Kinetics of the oxidation of L-cysteine by trans- and cis-cobalt(III) and iron(III) complexes

Article abstract of DOI:10.1515/znb-2006-1105

Kinetics of oxidation of L-cysteine by pairs of trans and racemic cis isomers of cobalt(III) and iron(III) based transition metal complexes have been studied in aqueous solution. Kinetics measurements were run under pseudo first order conditions in which the concentration of cysteine is between one and two orders of magnitude greater than that of the isomers of the transition metal complex. The orders of the reaction with respect to both cysteine and the isomer were determined. The observed rate constants and the overall rate constants of the oxidation process were measured. For all geometrical isomers, it was found that the rate constant of oxidation of L-cysteine by the trans isomer is between one to three orders of magnitude greater than that by the CM isomer. The difference in rates can be explained by a geometric factor around the metal ion center in the complex. The less crowded isomer (trans) makes electron transfer easier and hence facilitates the oxidation process which leads to a higher oxidation rate.

Full text of DOI:10.1515/znb-2006-1105

Kinetics of the Oxidation of L-Cysteine by trans- and cis-Cobalt(III)
and Iron(III) Complexes
a
a
b
Hamzeh M. Abdel-Halim , Adnan S. Abu-Surrah , and Hutaf M. Baker
a
Department of Chemistry, Faculty of Science, Hashemite University,
P.O. Box 330127, Zarqa 13133, Jordan
Department of Chemistry, Faculty of Arts and Science, Al al-Bayt University,
b
P.O. 130091, Mafraq 25113, Jordan
Reprint requests to Dr. Hamzeh M. Abdel-Halim. E-mail: hamzehah@hu.edu.jo
Z. Naturforsch. 61b, 1346 – 1350 (2006); received March 27, 2006
Kinetics of oxidation of L-cysteine by pairs of trans and racemic cis isomers of cobalt(III) and
iron(III) based transition metal complexes have been studied in aqueous solution. Kinetics measure-
ments were run under pseudo first order conditions in which the concentration of cysteine is between
one and two orders of magnitude greater than that of the isomers of the transition metal complex. The
orders of the reaction with respect to both cysteine and the isomer were determined.
The observed rate constants and the overall rate constants of the oxidation process were measured.
For all geometrical isomers, it was found that the rate constant of oxidation of L-cysteine by the trans
isomer is between one to three orders of magnitude greater than that by the cis isomer. The difference
in rates can be explained by a geometric factor around the metal ion center in the complex. The
less crowded isomer (trans) makes electron transfer easier and hence facilitates the oxidation process
which leads to a higher oxidation rate.
Key words: Co(III), Fe(III), Kinetics, Cysteine
Introduction
TolSO)(Py)Cl ] with several nucleophilic reagents
2
(
Py, Ph PS, Ph P, Ph As, and MeSO). The effect of
3 3 3
Geometric isomerisms are common among coordi-
nation compounds. They contain the same type and
the same number of ligands, but arranged differently
around the central transition metal ion in the complex.
Study of isomers provides much of the experimental
knowledge used to develop and defend coordination
theory. For four-coordinated compounds, trans and
cis isomers are common in square planar molecules.
For the octahedral geometry of six-coordinated com-
pounds, with monodendate ligands, several geometric
isomerisms are possible. However, if some of the lig-
ands are bidendates, the number of possible isomers
is limited. In the present work, octahedral transition
geometric isomerism in platinum complexes used as
anti-tumor agents has been studied by Farrell et al. [2].
They found that the binuclear platinum complexes
[
(
{trans-PtCl(NH ) } -µ-{NH (CH )nNH }](NO )
3
2
2
2
2
2
3 2
n = 4,6) and [{cis-PtCl(NH ) } -µ-{NH (CH )n-
3
2
2
2
2
NH }](NO ) (n = 4,6)] exhibit anti-tumor activity
2
3 2
comparable with cisplatin. Also, they found that,
◦
at 37 C, the initial binding and reaction of the
cis isomer was slower than that of the trans iso-
mer. Toma et al. [3] reviewed linkage isomerization
reactions from the aspect of kinetics and mecha-
nisms involved, some focusing on selected cases
of direct formation, as well as on electrochemical,
photochemical, thermal and pH-induced generation
of linkage isomers. The biodegradation kinetics of
geometric isomers of model naphthenic acid in water
metal complexes of the type [M(L-L) Cl ]Cl (where
2
2
L-L is a bidendate ligand) were selected for a kinetic
study.
