# Kinetic and Mechanistic Studies on the Reaction of DL-Methionine with[(H2O)(tap)2RuORu(tap)2(H2O)]2+ in Aqueous Medium at Physiological pH

Authors: Tandra Das; A.  K. Datta; A. K. Ghosh
Journal: Research Letters in Inorganic Chemistry
 (2009)

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## Abstract

The reaction has been studied spectrophotometrically; the reaction shows two steps, both of which are dependent on ligand concentration and show a limiting nature. An associative interchange mechanism is proposed. Kinetic and activation parameters (k1∼10−3s−1 and k2∼10−5s−1) and (ΔH1≠=13.8±1.3kJmol−1, ΔS1≠=−250±4JK−1mol−1, ΔH2≠=55.53±1.5kJmol−1, and ΔS2≠=−143±5JK−1mol−1) have been calculated. From the temperature dependence of the outer sphere association equilibrium constant, thermodynamic parameters (ΔH1∘=16.6±2.3kJmol−1 and ΔS1∘=95±7JK−1mol−1; ΔH2∘=29.4±3.2kJmol−1 and ΔS2∘=128±10JK−1mol−1) have also been calculated.


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## Body

## 1. Introduction

The  binding of  the antitumor drug cisplatin and other platinum group
metal complexes, especially ruthenium(II), rhodium(III), iridium(III),
platinum(II), and palladium(II) to amino acids, nucleosides, nucleotides, and
particularly to DNA is still an interesting subject and has given considerable
impetus to research in the area of metal ion interactions with nucleic acid
constituents. Ruthenium complexes are an order of magnitude less toxic than
cisplatin, and aqua complexes if used directly will be less toxic as some
hydrolyzed side products are responsible for toxicity. From a literature survey
[1–3], it is
revealed that many potential alternative metallopharmaceuticals have been
developed, ruthenium being one of the most promising, and are currently
undergoing clinical trials [4–7]. Another point
of interest is that DNA is not the only target. Binding to proteins, RNA [8–10] and several
sulphur donor ligands, present in the blood, are available for kinetic and
thermodynamic competition [11, 12].Keeping this in mind, in this paper, we have
studied the kinetic details of the interaction of our chosen complex (an aqua-amine
complex of ruthenium(II)) with an S-containing amino acid DL-methionine at pH 7.4 in aqueous
medium and a plausible mechanism is proposed.The importance of the work lies in the fact that (a) the
reaction has been studied in an aqueous medium, (b) the reaction has been
studied at pH (7.4) which is the physiological pH of the human body, (c) the
aqua-amine complex is chosen, (d) ruthenium(II) than ruthenium(III) is chosen,
as ruthenium(III) is a prodrug which is reduced in the cell to ruthenium(II),
and (e) the title complex maintains its +2 oxidation state even at pH 7.4 due
to the presence of a strong pi-acceptor ligand tap (tap={2-(m-tolylazo)pyridine}), where most of the other ruthenium(II)
complexes are oxidized to ruthenium(III).


## 2. Materials and Methods

Reported
method [13, 14] was used to isolate cis-[Ru(tap)2(H2O)2](CIO4)2⋅H2O.
The reacting complex ion [(H2O)(tap)2RuORu(tap)2(H2O)]2+ (1) was generated in situ by adjusting
the pH at 7.4. The reaction product [(tap)2Ru(μ-O)(μ-meth)Ru(tap)2]2+ (complex 2) of DL-methionine, and complex 1 is shown in Figure 1.  The
composition of 2 in solution was determined by Job's method of continuous
variation and the metal:  ligand ratio was found to be 2:1.  The pH of the
solution was adjusted by adding NaOH/HClO4, and the measurements
were carried out with the help of a Sartorius make digital pH meter (PB 11)
with an accuracy of ±0.01 unit.
Doubly distilled water was used to prepare all the kinetic solutions. All
chemicals used were of AR grade, available commercially. The reactions were
carried out at constant ionic strength of (0.1 M NaClO4).Figure 1
Difference in spectrum between complex1 and product complex (2); [1]=1.0×10−4moldm−3, [DL-methionine]=2.0×10−3moldm−3, cell used 1 cm quartz.


## 3. Kinetics

The kinetic studies were done on a Shimadzu UV-2101PC
spectrophotometer attached to a thermoelectric cell temperature controller (model
TB 85, accuracy±0.1°C). The progress of the reaction was monitored by following the
decrease in absorbance at 600 nm using mixing technique and maintaining pseudo-first-order
conditions. In Figure 2, plot of ln(At−A∞) versus time  shows a consecutive nature
of the reaction. Initially, it is curved and shows linear behavior in the
latter stage. The rate constants were calculated using the method of Weyh and Hamm [15] as described in
an earlier paper [1] using the following equation:(1)lnΔ=constant−k1(obs)t,whentis small.The meaning of Δ is shown in Figure 2 (Δ=X−Y). k2(obs) is calculated from the latter linear portion.Figure 2
A typical plot ofln(At−A∞) versus time.


