Electron transfer reactions of photochemically generated ruthenium(III)-polypyridyl complexes with methionines

Article abstract of DOI:10.1002/kin.20874

The oxidation of methionine (Met) plays an important role during biological conditions of oxidative stress as well as for protein stability. Ruthenium(III)-polypyridyl complexes, [Ru(NN)₃]³⁺, generated from the photochemical oxidation of the corresponding Ru(II) complexes with molecular oxygen, undergo a facile electron transfer reaction with Met to form methionine sulfoxide (MetO) as the final product. Interaction of [Ru(NN)₃]³⁺ with methionine leads to the formation of >S^+? and (>S∴S<)⁺ species as intermediates during the course of the reaction. The interesting spectral, kinetic, and mechanistic study of the electron transfer reaction of four substituted methionines with six [Ru(NN)₃]³⁺ ions carried out in aqueous CH₃CN (1:1, v/v) by a spectrophotometric technique shows that the reaction rate is susceptible to the nature of the ligand in [Ru(NN)₃]³⁺ and the structure of methionine. The rate constants calculated by the application of Marcus semiclassical theory to these redox reactions are in close agreement with the experimental values.

Full text of DOI:10.1002/kin.20874

Electron Transfer Reactions
of Photochemically
Generated
Ruthenium(III)–Polypyridyl
Complexes with
Methionines
DHARMARAJ THIRUPPATHI,1 PERIYAKARUPPAN KARUPPASAMY,1 MUNIYANDI GANESAN,1
VELUCHAMY KAMARAJ SIVASUBRAMANIAN,1 THANGAMUTHU RAJENDRAN,1,2
SEENIVASAN RAJAGOPAL3
1Postgraduate and Research Department of Chemistry, Vivekananda College, Thiruvedakam West, Madurai 625 234, India
2Department of Chemistry, PSNA College of Engineering and Technology, Dindigul 624 622, India
3School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
Received 3 January 2014; revised 5 July 2014; accepted 14 July 2014
DOI 10.1002/kin.20874
Published online 27 August 2014 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The oxidation of methionine (Met) plays an important role during biological condi-
tions of oxidative stress as well as for protein stability. Ruthenium(III)–polypyridyl complexes,
[Ru(NN)3]3+, generated from the photochemical oxidation of the corresponding Ru(II) com-
plexes with molecular oxygen, undergo a facile electron transfer reaction with Met to form
methionine sulfoxide (MetO) as the final product. Interaction of [Ru(NN)3]3+ with methionine
leads to the formation of >S+• and (>SࢺS<)+ species as intermediates during the course of
the reaction. The interesting spectral, kinetic, and mechanistic study of the electron transfer
reaction of four substituted methionines with six [Ru(NN)3]3+ ions carried out in aqueous
CH3CN (1:1, v/v) by a spectrophotometric technique shows that the reaction rate is susceptible
to the nature of the ligand in [Ru(NN)3]3+ and the structure of methionine. The rate constants
Correspondence to: Muniyandi Ganesan; e-mail: mganesan58@
ymail.com. Seenivasan Rajagopal; e-mail: rajagopalseenivasan@
yahoo.com.
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
C
ꢀ
2014 Wiley Periodicals, Inc.

PHOTOCHEMICALLY GENERATED RUTHENIUM(III)–POLYPYRIDYL COMPLEXES
607
calculated by the application of Marcus semiclꢀaCssical theory to these redox reactions are in
close agreement with the experimental values. 2014 Wiley Periodicals, Inc. Int J Chem Kinet
46: 606–618, 2014
ticancer activity of ruthenium complexes, and one of
them is their redox reactivity [35]. Generally metallo-
drugs undergo the ligand exchange and redox reaction
before they reach the target site. The disturbance of
redox balance in living organism is at the center of
many diseases, including cancer [36] and neurological
disorders [37]. It is proposed that the binding of met-
allodrugs to sulfur-containing amino acid side chains
(methionine and cysteine) of proteins can deactivate
their activity [38]. Thus the study of reactivity of re-
dox active ruthenium complexes with Met is important
from biochemical and medicinal point of view because
ruthenium and iron belong to the same group in the
periodic table [39].
INTRODUCTION
Methionine (Met) is the most important essential
sulfur-containing amino acid in human nutrition, that
is only available from food sources [1]. It assists in
metabolic function, breaks down fats, and is a primary
source of sulfur in the living system [2,3]. Met residues
in proteins and peptides are susceptible to oxidation
by reactive oxygen species to form the correspond-
ing methionine sulfoxide (MetSO) [4] as a result of
two-electron oxidation of sulfur. The oxidation of Met
residues is related to the pathogenesis of specific dis-
eases such as Alzheimer’s disease [5–12], Parkinson’s
disease [13], and even the aging process [14]. Oxida-
tive damage to Met residue is also considered to be a
main reason for the development of cataracts, a leading
cause of blindness. Thus Met oxidation is likely to alter
physical as well as biochemical properties of proteins
of pathophysiological relevance. Although the forma-
tion of MetSO from Met is a physiologically reversible
process, further oxidation to MetSO2 is believed to be
biologically irreversible [15–18]. The first step in the
oxidation of Met is the formation of the Met sulfur
radical cation, MetS•+ due to the one-electron oxi-
dation initiated by species such as peroxyl (ROO•),
alkoxyl (RO•), and hydroxyl (OH•) radicals. In par-
ticular, the oxidation of Met residues initiated by OH•
and other free radical species has attracted a lot of
interest [19,20].
