Oxidation of cysteine and glutathione by soluble polymeric MnO2

Article abstract of DOI:10.1021/es0340634

The kinetics of reduction of soluble polymeric Mn02 by cysteine and glutathione has been studied in the pH range of 4.0-9.0. The concentration of thiols was varied between 1 and 2 mM, while the Mn02 concentration was varied between 2 and 12 μM. In this pH range, the reaction products were identified as MN(II) and the corresponding disulfides (cystine and glutathione disulfide). Cysteic or cysteinesulfonic acid was formed only when pH < 2. Experimental data indicate that the rate law over the pH range of 4-9 is first-order in both Mn02 and thiol concentration. Eyring plots for both thiols reacting with Mn02 indicate that the reaction is associative (ΔS^?～ -160 J mol^-1 K^-1) and proceeds via an inner-sphere redox process. The reaction proceeds via the formation of two different inner-sphere complexes ≡Mn^IVSR^- and ≡Mn^IVSR and their further reaction to products. Both surface species are linked to each other via acid-base equilibria, and the rate constant decreases as pH increases, The presence of two ligand surface species is determined using surface complexation modeling, A reaction mechanism in agreement with the experimental results is proposed.

Full text of DOI:10.1021/es0340634

Environ. Sci. Technol. 2003, 37, 3332-3338
sources of cysteine, an essential amino acid for protein
synthesis, are microbial protein degradation (14) and as-
similatory sulfate reduction (15).
In aquatic environments, hydrogen sulfide can be readily
oxidized by manganese and/ or iron oxides, either biotically
or abiotically, leading to the formation of several oxidized
sulfur species such as sulfite, thiosulfate and polysulfides
Oxidation of Cysteine and
Glutathione by Soluble Polymeric
MnO2
J U L I AÄ N H E R S Z A G E A N D
M A R IÄ A D O S S A N T O S A F O N S O *
INQUIMAE and Departamento de Qu ´ı mica Inorg a´ nica,
Anal ´ı tica y Qu ´ı mica F ´ı sica, Facultad de Ciencias Exactas y
Naturales, Universidad de Buenos Aires, Ciudad Universitaria
Pabell o´ n II, (C1428EHA) Buenos Aires, Argentina
(
16-21). Although the reductive dissolution of different iron
oxides with several thiols has been studied in water (22-25),
this is not the case for the manganese oxides.
To the best of our knowledge, the only report for this kind
of reaction in the aqueous phase is included in the study of
the reactivity of manganese(III/ IV) oxides toward many
different organic molecules done by Stone and Morgan (26).
The authors used thiosalicylic acid as an example of a thiol
and only reported an apparent second-order rate constant.
There are a few other reports in the literature for the
reductive dissolution of manganese oxides with thiols in
organic solvents (27-29) and with metallic oxides other than
G E O R G E W . L U T H E R , I I I
College of Marine Studies, University of Delaware,
Lewes, Delaware 19956
manganese and iron oxides, such as PbO
2 3 2 3
, CrO , and Co O
The kinetics of reduction of soluble polymeric MnO2 by
cysteine and glutathione has been studied in the pH range
of 4.0-9.0. The concentration of thiols was varied
between 1 and 2 mM, while the MnO2 concentration was
varied between 2 and 12 µM. In this pH range, the
reaction products were identified as Mn(II) and the
corresponding disulfides (cystine and glutathione disulfide).
Cysteic or cysteinesulfonic acid was formed only when
pH < 2. Experimental data indicate that the rate law over
the pH range of 4-9 is first-order in both MnO2 and
thiol concentration. Eyring plots for both thiols reacting
(
30). The study of the reductive dissolution of manganese
oxides with thiols in water could contribute to a deeper and
more complete knowledge of both the Mn and S bio-
geochemical cycles. Luther and Church (31) proposed that
thiols could be oxidized to disulfides and even to sulfonic