## 4. Results and Discussion

At
a fixed excess [DL-methionine](2.0×10−3moldm−3), pH 7.4,
temperature 50°C, and ionic strength (0.1moldm−3NaClO4)
the reaction was found to be first order in [complex 1], that is, d
[complex 2]/dt=kobs [complex 1].ThepKa1 and pKa2 values [16] of DL-methionine are 2.24 and 9.07,
respectively, at 25°C. Thus, at pH 7.4, the ligand exists mainly as
a neutral molecule, that is, as a zwitterion (LH2+→LH→L−). On the other
hand, first acid dissociation equilibrium of the complex [Ru(tap)2(H2O)2]2+ is 6.6 [17] at 25°C. At pH 7.4, the
complex ion exists in dimeric oxo-bridged form, [(H2O)(tap)2RuORu(tap)2(H2O)]2+ [18–21]. At pH 7.4, the mononuclear species
exists in the hydroxoaqua form. Two such species assemble to form the dinuclear
oxo-bridged diaqua complex due to thermodynamic force mainly arising from
pi-bonding [22] (O2− donor, RuII acceptor) which is
favorable for 4d ion, RuII. Now, such strong covalency reduces the
acidity of the coordinated water. The oxo-bridge formation is solely
dependent on pH. Electrochemical studies show that there is pH potential domain,
where the μ-oxo structures stay intact. Variable
temperature study does not show any effect, which is in line with the fact that
oxo-bridge formation is solely pH-dependent [23, 24]. The rate constant for such
process can be evaluated by assuming the following scheme (2)(1)→k1B→k2(2),  where B is [(H2O)(tap)2RuORu(tap)2(LH)]+.


### 4.1. Calculation ofk1 and k2 Values for Step (1)→B and for (B)→(2) Step

The rate  constants,k1(obs) for (1)→B and k2(obs) for (B)→(2),
were calculated following the technique described in an earlier paper
[25], and the values are collected in Tables 1 and 2. The rate increases with the
increase in [ligand] and reaches a limiting value for both steps. Details of
the mechanism are
discussed in “Mechanism and Conclusion" section. The k1,k2,KE′, and KE′′ for the two steps are
calculated similarly and collected in Table 3.Table 1
103k1(obs) values for different ligand
concentrations at different temperatures. [Complex]=1×10−4moldm−3,
pH = 7.4, ionic strength =0.1moldm−3NaClO4.
103 [ligand](moldm−3)Temperature(C°)455055602.00.700.871.021.183.01.01.221.41.554.01.231.451.621.905.01.401.652.02.0810.01.672.02.252.63Table 2
105k2(obs) values for different ligand concentrations at different temperatures.
[Complex]=1×10−4moldm−3, pH = 7.4, ionic strength =0.1moldm−3NaClO4.
103 [ligand](moldm−3)Temperature(C°)455055602.02.273.415.958.933.02.924.58.211.634.03.855.9810.015.05.05.137.5812.218.210.07.410.5217.8625.0Table 3
Thek1,KE′,k2, and KE′′ values for the interaction of
methionine with (1).
Temperature(C°)103k1(s−1)KE′(dm3mol−1s−1)105k2(s−1)KE′′(dm3mol−1s−1)453.0615618.070503.3817925.078553.7019736.098604.0620748.0113


### 4.2. Effect of Change in pH on the Reaction Rate

This was studied at five different pH
values.103k1(obs)(s−1) and 105k2(obs) values are 0.73, 0.76, 0.83, 1.04 and 1.55 (s−1), and 3.3,
3.7, 4.16, 6.6, and 11.32 (s−1) at pH 5.5, 6.0, 6.5, 7.0, and 7.4,
respectively. In the studied pH range (pH 5.5 to 7.4), the percentage of diaqua
species is reduced with the increase in pH, and the percentage of the dimer is
predominant. The dimer with its two metal centers is a better center to the
incoming nucleophiles. On the other hand, the pK1 and pK2 values
of the ligand DL-methionine are 2.24 and 9.07 at 25°C. With the increase
in pH from 5.0 to 7.4, the amount of the deprotonated form increases, and the
zwitterionic form (LH) predominates which also partly accounts for the enhancement of the rate
with increase in pH.


### 4.3. Effect of Temperature on the Reaction Rate

Four different temperatures with
varied ligand concentrations were chosen, and the results are listed in Tables1 and 2. The activation parameters for the steps (1)→B and (B)→(2), evaluated
from the linear Eyring plots and compared with the analogous systems [1], support the proposition.