The lifetime of the sulfur-centered radical cation
formed due to the oxidation of Met is very short, but
it is stabilized by complexation with the heteroatoms
(O, N, and S) of the peptidic bond or with a sulfur
atom of another thioether group [21]. Met sulfur radical
cations can be stabilized by an intramolecular sulfur–
nitrogen (>SࢺN<)+ or sulfur–oxygen (>SࢺO<)+
three-electron bond species or the intermolecular bond-
ing with the unoxidized sulfur atom of the other Met
to form a sulfur–sulfur (>SࢺS<)+ two-center three-
electron–bonded dimeric radical cation [12,21–24].
Octahedral ruthenium(II)–polypyridyl complexes,
a versatile group of compounds with unique electro-
chemical and photophysical properties, have extensive
applications as oxidation catalysts, dye sensitizers for
solar cells, for DNA intercalation, and protein-binding
properties [25–29]. Ruthenium complexes with lig-
ands of various structures have been shown to dis-
play promising anticancer activities [30–34]. Different
mechanisms have been proposed to account for the an-
In the present study, we have used six [Ru(NN)3]3+
complexes as electron acceptors to understand the role
of electrostatic and hydrophobic interactions on the re-
activity of these metal complexes with Met. To confirm
the formation of the sulfide radical cation as an in-
termediate including an intermolecular (SࢺS)-bonded
radical cation and bipyridyl anion radical of the reac-
tion, we have followed the reaction by a time-resolved
technique from which we are able to get the absorption
spectrum of the transients supporting the formation
of the monomeric sulfide radical cation (>S+•) and
the intermolecular dimeric radical cation [>SࢺS<]+
species during the course of the reaction [40]. From
these spectral observations for the formation of the
sulfur radical cation and successful application of
Marcus theory, we conclude that the electron trans-
fer (ET) process is the rate-controlling step for the
reaction between [Ru(NN)3]3+ and Met. The results of
the spectral and kinetic studies on the reaction of six
[Ru(NN)3]3+complexes with four Mets are presented
in this article. Although different methods are available
for the generation of Ru(III) from Ru(II) complexes,
in the present study Ru(III) complexes have been gen-
erated by the visible light oxidation of [Ru(NN)3]2+
in the presence of molecular oxygen [41–45], a green
process for the generation of an active oxidant.
EXPERIMENTAL
Materials
The ligands, 2,2ꢁ-bipyridine (bpy), 4,4ꢁ-dimethyl-2,2ꢁ-
bipyridine (dmbpy), 4,4ꢁ-di-tert-butyl-2,2ꢁ-bipyridine
(dtbpy), 1,10-phenanthroline (phen), 4,7-diphenyl-1,
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

608
THIRUPPATHI ET AL.
10-phenanthroline (dpphen), and 4,7-diphenyl-1,10-
phenanthroline-disulfonate (BPS), and RuCl3·3H2O
were obtained from Aldrich (Bangalore, India)
and methionine, ethionine, buthionine and N-
acetylmethionine (purity >99%) from Sigma–Aldrich
(Bangalore, India) and used without further purifica-
tion. HPLC-grade CH3CN was obtained from Merck
(Mumbai, India).
Synthesis and Characterization of
Ruthenium(II)–Polypyridyl Complexes
The [Ru(NN)3]2+complexes (where NN is bpy, dmbpy,
dtbpy, phen, dpphen, and BPS) were used in the present
study synthesized by known procedures [46–49]. All
the six Ru(II) complexes synthesized were charac-
terized by UV–vis, IR, 1H NMR, 13C NMR, and
ESI-MS spectral techniques (spectral data are given
in the Supporting Information). The spectral data
are in close agreement with the reported values
[46–49].
Figure 1 (a) The absorption spectrum of a solution of the
2 × 10−5 M [Ru(bpy)3]2+ complex in aqueous CH3CN
(1:1, v/v) oxygen-saturated 2.3 M HClO4. (b) The absorption
spectrum of [Ru(bpy)3]3+ obtained from the irradiation of
2 × 10−5 M [Ru(bpy)3]2+ in the oxygen-saturated solution.
Photochemical Oxidation of [Ru(NN)3]2+ to
[Ru(NN)3]3+
Kinetic Measurements
The steady-state photolysis of [Ru(NN)3]2+ (2 ×
10−5 M) in 2.3 M HClO4, in the presence of molec-
ular oxygen, for 20 min using a 500-W tungsten-
halogen lamp led to the formation of the correspond-
ing Ru(III) complex. The IR and UV radiations were
cut off by passing the light beam through a 30-cm
quartz cell filled with water. It was observed that the
color of the solution readily changed from orange-
yellow to green during irradiation. The formation of
[Ru(NN)3]3+ complex was confirmed by recording the
absorption spectrum of the irradiated solution. Ow-
ing to [Ru(bpy)3]2+, the absorption peak at 450 dis-
appeared quickly on irradiation for 20 min, resulting
in the formation of [Ru(bpy)3]3+ having peaks around
420–430 and 650–670 nm, matching with the reported
wavelength of maximum absorption of Ru(III) in the
acidic aqueous solution [43,50]. These peaks are as-
signed to charge transfer transitions from the bipyridyl
π ligands to the electron-deficient metal (t2g) [51].