acids by reaction with manganese and or iron oxides.
Sulfonates have been confirmed in sediments in an XANES
study (32). According to the results reported for the reaction
of thiols with iron oxides in water, the only oxidation products
are the corresponding disulfides (24, 25). The same products
were reported in the oxidation with manganese oxides in
organic solvents (27-29). In this work the study of the
reduction of soluble polymeric manganese(IV) dioxide by
cysteine and glutathione has been undertaken to find a rate
law, identify the reaction products, and elucidate the
elementary steps involved in this process over the pH range
of 4.0-9.0.
q
with MnO2 indicate that the reaction is associative (∆S ∼
-
1
-1
-
160 J mol K ) and proceeds via an inner-sphere
redox process. The reaction proceeds via the formation
of two different inner-sphere complexes tMn SR and
tMn SR and their further reaction to products. Both surface
IV
-
IV
species are linked to each other via acid-base equilibria,
and the rate constant decreases as pH increases. The
presence of two ligand surface species is determined using
surface complexation modeling. A reaction mechanism
in agreement with the experimental results is proposed.
Experimental Section
The soluble manganese dioxide used was a stable colloidal
suspension, which we term polymeric. It was prepared
following the technique described by Perez-Benito et al. (33)
by mixing the appropriate amounts of KMnO
4
(Aldrich) and
Na S O (Aldrich) stock solutions, according to the following
2 2 3
stoichiometry:
Introduction
3S2O3 - + 8MnO4 + 2H f 8MnO2 + 6SO4 + H2O
2
-
+
2-
Thiols are widespread in the environment. Many have been
identified and quantified in a variety of samples from suboxic
and anoxic waters and sediments, using electrochemical
techniques (1-4) and/ or chromatographic techniques (3,
Although this soluble phase is unlikely to exist in marine
environments for extended periods, it is likely present in
freshwater systems (34, 35). The polymeric manganese
dioxide was electrochemically inactive (35) but showed a
large absorption band covering the whole visible region of
the spectrum with absorbance uniformly increasing with
decreasing wavelength that gave a broad maximum at 300-
400 nm (Figure 1). Dilutions of this polymeric oxide followed
the Lambert-Beer law in the concentration range used (ꢀ400nm
5
-9). Particularly in sediments, thiols are found in the nano-
to micromolar concentration range (2, 3, 5). Thiols are
suspected to affect the bioavailability of essential trace metals
(
e.g., Cu and Zn) in sulfidic environments where they are
strongly bound as solid sulfide minerals (3). Among the
nonvolatile thiols reported in the literature, cysteine and
glutathione are two of the most frequently found. Glutathione
is a tripeptide formed by the amino acids glycine, cysteine,
and glutamic acid and is thought to be the most abundant
nonprotein thiol in animals, plants, and several bacteria (10,
4
-1
-1
) 1.77 × 10 M cm ). The UV-Vis spectra were performed
using a Hewlett-Packard 8453 diode array spectrophotometer.
The stoichiometry of the oxide was determined to be MnO
2
1
1). Glutathione is a redox cofactor for proteins involving
(average of three replicates) by iodometric techniques and
flame atomic absorption spectroscopy using a Varian AAS5
instrument for total Mn measurement. The point of zero
charge (pzc) of the oxide was determined to be 1.93 by
potentiometric titrations using a 716 Metrohm Titrino
equipped with a Metrohm pH glass combination electrode.
disulfide bonds (12, 13), works as a cysteine repository, and
is possibly involved in the S metabolism (12). The main
0
*
Corresponding author fax: +5411-4576-3341; e-mail: dosantos@
qi.fcen.uba.ar.
3
3 3 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 15, 2003
10.1021/es0340634 CCC: $25.00