Several investigators have studied the effect of has been studied by Peru et al. [4]. The rates of
geometric isomerism on the reactivity of some isomers biodegradation of six model naphthanic acids by het-
with various reagents. Using optical rotatory disper- erotrophic bacteria were compared. Specifically, they
sion, IR and NMR spectroscopy, de Vekki et al. [1] monitored by gas chromatography in surface water the
studied the reaction of the optically active geometric biodegradation of cis- and trans-isomers of 4-meth-
isomers of the platinum(II) complex (-)-[Pt(Me-p- ylcyclohexenylacetic acids, 4-methylcyclohexanyl-
0932–0776 / 06 / 1100–1346 $ 06.00 © 2006 Verlag der Zeitschrift f u¨ r Naturforschung, T u¨ bingen · http://znaturforsch.com
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H. M. Abdel-Halim et al. · Kinetics of Oxidation of L-Cysteine
1347
carboxylic acids, and 3-methylcyclohexenylcarboxylic metal ion, the observed rates and the rate constants
acids. of oxidation processes is given. Correlation has been
Oxidation of amino acids by a transition metal com- made between the rate of the oxidation process and the
plex has been studied using different methods, includ- geometry around the metal ion (cis and trans).
ing stopped-flow spectrophotometry, chemical analy-
sis of products, and the use of radioactive and sta-
ble isotope tracers. Discussion of these methods, along
Experimental Section
with much of the data produced and interpretation
Materials: L-cysteine (minimum assay 99%) was pur-
of the results according to different pathways in this chased from BDH Laboratory Supplies (England) and was
used without further purification. The cobalt(III) and iron(III)
field, is given elsewhere [5]. Several investigators re-
ported studies on the oxidation process of amino acids
by various transition metal complexes [6 – 15]. Kinet-
ics and mechanism of oxidation of L-methionine by
a 1,10-phenanthroline iron(III) complex in perchloric
acid medium have been studied by Vani et al. [6].
Sharma et al. [7] reported the reactivity of iron(V) and
iron(VI) with both glycine and α-alanine, at pH 12.4
trans- and cis-pairs complexes: trans-[Co(en) Cl ]Cl
2
2
(
1), cis-[Co(en) Cl ]Cl (2) (en: 1,2-ethylenediamine),
2 2
trans-[Co(bipy) Cl ]Cl (3), cis-[Co(bipy) Cl ]Cl (4)
2
2
2
2
(
bipy: 2,2’-bipyridine), trans-[Co(phen) Cl ]Cl (5),
2 2
cis-[Co(phen) Cl ]Cl (6) (phen: 1,10-phenanthroline),
2
2
trans-[Fe(en) Cl ]Cl (7), cis-[Fe(en) Cl ]Cl (8), trans-
2
2
2
2
[Fe(bipy) Cl ]Cl (9), cis-[Fe(bipy) Cl ]Cl (10), trans-
2 2 2 2
[
Fe(phen) Cl ]Cl (11), and cis-[Fe(phen) Cl ]Cl (12), were
2
2
2
2
◦
and 23 – 24 C, using stopped-flow and pulse radiolysis
prepared according to literature procedures [20] with some
modification. The trans-isomers (1, 3, 5, 7, 9, and 11)
were prepared by the reaction of stoichiometric amounts
techniques. Laloo et al. [8] studied the kinetics of the
oxidation of three amino acids (lysine, arginine, and
histidine) by alkaline hexacyanoferrate(III) at constant of ligands with CoCl · 6H O or FeCl · 4H O in water.
2
2
2
2
ionic strength over the temperature range 318– 338 K.
The resulting M(II) complexes were oxidized to M(III)
Jameson et al. [9, 10] reported anaerobic oxidation of complexes by the addition of a stoichiometric amount of
cysteine to cystine by iron(III) in acidic solution using H2O2. The product was then treated with excess HCl to form
the desired dichloro species of the form [M(L-L) Cl ]Cl.
stopped-flow high-speed spectrophotometry. The oxi-
dation of amino acids to aldehydes, in the presence
of Os(VIII) as a catalyst, by alkaline hexacyanofer-
rate(III) has been reported by Mehrotra et al. [11].