The absence of a peak at 450 nm in the absorp-
tion spectrum of Ru3+ confirms the complete con-
version of Ru2+ to Ru3+. The absorption spectra of
[Ru(bpy)3]2+ and [Ru(bpy)3]3+ are shown in Fig. 1.
The percentage conversion of Ru2+ to Ru3+ deter-
mined from the absorption intensity at 650–670 nm
is ꢀ95% in all cases (vide infra). The molar extinc-
tion coefficient (ε) of [Ru(bpy)3]3+ at 650–670 nm is
A Jasco UV–vis absorption spectrophotometer (model
V530) was employed to record the absorption spec-
tra of [Ru(NN)3]2+ and [Ru(NN)3]3+ complexes used
in the present study and to follow the kinetics of ET
reactions of [Ru(NN)3]3+ complexes with Met. The
kinetic study was carried out in aqueous CH3CN (1:1,
v/v) under the pseudo–first-order condition by taking
an excess substrate over the oxidant. The progress of
the reaction was monitored by following the increase
in the absorbance of [Ru(NN)3]2+ (λ = 450 nm)
abs
max
at definite time intervals at 298 K (Fig. 2) [52]. The
pseudo-first–order rate constant (k1) for each kinetic
run was evaluated from the slope of a linear plot of
log Abs versus time by the method of least squares.
The linearity of each fit was confirmed from the values
of correlation coefficient (r) and standard deviation(s).
The second-order rate constant (k2) was evaluated from
the relation k2 = k1/ [substrate].
Electrochemical Measurements
The redox potentials of [Ru(NN)3]3+ complexes in
aqueous CH3CN (1:1, v/v) medium were measured by
the cyclic voltammetric technique using a computer-
controlled potentiostat (CH Instruments; model 680
AMP Booster). HClO4 was used as the supporting
electrolyte. The oxidation potentials of Mets were
680 M−1 cm−1
.
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PHOTOCHEMICALLY GENERATED RUTHENIUM(III)–POLYPYRIDYL COMPLEXES
609
II. The values of the oxidation potential of methionines
and the reduction potential of [Ru(NN)3]3+ are in good
agreement with the reported values [43,53,54].
Laser Flash Photolysis Experiments
The short-lived intermediates formed during the course
of reaction is followed using the transient absorp-
tion spectral study with a laser flash photolysis tech-
nique using an Applied Photophysics SP-Quanta Ray
GCR-2(10) Nd:YAG laser–generating 355 nm pulses
(ꢀ8 ns pulse width) [55]. The transient absorption
at preselected wavelengths was monitored using a
Czerny–Turner monochromator with a stepper motor
control and a Hamamatsu R-928 photomultiplier tube.
A 250-W xenon arc lamp was used as the monitor light
source. Experiments were carried out using a quartz
cell (1 cm) with an optical path length of 0.5 cm for the
monitoring beam. The concentration of [Ru(bpy)3]3+
(1 × 10−3 M) and Met (0.05 M) was used for all
experiments. Typically 3–5 laser shots were averaged
for each kinetic trace. All the measurements were car-
ried out at 22 2°C. The kinetic traces were taken at
10 nm intervals, usually between 320 and 800 nm. The
change in the absorbance of the sample on laser irradi-
ation was used to record the time-resolved absorption
transient spectrum. The change in the absorbance on
flash photolysis was calculated using the following ex-
pression:
Figure 2 Absorption spectral changes in the reaction be-
tween [Ru(bpy)3]3+ (2 × 10−5 M) and methionine (5 ×
10−4 M) at 298 K with a 2-min time interval.
measured by differential pulse voltammetry, and tetra-
butylammonium perchlorate (0.1 M) was used as the
supporting electrolyte. A glassy carbon (working elec-
trode) and a standard (Ag/AgCl) electrode (reference
electrode) were used in the electrochemical measure-
ments. The sample solutions were deaerated by purg-
ing dry nitrogen gas for about 30 min before each
measurement. The values of the oxidation potential of
four-substituted methionines and the reduction poten-
tial of six [Ru(NN)3]3+ are presented in Tables I and
ꢀA = log I0/ (I0 − ꢀI)
ꢀI = (I − It )
Table I Second-Order Rate Constants (k2; M−1s−1) for the Oxidation of Methionines by [Ru(NN)3]3+ in Aqueous
CH3CN 1:1 (v/v) at 298 K
k2 (M−1 s−1
)
Ru(bpy)3]3+
(0.92 V) (I)
[Ru(dmbpy)3]3+
(0.75 V) (II)
[Ru(dtbpy)3]3+
(0.76 V) (III)
Oxidation
Potential, V
(Ag/AgCl)
Observed
Calculated
Observed
Calculated
Observed
Calculated
No.
1
Met
ꢀGo (eV)
ꢀGo (eV)
ꢀGo (eV)
Methionine
1.34
1.47
1.49
1.42
6.9
0.21
0.23
0.41
0.54
0.42
0.38
0.42
0.54
0.62
0.01
0.01
0.01
0.02
0.59
0.19
0.23
0.27
0.32
0.01
0.01
0.01
0.01
0.58
8.2
0.33
0.31
0.82
0.96
0.21
0.18
0.20
0.46
2
3
4
Ethionine
8.3
0.55
0.57
0.50
0.72
0.74
0.67
0.71
0.73
0.66
10.1
16.2
19.9
Buthionine
13.8
17.5
N-acetyl
methionine
General conditions: [Ru(NN)3]3+ = 2 × 10−5 M, [Mets] = 5 × 10−4 M, and [H+] = 2.3 M.