## 4.1. Calculation ofk1 and k2 Values for Step (1)→B and for (B)→(2) Step

The rate  constants,k1(obs) for (1)→B and k2(obs) for (B)→(2),
were calculated following the technique described in an earlier paper
[25], and the values are collected in Tables 1 and 2. The rate increases with the
increase in [ligand] and reaches a limiting value for both steps. Details of
the mechanism are
discussed in “Mechanism and Conclusion" section. The k1,k2,KE′, and KE′′ for the two steps are
calculated similarly and collected in Table 3.Table 1
103k1(obs) values for different ligand
concentrations at different temperatures. [Complex]=1×10−4moldm−3,
pH = 7.4, ionic strength =0.1moldm−3NaClO4.
103 [ligand](moldm−3)Temperature(C°)455055602.00.700.871.021.183.01.01.221.41.554.01.231.451.621.905.01.401.652.02.0810.01.672.02.252.63Table 2
105k2(obs) values for different ligand concentrations at different temperatures.
[Complex]=1×10−4moldm−3, pH = 7.4, ionic strength =0.1moldm−3NaClO4.
103 [ligand](moldm−3)Temperature(C°)455055602.02.273.415.958.933.02.924.58.211.634.03.855.9810.015.05.05.137.5812.218.210.07.410.5217.8625.0Table 3
Thek1,KE′,k2, and KE′′ values for the interaction of
methionine with (1).
Temperature(C°)103k1(s−1)KE′(dm3mol−1s−1)105k2(s−1)KE′′(dm3mol−1s−1)453.0615618.070503.3817925.078553.7019736.098604.0620748.0113


## 4.2. Effect of Change in pH on the Reaction Rate

This was studied at five different pH
values.103k1(obs)(s−1) and 105k2(obs) values are 0.73, 0.76, 0.83, 1.04 and 1.55 (s−1), and 3.3,
3.7, 4.16, 6.6, and 11.32 (s−1) at pH 5.5, 6.0, 6.5, 7.0, and 7.4,
respectively. In the studied pH range (pH 5.5 to 7.4), the percentage of diaqua
species is reduced with the increase in pH, and the percentage of the dimer is
predominant. The dimer with its two metal centers is a better center to the
incoming nucleophiles. On the other hand, the pK1 and pK2 values
of the ligand DL-methionine are 2.24 and 9.07 at 25°C. With the increase
in pH from 5.0 to 7.4, the amount of the deprotonated form increases, and the
zwitterionic form (LH) predominates which also partly accounts for the enhancement of the rate
with increase in pH.


## 4.3. Effect of Temperature on the Reaction Rate

Four different temperatures with
varied ligand concentrations were chosen, and the results are listed in Tables1 and 2. The activation parameters for the steps (1)→B and (B)→(2), evaluated
from the linear Eyring plots and compared with the analogous systems [1], support the proposition.


## 5. Mechanism and Conclusion

The lowΔH≠ value, together with negative ΔS≠ value, suggests ligand participation in the transition state,
and an associative interchange mechanism is proposed (Scheme 1)  for the interaction of
DL-methionine with the title complex. The bonding mode of methionine is not
fully understood, as it was not possible to isolate the solid product. In the
studied reaction condition, that is, at pH 7.4, methionine exists in the
deprotonated form. At first S attacks on one of the two ruthenium(II), centers are assumed. This step is
ligand dependent, and with increasing the ligand concentration, a limiting rate
is reached. This may be due to the formation of outersphere association complex,
which is possibly stabilized through hydrogen bonding. The spontaneous
formation of an outersphere association complex is also supported from a
negative ΔG° value calculated from the
temperature dependence of the KE values. The corresponding
thermodynamic parameters are ΔH1°=16.6±2.3kJmol−1 and ΔS1°=95±7JK−1mol−1,ΔH2°=29.4±3.2kJmol−1 and ΔS2°=128±10JK−1mol−1.Scheme 1The coordinated
methionine in any of the ruthenium(II) centers now attacks the second
ruthenium(II) center like a metalloligand, and we observe two distinct ligand
dependent steps. For the ligand to behave as a bridging ligand with the oxo-bridging
complex, the mono atom sulphur [26, 27] bridging has the best prospects. It is to
be noted here that the second step is not a normal cyclisation step as occurs
in chelation in a single central atom. Here, two metal centers are available,
and after attachment of the ligand to one of the metal centers, the environment
of the two centers will no longer remain the same, and when the difference in
rate between two steps is larger, we observe the dependence of rate on ligand
concentration carried to the second step. But when the difference between two
steps is comparatively smaller as is found in a system earlier [2], the second
step is found to be independent on ligand concentration. A plausible mechanism
is shown here to commensurate with the experimental findings.



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*Source: 314672-2009-04-16.xml*