From our laboratories, several papers have been
published [12 – 15] on the oxidation of some amino
acids by transition metal complexes. Of particular
interest and relation to the present work, Abdel-
Halim et al. [12] reported recently kinetic measure-
ments of L-cysteine oxidation by chromium(III), man-
2
2
(
L-L = 1,2-ethylenediamine, 2,2’-bipyridine, 1,10-phen-
anthroline; and M = Co, Fe). The product was filtered,
washed with ethanol and diethylether, and dried. Slow
evaporation of the filtrate for several days gave crystals
of the corresponding racemic cis-isomers (2, 4, 6, 8, 10,
and 12). The geometry of each isomer was confirmed by
its physical properties (melting point and color) and by its
UV-vis spectrum: trans-[Co(en) Cl ]Cl (1): Deep-green. –
M. p. (dec) 229 C. – UV/vis (MeOH): λmax (lgε) = 608 nm
1.49). cis-[Co(en) Cl ]Cl (2): Violet. – M. p. (dec) 221 C.
2
2
◦
◦
(
2
2
ganese(III), iron(III), and cobalt(III) based transition – UV/vis (H O): λ (lgε) = 536 nm (1.81). trans-
2
max
◦
metal complexes. The authors [13– 15] also reported [Co(bipy) Cl ]Cl (3): Green. – M. p. (dec) 339 C. – UV/vis
2
2
kinetic studies on the oxidation of L-cysteine and pro- (MeOH): λmax (lgε) = 544 nm (1.75). cis-[Co(bipy)2Cl2]Cl
◦
(
4): Deep-violet. – M. p. (dec) 292 C. – UV/vis (H O):
line by some cobalt(III) and iron(III) based complexes
2
λmax (lgε) = 518 nm (1.74). trans-[Co(phen) Cl ]Cl
5): Green. – M. p. (dec) 250 C. – UV/vis (MeOH):
containing various ligands such as CN, NO , acac,
2
2
2
◦
(
NH , urea, en, and 1,10-phenanthroline. They inves-
3
λmax (lgε) = 465 nm (1.84). cis-[Co(phen) Cl ]Cl (6): Gray.
2
2
tigated the effect of the nature of the ligand in the tran-
sition metal complex on the rate of oxidation of cys-
teine. Previous studies on the coordination chemistry
of heteroatom-containing ligands [16, 17], and their
catalytic applications [18, 19], were also reported by
these authors.
◦
M. p. (dec) 200 C. – UV/vis (H O): λmax (lgε) = 516 nm
2
–
(
2
1.97). trans-[Fe(en) Cl ]Cl (7): Orange-red. – M. p. (dec)
2 2
◦
31 C. – UV/vis (MeOH): λmax (lgε) = 368 nm (2.25).
cis-[Fe(en) Cl ]Cl (8): Pink. – M. p. (dec) 258 C. – UV/vis
◦
2
2
(
(
H O): λmax (lgε) = 477 nm (0.98). trans-[Fe(bipy) Cl ]Cl
2
2
2
◦
9): Gray. – M. p. (dec) 300 C. – UV/vis (MeOH):
In the present work, kinetics of oxidation of
L-cysteine by pairs of trans and racemic cis isomers
λmax (lgε) = 557 nm (1.72). cis-[Fe(bipy) Cl ]Cl (10): Or-
2
2
◦
ange. – M. p. 220 C. – UV/vis (H O): λmax (lgε) = 489 nm
2
of cobalt(III) and iron(III) based transition metal com- (1.20). trans-[Fe(phen) Cl ]Cl (11): Purple. – M. p. (dec)
2
2
◦
plexes have been studied in aqueous solution. For each 304 C. – UV/vis (MeOH): λmax (lgε) = 504 nm (2.71).
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1
348
H. M. Abdel-Halim et al. · Kinetics of Oxidation of L-Cysteine
◦
cis-[Fe(phen) Cl ]Cl (12): Brown. – M. p. (dec) 215 C. – b ≈ 1, in agreement with previous studies [6, 8, 12 –
2
2
UV/vis (H O): λmax (lgε) = 507 nm (2.90).