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

610
THIRUPPATHI ET AL.
Table II Second-Order Rate Constants (k2, M−1s−1) for the Oxidation of Methionines by [Ru(NN)3]3+ in Aqueous
CH3CN 1:1 (v/v) at 298 K
k2 (M−1 s−1
)
Ru(phen)3]3+
(0.92 V) (IV)
[Ru(dpphen)3]3+
(0.89 V) (V)
[Ru(dspphen)3]3+
(0.91 V) (VI)
Oxidation
Potential,V
(Ag/AgCl)
Observed
Calculated
Observed
Calculated
Observed
Calculated
No.
1
Met
ꢀGo (eV)
ꢀGo (eV)
ꢀGo (eV)
Methionine
1.34
1.47
1.49
1.42
9.2
12.6
17.5
25.3
0.25
0.37
0.48
0.75
0.42
5.8
8.3
0.18
0.23
0.42
0.45
0.45
11.5
14.7
21.2
28.6
0.32
0.41
0.67
0.85
0.43
13.5
11.7
19.9
26.8
4.5
10.6
12.3
26.5
34.8
2
3
4
Ethionine
0.55
0.57
0.50
0.58
0.60
0.53
0.56
0.58
0.51
11.8
14.2
18.4
Buthionine
13.8
15.4
N-acetyl
methionine
General conditions: [Ru(NN)3]3+ = 2 × 10−5 M, [Mets] = 5 × 10−4 M, and [H+] = 2.3 M.
where ꢀA is the change in the absorbance at time t,
I0, I , and It are the voltage after flash, the pretrigger
voltage, and the voltage at particular time, respectively.
The time-resolved transient absorption spectrum was
recorded by plotting the change in the absorbance at
a particular time versus wavelength. The experiment
was carried out in the absence and presence of molec-
ular oxygen. The presence of oxygen is necessary to
generate the Ru3+ species from Ru2+ photochemically
[43].
RESULTS AND DISCUSSION
The generation of the Ru3+ ion from the corresponding
[Ru(NN)3]2+ ion is carried out as detailed below. The
steady-state photolysis of [Ru(NN)3]2+ (2 × 10−5 M)
in 2.3 M HClO4, in the presence of molecular oxy-
gen, for 20 min using a 500-W tungsten–halogen lamp
led to the formation of the corresponding Ru3+ ion.
The IR and UV radiations were cut off by passing
the light beam through a 30-cm quartz cell filled with
water. It was observed that the color of the solution
readily changed from orange-yellow to green during
irradiation. The formation of [Ru(bpy)3]3+ complex
was confirmed by recording the absorption spec-
trum of the irradiated solution. The absorption spec-
tra of [Ru(bpy)3]2+ and [Ru(bpy)3]3+ are shown in
Fig. 1. To learn the concentration of Ru3+ formed
from the photochemical oxidation of Ru2+, we es-
timated the concentration of the Ru3+ ion from the
absorbance (Abs) values at 420 and 670 nm. Since
the band at 670 nm (ε = 680 M−1cm−1) corresponds
to Ru3+ with no interference from Ru2+, we con-
sidered the concentration estimated from the Abs at
670 nm as more reliable. This estimation shows that
the chemical conversion of Ru2+ to Ru3+ is more than
95% under the present experimental conditions, and
we have used this Ru3+ solution generated from the
photochemical oxidation of Ru2+ for kinetic studies.
When [Ru(NN)3]2+ complexes are irradiated in
the presence of molecular oxygen, two processes, en-
ergy and ET from the excited state [Ru(NN)3]2+ to
Product Analysis
A sample of 0.05 M of substrate (Met) was added to
a 0.005 M solution of [Ru(bpy)3]3+ complex in 5 mL
aqueous CH3CN (1:1, v/v). The solution was stirred
at 298 K for ꢀ1 h. The products of the reaction were
extracted with chloroform and dried, and the solvent
was removed. The final compound was then analyzed
using IR and 1H NMR spectroscopy, which confirmed
Met sulfoxide as the only major product under the
present experimental conditions. The IR spectrum of
the product (sulfoxide) was found to have a stretch-
ing frequency in the characteristic region 1030 cm−1
for the product. 1H NMR (200 MHz, D2O): δ = 2.1–
2.24 (2H, q), 2.73–3.03 (2H, m), 3.71–3.79 (1H, m),
and 2.62 (s, SOCH3) values are in good agreement
with reported values of methionine sulfoxide [18,56].
1H NMR and IR spectra of methionine sulfoxide are
given in the Supporting Information (see Figs. S1 and
S2).
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PHOTOCHEMICALLY GENERATED RUTHENIUM(III)–POLYPYRIDYL COMPLEXES
611
molecular oxygen, take place to form singlet oxygen
L =
and superoxide anion radical, respectively (Eqs. (1) and
(2)). Under high acid concentration, the oxidation of
Ru2+ to Ru3+ is highly favored as indicated in Eq. (3).