2003 Am erican Chem ical Society
Published on Web 06/26/2003

2
FIGURE 1. UV-Vis spectra of soluble polymeric MnO .
FIGURE 2. Initial rate (R
0 2
) of the reduction of MnO by thiols for
This polymeric phase is easy to syntethize, is stable for
several months, is electrochemically inactive, and has been
previously characterized (33). Some other properties of the
polymeric oxide were previously reported (33, 36, 37):
Negative particles roughly spherical, 90% of the particles had
a diameter in the range of 89-193 nm, most of them had a
diameter of 120 nm, and ú-potential was -47( 3 mV (33).
Thiol stock solutions were prepared fresh daily from the
corresponding solids, cysteine (CSH) (Aldrich) and glutahione
different [MnO ] concentrations: cysteine (2) and glutathione (9),
2
pH 4.1; T ) 25 °C; I ) 0.02 M, [thiol] ) 1 mM.
a Shimadzu SPD-6AV UV-Vis detector. The samples were
injected into a Rheodyne injector 77251 with a 50-µL loop
and a J&W Scientific ACCU ODS 5-µm analytical chromato-
graphic column. The retention time for GHS and GSSG under
these operational conditions was 4.72 and 21.49 min,
respectively. The analytical precision was usually within 1%.
In experiments performed below pH 2, cysteine was totally
oxidized to an electrochemically inactive compound identi-
fied as cysteic (cysteinesulfonic) acid. The amount of cysteic
acid formed was evaluated by ion chromatography using a
DIONEX DX-100 instrument with a conductivity detector, a
sample injection valve, and a 25-µL sample loop. Two plastic
anion columns were coupled in series to serve both as
precolumn (DIONEX AG-9) and analytical chromatographic
column (DIONEX AS-9). The suppressor was regenerated
(
(
GSH) (Merck), using degassed distilled deionized water
DDDW) and were standardized using Ellmann’s reagent (38).
Oxidation-reduction experiments were performed under a
nitrogen atmosphere. The temperature was held constant to
(
0.1 °C with a constant temperature circulation bath. The
pH was held constant between 4.0 and 9.0 using the
appropriate buffer solutions (acetate, borate, and Tris). In
all the experiments, the ionic strength was 0.02 M using
4
NaClO .
In one set of experiments designed to evaluate the
influence of thiol concentration on the rate law, thiol
with 50 mN H
2
4
SO with a flow rate of 12.5 mL/ min. A mixture
4 mM was chosen as eluent with a flow rate
-
-2
of HCO
3
/ CO
3
concentration was varied between 1 and 2 mM, while MnO
concentration was held constant. In another set of experi-
ments, the MnO concentration was varied between 2 and
2 µM, while thiol concentration was held constant.
Under the experimental conditions used, the reaction was
2
of 1 mL/ min. The retention time for cysteic acid under these
operational conditions was 3.92 min.
Standard cystine and cysteic acid solutions were prepared
by dissolving the corresponding amount of solid (used as
received from Sigma) in DDW. Glutathione disulfide stan-
dards were prepared according to Luther et al. (1) by oxidizing
2
1
completed in less than 1 min. The course of the reaction was
followed using a stopped-flow system coupled to a HP 8451
UV-Vis diode array spectrophotometer set to follow the decay
2
a standardized solution of glutathione with I . Mn(II) standard
solutions were prepared by diluting a 1000 ppm Mn(II) AAS
standard (Fluka) in acidified distilled deionized water.
Errors for the k values were obtained from the fitting tool
included in the HP8453 (diode array spectrophotometer)
software package. Errors for the fitting parameters were
obtained using the nonlinear curve fit tool in Microcal Origin
software using the Levenberg-Marquardt fitting routine.
2
in absorbance at 400 nm due to MnO consumption. The
absorbance versus time data were fitted using the data
analysis package included in the spectrophotometer software.
The resulting rate constants for different experimental
conditions are the average of at least three different runs.
Square-wave voltammetry (SWV) was used in separate
experiments to evaluate the formation of Mn(II), Mn(III) (35,
3
9), and cystine (cysteine disulfide) (1, 35) using an Analytical
Results and Discussion
Experiments performed in the absence of thiols showed no
Instrument Systems, Inc. DLK 100 potentiostat with a EG&G
PAR 303 static dropping mercury electrode stand (DME).
The electrode stand was modified to use a saturated calomel
electrode (SCE) rather than the Ag/ AgCl reference supplied.
The SCE electrode was connected to the solution through a
NaCl salt bridge. Instrumental parameters for the SWV mode
were typically 200 mV s scan rate over the potential range
of -0.1 to 1.5 V with a 24-mV pulse height, which gave a
detection limit lower than 1 µM. Because pyrophosphate
forms stable complexes with Mn(III) at near neutral or slightly
alkaline pH (39), separate electrochemical and spectropho-
tometric experiments were done in the presence of pyro-
phosphate (pH 8) in order to detect the formation of Mn(III).
Glutathione disulfide (GSSG) was identified and quantified
according to the HPLC technique described by Winterbourn
and Brennan (40) using a Shimadzu LC-6A instrument with
change in the MnO
of the kinetic experiments, indicating that there is no reaction
of the MnO with the buffers used to keep pH constant. Similar
2
UV-Vis spectra during the time scale
2
results were obtained when cysteine was replaced by glycine,
indicating that only the sulfhydryl group of cysteine is
oxidized.
At a given pH value and thiol concentration, the initial
rate is proportional to the amount of MnO
-
1
2
added (Figure
2
). At constant amount of manganese oxide added and pH,
a linear correlation between the initial rate and the thiol
concentration is observed (Figure 3). The experimental rate
law is expressed as follows:
R ) k[thiol][MnO ]
(I)
o
2 T
VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 3 3 3