15]. Since M(III) complexes are all one-electron oxi-
2
Kinetic measurements: Freshly prepared aqueous solu- dants, the oxidation of cysteine would give a radical in-
tions of the desired concentrations of complexes and of termediate, and the following mechanism is proposed:
L-cysteine were used for the kinetic measurements. The
−
+
measurements were carried out using a Diode Array Spec-
trophotometer model 8453E from HP Agilent Technologies.
The reactions were monitored by following the change in
the absorbance of the trans- or the racemic cis-isomers of
Co(III) and Fe(III) metal complex (MC) with time at a pre-
determined wavelength. This wavelength was determined by
recording the absorption spectral curves, for the MC and for
its mixture with cysteine (Cys) after the completion of the
reaction. The reaction progress and the reaction rate were
RSH ⇔ RS +H
−
•
−
RS → RS +e
(4)
•
2
RS → RSSR
where the second step is the rate-determining step.
Under pseudo first order conditions, in which
Cys] ꢀ [MC], the concentration of the cysteine is es-
[
sentially constant throughout the reaction. The reaction
monitored at the wavelength of maximum absorbance dif- rate is then given by
ference (λmad) between the absorption of MC and that of the
d[MC]
dt
mixture at the end of the reaction. A list of λmad for various
complexes is shown in Table 1. All oxidation reactions were
studied under pseudo first order conditions. The concentra-
Rate ≈ −
= kobs[MC]
(5)
where kobs is the observed rate for the reaction, given
−
2
−1
−3
tions of Cys used [10 – 10 mol dm ] were chosen to be
−4
by
1
1
-2 orders of magnitude larger than that of the MC [10
–
−2
−3
mol dm ]. The ionic strength of the solutions was kept
0
kobs = k[Cys]
(6)
−3
constant at 0.20 mol dm using NaClO . The temperature
4
◦
of the solution and its pH were both maintained at 25± 0.1 C
where k is the second-order rate constant for reac-
tion (2) above.
and 7.0± 0.1, respectively.
For a first-order dependence of the reaction on
MC], the experimental absorbance-time data pairs
Results and Discussion
[
were fit to the exponential function:
Oxidation of cysteine (RSH) leads to formation of
cystine (RSSR), as shown below [21].
At = (A −A∞)exp(−kobst)+A∞
(7)
0
+
−
2
RSH → RSSR+2H +2e
(1)
where At is the absorbance of the MC at time t through
the reaction, A is the initial absorbance of the complex
0
Estimation of residual oxidant suggested that
moles of cysteine consumed 2 moles of the MC,
(t = 0) and A∞ is the final absorbance of the reaction
2
mixture at the end of the reaction (t = ∞).
III
m+
[M (L-L) Cl ] , where M is the transition metal,
−1
2
2
The value of kobs (in s ) can be obtained from a plot
such that
of ln(At) versus time. A plot of kobs versus [Cys] gives
the value of the second-order rate constant, k, in units
III
m+
2
[M (L-L) Cl ] +2RSH
3
−1 −1
2
2
of dm mol s . Kinetics results for the oxidation of
(2)
II
(m−1)+
→
2[M (L-L) Cl ]
+RSSR
cysteine by various trans- and cis-pairs are shown in
Table 1.
2
2
where m is the charge on the M(III)-L complex.
The rate of the reaction is given by
Table 1 shows experimental results for various
Co(III) and Fe(III) trans- and racemic cis-geometric
pairs. For all complexes studied, the observed rate con-
a
b
Rate = k[Cys] [MC]
(3) stant for the trans-isomer is found to be one to two or-
ders of magnitude greater than that for the cis-isomer,
where k is the reaction rate constant and a, b are the while the second-order rate constant for the trans-
orders of the reaction with respect to the concentra- reaction is two to three orders of magnitude greater
tion of Cys and MC, respectively. It was found that the than that of the racemic cis-isomer. This difference in
rate is, in general, dependent on the first power of both rates can be explained by the reaction mechanism and
the concentrations of substrate and oxidant, i. e., a ≈ geometric factors.
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H. M. Abdel-Halim et al. · Kinetics of Oxidation of L-Cysteine
1349
MC
No.