More than 95% conversion of Ru2+ to Ru3+ in our
present experimental condition also supports our ar-
gument that ET from *[Ru(NN)3]2+ to O2 to produce
N
N
N
N
[Ru(phen)3]3+
[Ru(bpy)3]3+
IV
I
N
N
N
N
O2 is the predominant reaction here. The high [H+]
•−
[Ru(dpphen)3]3+
[Ru(dmbpy)3]3+
chosen in the present study facilitates the generation
of Ru3+ from the cage as shown in Eq. (3). Thus we
propose that under the conditions used in the present
study the formation of singlet oxygen through energy
transfer is less favored compared to ET, and most of
the Ru2+ is converted to Ru3+, which is responsible for
the redox reactions of methionines:
II
V
-
-O3 S
Na+
SO3
Na+
N
N
N
N
[Ru(dspphen)3]3+
[Ru(dtbpy)3]3+
VI
III
∗ ꢀ
ꢁ
ꢀ
ꢁ
energy
2+
2+
3
Ru (NN)3
+ O2 −−−→ Ru (NN)3
+
1 O2
(1)
transfer
O
O
S
S
HO
HO
NH2
NH2
ꢂ
ꢃ
∗ ꢀ
ꢁ
ꢀ
ꢁ
electron
2+
3
Methionine
Ethionine
Ru (NN)3
+ O2 −−−→ Ru (NN)3 3+ − O2•−
transfer
O
H
N
(2)
S
S
HO
ꢂ
ꢃ
O
ꢀ
ꢁ
ꢀ
ꢁ
NH2
•
3+
3+
O
OH
−
Ru (NN)3
. . . O2 + H+ ꢀ Ru (NN)3 + HO•2
Buthionine
N-Acetylmethionine
cage
Chart 1 Structure of ligands of [Ru(NN)3]3+, methionine,
(3)
and substituted methionines.
The dependence of the rate constant of the ET reac-
respect to Ru(III) complex, which is evident from the
linear log Abs versus time plot shown in Fig. S3 in the
Supporting Information.
The values of pseudo–first-order rate constant, k1
plotted as a function of the concentration of Met, are
shown in Fig. 3. The linear relationship between k1 and
[Met] and constant k2 values at different [Met] also
points out the first-order dependence in the substrate.
The reaction is thus overall second order.
The second-order rate constants determined for the
reaction of six [Ru(NN)3]3+ complexes with four-
substituted methionines are presented in Tables I and II.
It is observed that the introduction of electron-donating
groups, such as methyl and tert-butyl at the 4- and 4ꢁ-
position of 2,2ꢁ-bipyridine of the Ru(III) complexes,
4,4ꢁ-dimethyl-2,2ꢁ-bipyridine, and 4,4ꢁ-tert-butyl-2,2ꢁ-
bipyridine ligands, retards the rate of oxidation enor-
mously. When we look at the ꢀGo values presented
in Table I, we realize that the introduction of the alkyl
group in the ligand of [Ru(NN)3]3+ changes the ꢀGo
values by 0.17–0.18 eV. Thus the main reason for the
substantial change in the rate constant with the change
in the structure of the ligand is the change in exer-
gonicity (ꢀGo) of the reaction. On the other hand, the
tion from methionines to [Ru(NN)3]3+ on the reduction
potentials of [Ru(NN)3]3+ also supports the formation
of Ru3+ as the major product when Ru2+ is irradi-
ated with visible light in the presence of O2 under our
experimental condition (vide infra).
The structure of ligands of [Ru(NN)3]3+ and Mets
used in the present study are shown in Chart 1. The ki-
netics of the redox reaction between [Ru(NN)3]3+ and
Met has been followed spectrophotometrically, under
the pseudo–first-order condition by taking excess Met
over the [Ru(NN)3]3+ ion in the aqueous CH3CN (1:1,
v/v) mixture. The progress of the reaction has been
followed by measuring the increase in the absorbance
(A) of the Ru(II) ion formed (450 nm) as one of the
products of the reaction, and a sample kinetic run is
shown in Fig. 2. Interestingly, the observation of isos-
bestic point at 538 nm for the reduction of Ru(III) to
Ru(II) as shown in Fig. 2, points out that the conversion
proceeds neatly and confirms that the reaction follows
simple kinetics without involving any complex mecha-
nism. These experimental observations are strongly in
favor of ET from Met to Ru(III) to form Ru(II) in the
rate-determining step. The reaction is first order with
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

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THIRUPPATHI ET AL.
Table III Effect of Various Solvent Compositions on
the Reaction of [Ru(NN)3]3+ with Methionine and
Ethionine at 298 K
k2 (M−1 s−1
)
[Ru(bpy)3]3+
[Ru(phen)3]3+
Solvent Composition
CH3CN:H2O (v/v)
Methionine
Ethionine
80:20
70:30
60:40
50:50
40:60
20:80
4.1
4.8
5.3
6.9
7.2
8.5
0.12
0.13
0.15
0.21
0.22
0.24
9.3
10.7
11.1
12.4
14.2
16.5
0.25
0.32
0.31
0.35
0.38
0.50
General conditions: [Ru(NN)3]3+ = 2 × 10−5 M, [Mets] = 5 ×
10−4 M, and [H+] = 2.3 M.
Figure 3 Plot of k1 versus [Mets] for the oxidation of 1–4
with [Ru(phen)3]3+ in aqueous CH3CN (1:1, v/v) at 298 K.
