, 71
(in Russian).
18. C. N. Trumbore and J. Kraljic, J. Amer. Chem. Soc., 1965,
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Received October 17, 2001;
in revised form February 13, 2002