1
2
3
4
5
6
7
Complex
(MC)
Ligand
λmad
(nm)
400
400
400
375
400
375
355
355
400
521
520
521
kobs
k *
(dm mol
Table 1. Rates of oxidation of
cysteine by various trans- and
racemic cis-pairs of Co(III)
and Fe(III) complexes in
−
(s 1)
3
−1 −1
−2
s
)
−
3
5
2
3
3
4
3
5
1
2
1
2
trans-[Co(en)2Cl2]Cl
cis-[Co(en)2Cl2]Cl
trans-[Co(bipy)2Cl2]Cl
cis-[Co(bipy)2 Cl2]Cl
trans-[Co(phen)2Cl2]Cl
cis-[Co(phen)2Cl2]Cl
trans-[Fe(en)2Cl2]Cl
cis-[Fe(en)2Cl2]Cl
trans-[Fe(bipy)2Cl2]Cl
cis-[Fe(bipy)2Cl2]Cl
trans-[Fe(phen)2Cl2]Cl
cis-[Fe(phen)2Cl2]Cl
trans-en
cis-en
1.22×10
1.72×10
5.32×10
8.37×10
2.11×10
1.62×10
7.32×10
1.46×10
1.85×10
9.88×10
3.44×10
9.74×10
2.87×10
−
−5
−1
−2
−2
−4
−2
−4
−1
−2
−1
−1
4.17×10
◦
−
aqueous medium at 25 C,
trans-bipy
cis-bipy
trans-phen
cis-phen
trans-en
cis-en
trans-bipy
cis-bipy
trans-phen
cis-phen
7.43×10
−
pH = 7.0 and ionic strength =
4.67×10
−
−3
2.99×10
0.20 mol dm
.
−
4.58×10
−
1.13×10
* Experimental errors are esti-
mated to be 20%.
−
8
9
7.50×10
−
5.11×10
−
1
1
1
0
1
2
4.12×10
−
5.13×10
−
2.34×10
Deciding via which mechanism the oxidation pro-
cess of cysteine by a transition metal complex will
proceed may be difficult. If the ligand in the complex
has extra lone pair(s) with which to form “links” to
cysteine and if the geometry around the metal center
leaves enough space for cysteine to bind to this lig-
and, then the reaction is more likely to proceed via an
inner-sphere mechanism. In this case, a substitution re-
action that leaves cysteine and MC linked by the bridg-
ing ligand takes place, and the actual transfer of the
electron is frequently accompanied by transfer of the
ligand followed by separation of the products. How-
ever, when the ligands in the MC have no extra lone
pairs with which to form bonds to cysteine, or when
there is a “closed” or crowded geometry around metal
center, then the ligands are tightly held and there is no
change in the coordination sphere on reaction, the re-
action proceeds by outer-sphere electron transfer [22].
Looking at ligands and structures of MC’s stud-
ied in the present work (Fig. 1) reveals that they all
have closed octahedral geometries that do not permit
linking between cysteine and MC. Also, according to
Taube [5], an inner-sphere redox reaction requires two
metal centers, which is not the case in any of the MC’s
used. Therefore, we believe that all reactions studied
in the present work proceed via an outer-sphere mech-
anism.
Fig. 1. Suggested structures for the complexes used in the
present study.
Since direct linkage between Cys and MC is not
possible, then the rate of electron transfer from Cys
to M(III) in the MC, according to (2) above, will de-
pend on the closest distance the two reactants can ap-
proach each other. Obviously, the trans-isomer, which
is less crowded than the cis-isomer, can provide a
shows that this is the case for all trans- and cis-isomers
pairs studied.
The differences in rates due to the nature of the lig-
closer distance of approach from both sides (“top” and and [en, bipy and phen] and due to the metal center
bottom”) of the MC. This will lead to higher rates [Co(III) and Fe(III)] were discussed in our previously
for trans-isomers than for cis-isomers. Table 1 clearly published work [12].
“
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1
350
H. M. Abdel-Halim et al. · Kinetics of Oxidation of L-Cysteine
Conclusion
were found to be one to three orders of magnitudes
higher for the trans-isomer. The differences are at-
The rates of oxidation of L-cysteine by pairs of tributed to steric factors. The less crowded trans-
trans- and racemic cis-isomers of Co(III) and Fe(III) isomers facilitate electron transfer making the oxida-
complexes with various ligands were studied. Rates tion process faster than the cis-isomers.
[
[
[
[
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