(The plots 1–4 refer to the Mets given in Table II.)
[Ru(NN)3]3+ with methionine and not from the re-
action of the excited state [Ru(NN)3]2+, we have de-
signed the transient spectral study in the following way:
We carried out the transient spectral study in the ab-
sence and presence of O2. When we irradiated a mix-
ture of [Ru(bpy)3]2+ and methionine in the absence of
O2 though the excited state, [Ru(bpy)3]2+ was formed;
no evidence for the formation methionine sulfur radi-
cal was observed. This clearly shows that the excited
state [Ru(bpy)3]2+ has no reaction with methionine.
But when the reaction of [Ru(bpy)3]2+ with methion-
ine in 2.3 M [H+] in the presence of molecular O2 was
followed by transient absorption spectroscopy, we are
able to get evidence for the formation of the methion-
ine sulfur cation radical. The results of transient spec-
tral study are shown in Figs. 4 and 5. When we look
at the spectral changes in the region of 320–800 nm
in the absence of molecular O2, the formation of one
intermediate species and bleaching of two species can
be noticed (Fig. 4). A sharp absorption band at 370 nm
attributed to the formation of bipyridyl radical an-
ion (bpy•−) and the bleaching at 450 nm correspond
to a relative decrease in the MLCT (metal to lig-
and charge transfer) transition of the complex in the
ground state [57–59]. The bleaching around 600–
700 nm corresponds to the light emission from the
ground state [60]. In the present experiment, it is perti-
nent to mention that a lack of absorption band at 480 nm
infers a nonformation of the sulfur–sulfur dimeric rad-
ical cation (SࢺS)+ even at longer timescales followed
in the experiment. Figure 5 shows the spectra of tran-
sients formed due to the reaction between the photo-
chemically generated [Ru(bpy)3]3+ ion and methion-
ine in aqueous CH3CN (1:1, v/v) solution at 298 K. We
introduction of the electron-withdrawing group, disul-
fonate, in the 1,10-phenanthroline enhances the rate of
the reaction slightly (Table II). This is again due to a
little change in the ꢀGo value. Thus the effect of chang-
ing the substituent in the ligand of the Ru(III) complex
on the rate of ET can be accounted for in terms of
change in reduction potentials of [Ru(NN)3]3+. Simi-
larly, the change in the structure of methionine has a
slight effect on the rate of the reaction, which may be
due to the change in the electronic effect of the alkyl
group attached to the sulfur atom.
To understand the influence of other parameters on
the rate of the reaction, the kinetics of the reaction
has been followed at different solvent compositions
and the results are presented in Table III. Generally,
the increase in the water content favors the reaction
when charge development takes place in the transition
state of the reaction. Our research group established the
fact that an increase in the water content increases the
ET reaction between sulfur compounds and Ru(III)-
polypyridyl complexes [45b]. Thus the solvent effect
supports the formulation of ET in the rate-controlling
step.
Transient Absorption Spectral Studies
To understand the transient species formed due to the
reaction of [Ru(NN)3]3+ complexes with methionines,
the convenient method is monitoring the course of
this redox reaction by applying the transient absorp-
tion spectroscopy. To get the conclusive evidence for
the formation of the transients from the reaction of
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PHOTOCHEMICALLY GENERATED RUTHENIUM(III)–POLYPYRIDYL COMPLEXES
613
Figure 4 Transient difference spectra of the [Ru(bpy)3]2+ complex with 0.05 M methionine in a deoxygenated aqueous
CH3CN (1:1, v/v) solution obtained at different time delays: from 100 ns to 2 μs.
complete bleaching of Ru2+ at 450 nm and the for-
mation of Ru3+ (Figs. S4 and S5 in the Supporting
Information).
recorded the absorption spectra of transients formed at
different timescales from 100 ns to 2 μs. It exhibits
a sharp absorption band at 370 nm attributed to the
formation of the bipyridyl radical anion (bpy•−). The
bleaching at 450 and 650 nm corresponds to the disap-
pearance of [Ru(bpy)3]2+ and [Ru(bpy)3]3+ ions. Inter-
estingly, a new peak around 480 nm appeared at 500 ns,
which is assigned to the well-characterized two-center,
three-electron–bonded dimeric (SࢺS)+ radical cation
of methionine [61]. But this short-lived species almost
disappeared at 800 ns after the laser pulse (Fig. 5).
However, Schoneich et al. were able to get evidence
for the appearance of this type of the dimeric radical
cation at 8 μs (pH 4.0) [61a]. Since the conditions
used in the present study are different from those of
Schoneich et al., the lifetime of the transient formed is
different. The presence of the intermolecular (SࢺS)+-
bonded radical cation was previously identified by
pulse radiolysis and laser flash photolysis techniques
with sulfur-containing amino acids and methionine-
containing peptides [61–63]. The point we want to
emphasize from these results is that during the course
of this reaction the methionine sulfur radical cation
S•+ and dimeric radical cation (SࢺS)+ are formed
as the intermediates of the reaction. The flash pho-
tolysis study carried out in the absence of methion-
ine in the oxygen-saturated solution indicates almost
Mechanism of [Ru(NN)3]3+ Oxidation of
Methionines
As the progress of the reaction has been followed with
an increase in the absorption at 450 nm, the character-
istic absorption of the Ru(II) complex, we can presume
that the reaction proceeds through the ET mechanism
(Fig. 2). It is important to mention that the kinetic
data presented in Tables I and II have been obtained
using the isolated Ru3+ ion, which is obtained from
the photolysis of Ru2+ in the presence of molecular
oxygen. But the flash photolysis study has been car-
ried out by direct irradiation of Ru2+ in the presence
of molecular oxygen and methionine. Our results sug-
gest that excited state [Ru(NN)3]2+ has no reaction but
only Ru3+ is the predominant species responsible for
the reaction with methionine under high acidic con-
dition. Although Met contains amino, carboxyl, and
thioether parts, the site of attack of Ru(III) is on the
Met sulfur atom, i.e. the electron is transferred from
the sulfur center of Met to the Ru(III) ion. These
experimental observations, the formation of the Met
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

614
THIRUPPATHI ET AL.
Figure 5 Spectra of transients formed due to the reaction between the photochemically generated [Ru(bpy)3]3+ complex with
methionine (0.05 M) in an oxygen-saturated aqueous CH3CN (1:1, v/v) solution obtained at different time delays: from 100 ns
to 2 μs.
radical cation (>S•+) and Ru (II) as the products in the
rate-determining step (Step 5), are strongly in favor
of ET from Met to Ru(III). It has been proved earlier
that the sulfur radical cation formed from Met can be
stabilized either by the formation of the intermolecu-
lar (SࢺS)+ two-center three-electron–bonded dimeric
radical cation or intramolecular three-electron sulfur–
nitrogen (SࢺNH2)+ or sulfur–oxygen (SࢺO)+ bonded
species [21,22]. Since in the present study the reac-
tion has been carried out under high acidic condition
([H+] = 2.3 M), the carboxylic and amino groups are
protonated and thus the intramolecular sulfur–oxygen
and sulfur–nitrogen three-electron–bonded species not
identified. Thus the monomeric sulfur radical cation
(>S•+) is stabilized only through the formation of
the intermolecular sulfur–sulfur (SࢺS)+ two-center
three-electron–bonded dimeric radical cation (Step 6),
which is supported by the formation of absorption
band around 480 nm observed using the nanosecond
laser flash photolysis technique. The initially formed
Met sulfur cation radical further reacts with the wa-
ter molecule present in the solvent medium to form
the hydroxyl sulfuranyl radical (Step 7). Finally, the
formation of sulfoxide as the major product of the
reaction helps us to confirm that the major portion
of the Met sulfur radical cation is consumed by the
solvent, water even though the fragmentation and back
ET may be competing processes. The formation of
Met sulfoxide from the Met sulfur radical cation may
be shown as a three-step process (Eqs. (7)–(9) in
Scheme 1). Furthermore, a good agreement between
the experimentally observed second-order rate con-
stants and rate constants calculated from the Marcus
semiclassical theory also supports the proposed mech-
anism (Scheme 1). The reaction between [Ru(NN)3]3+
and Met also leads to the formation of the Ru(II) com-
plex in the excited state (*[Ru(NN)3]2+) (Eq. (8b)
in Scheme 1), which is supported by the peak ob-
served at 600–700 nm in the transient absorption
spectrum (Fig. 5). The mechanism proposed here is
similar to the one postulated for the [Ru(NN)3]3+
oxidation of organic sulfoxides and sulfides
[45].
Application of Marcus Semiclassical
Theory to the ET Reaction of [Ru(NN)3]3+
with Methionines
After proposing the ET as the rate-controlling step
of the reaction, we applied the semiclassical theory
of ET (Eq. (10)) [64–67] to the above redox reac-
tion. The rate of ET from a donor to an acceptor
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

PHOTOCHEMICALLY GENERATED RUTHENIUM(III)–POLYPYRIDYL COMPLEXES
615
+
oxygenated
solution
where E(D/D is the oxidation potential of electron
)
[Ru(NN)3]2++ hν
*[Ru(NN)3]2+
[Ru(NN)3]3+
(4)
−
donor and E(A/A ), the reduction potential of acceptor.
The reorganization energy (λ) is the sum of two contri-
butions, λ = λo + λi, where λi represents the activation
of the vibrational modes of the reactants and λo rep-
resents the changes in the solvent structure around the
reactants, which is strongly dependent on the solution
medium. The value of λo is evaluated by using the
dielectric continuum model (Eq. (12)):
+
k
R-S-R′ + [Ru(NN)3]3+
R-S-R′ + [Ru(NN)3]2+
(5)
(6)
+
+
R-S-R′ + R-S-R′
(>S S<)
OH
+
fast
fast
R-S-R′ + H2O
R-S-R′ + H+
(7)
λo = e2/4πεo (1/2rD + 1/2rA − 1/d)
OH
OH
R-S-R′ + [Ru(NN)3]3+
R-S-R′ + [Ru(NN)3]2+ (8a)
ꢅ
+
× 1/Dop − 1/Ds (12)
OH
OH
fast
fast
where e is the transferred electronic charge, εo the per-
mittivity of free space and Dop and Ds are the optical
and static dielectric constants, respectively. The terms
rD and rA are the radii of the electron donor and accep-
tor, respectively, and d is the sum of radii, rD + rA. The
values of rD and rA are estimated by the MM2 molecu-
R-S-R′ + [Ru(NN)3]3+
R-S-R′ + *[Ru(NN)3]2+
+
(8b)
OH
R-SO-R′ + H+
(9)
R-S-R′
+
˚
˚
lar model, and the values are 4.7–6.3 A and 6.1–12.6 A.
The value of λo calculated using Eq. (12) is 0.73 eV
for methionine, 0.70 eV for ethionine, 0.61 eV for
buthionine, and 0.69 eV for N-acetylmethionine. The
value of λi is found to be 0.2 eV and is employed in
the calculation of the rate constant for the ET reac-
tion [68–70]. Thus the total reorganization energy λ
value for this redox system is in the range of 0.81–
0.93 eV. Since ꢀG0 and λ values are known, the value
of the rate constant for ET from Met to [Ru(NN)3]3+
can be calculated using Eq. (10). The experimental and
calculated k2 values are presented in Tables I and II.
These data show a close agreement between the exper-
imental and calculated values. Thus, the semiclassical
theory of ET reproduces the experimental results fa-
vorably confirming the success of the theory of ET and
the operation of the ET mechanism of the reaction.
-CH2-CH2-CH-CO2H,
NH2
R=
-CH2-CH2-CH-CO2H
NHCOCH3
R'= -CH3, -CH2-CH3, -CH2-CH2-CH2-CH3
Scheme 1 Mechanism for the oxidation of Met by
[Ru(NN)3]3+
.
molecule in a solvent is controlled by free energy
change in the reaction (ꢀG0), the reorganization en-
ergy (λ) and the ET distance between the donor and the
acceptor.
α
ꢄ
ꢅ
ꢆ
ket = 4π2/h |HDA| (4πλkT)−1 2
e
−s Sm m!
2
/
m=0
ꢀ
ꢆ
ꢁ
× exp − (λ + ꢀG◦ + mhv)2 4λkT
(10)
Thermodynamic Parameters
The ET reaction of [Ru(NN)3]3+ with Mets has been
studied in four different temperatures, and the enthalpy
In Eq. (10), HDA is the electronic coupling matrix
element, the reorganization energy λ is composed of
solvational λo and vibrational λi contributions with
s = λi /hν, ν is the high-energy vibrational frequency
associated with the acceptor, and m is the density
of product vibrational levels. The terms h and k are
Planck’s and Boltzmann’s constants, respectively. The
free-energychange(ꢀG0)ofETcanbecalculatedfrom
(Eq. (11)):
ࣔ
ࣔ
(ꢀH ) and entropy (ꢀS ) of activation are evaluated
from the kinetic data. The rate constants obtained at
four temperatures and the thermodynamic data are pre-
sented in Table IV, and the Eyring plots are shown in
ࣔ
Fig. 6. The negative values of ꢀS indicate the com-
ࣔ
pactness of transition state. A comparison of ꢀS and
ࣔ
ꢀH values shows that with the change in the struc-
ࣔ
ture of methionine there is little change in the ꢀS but
there is a significant change in the ꢀH value. Thus it
seems that the reaction is enthalpy controlled.
ࣔ
ꢀG◦ = E
− E
(11)
−
D/D+
A/A
(
)
(
)
International Journal of Chemical Kinetics DOI 10.1002/kin.20874

616
THIRUPPATHI ET AL.
Table IV Second-Order Rate Constants (k2) and Activation Parameters for the Oxidation of Methionines by
[Ru(phen)3]3+ in Aqueous 1:1 (v/v) CH3CN at Different Temperatures
k2 (M−1 s−1
)
ࣔ
ࣔ
No.
Met
293 K
298 K
303 K
313 K
ꢀH (kcal mol−1
)
–ꢀS (cal K−1 mol−1
)
1
2
3
4
Methionine
Ethionine
Buthionine
N-acetyl-methionine
7.6
9.7
14.3
20.5
9.2
12.4
17.5
25.7
12.5
15.2
22.7
32.4
17.4
22.6
32.4
49.3
8.5
6.1
6.1
6.0
47.1
47.1
47.1
47.1
General conditions: [Ru(NN)3]3+ = 2 × 10−5 M, [Mets] = 5 × 10−4 M, and [H+] = 2.3 M.
Figure 6 Eyring’s plots for the oxidation of amino acids by [Ru(phen)3]3+ in aqueous CH3CN (1:1, v/v) at 298 K. (The plots
1–4 refer to the amino acids given in Table IV.)
Delhi, India, for a junior research fellowship. M.G. and D.T.
thank the Management, Principal, and Head of the Depart-
ment of Chemistry of Vivekananda College, Thiruvedakam
West, Madurai, India, for providing laboratory facilities. The
authors also thank Professor P. Ramamurthy and Dr. C. Sel-
varaj, National Centre for Ultrafast Processes, University of
Madras, Chennai, India, for their help in recording transient
spectra using the flash photolysis technique.
CONCLUSION
The oxidation of four substituted Mets with six
[Ru(NN)3]3+ ions proceeds through rate-determining
ET from the substrate to the oxidant. The ET nature
of the reaction is confirmed by the identification of the
sulfur cation radical using the nanosecond laser flash
photolysis technique and formation of the Ru(II) ion
as the product of the reaction. Met sulfoxide is the
major product of the reaction. The rate constants cal-
culated by using Marcus semiclassical theory are in
close agreement with the experimental values.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20874