FIGURE 3. Initial rate constant (k′) of reduction of MnO
for different [thiol] concentrations: cysteine (2) and glutathione
9), pH 4.4; T ) 25 °C; I ) 0.02 M, [MnO ] ) 20 µM.
2
by thiols
FIGURE 5. Variation of the rate constant with temperature. (A)
cysteine, pH 7, I ) 0.02 M; (B) glutathione, pH 5.4, I ) 0.02 M.
(
2
q
q
a
TABLE 1. E
a
, ∆H , and ∆S for Cysteine and Glutathione
∆Hq
∆S
q
Ea
pH
(kJ mol-1)
(kJ mol-1)
(J mol K )
-1
-1
cysteine
glutathione
7.0
5.4
17.4 ( 0.1
19.0 ( 0.1
15.0 ( 1.0
16.6 ( 1.0
-156 ( 15
-160 ( 15
a
In both cases I ) 0.02 M.
5
). The overall activation energy value obtained for cysteine
-
1
was (17.4 ( 0.1) kJ mol at pH 7 and I ) 0.02 M. For
gluthatione, the overall activation energy value obtained was
-
1
(
19.0 ( 0.1) kJ mol at pH 5.4 and I ) 0.02 M. The overall
activation energy for the rate constant k will be a combination
of the activation energies for elementary reactions.
Preexponential factors for bimolecular reactions com-
7
12
-1 -1
monly exhibit A values between about 10 and 10
M
s .
The dissolution rate of polymeric MnO has a preexponential
2
FIGURE 4. Rate constant for the reduction of MnO
different pH values for cysteine (2) and glutathione (9); T ) 25 °C;
I ) 0.02 M; [thiol] ) 1 mM; [MnO ] ) 20 µM.
2
by thiols at
5
4
-1 -1
factor of 1.1 × 10 and 7.5 × 10 M
s
for cysteine and
glutathione, respectively.
2
The preexponential factor values lower than normal are
showing the influence of steric effects due to the molecule
size and the presence of substituents. The different values
of A between both thiols would depend on the steric effects.
Consequently, this is due to the tripeptide structure of
glutathione where the large size of the glutathione molecule
may contribute to some steric effects that increase the
activation energy while decreasing the preexponential factor
and the dissolution rate.
The fact that the reaction rate for glutathione is lower than
that for cysteine can be attributed to steric hindrance, since
glutathione is a tripeptide formed by the amino acids
glutamate, cysteine, and glycine. On the time scale used in
2
the experiments, only the SH group is oxidized by MnO .
Figure 4 gives the dependence of the rate constant with
pH at 25 °C and demonstrates that the rate increases with
decreasing pH. These data indicate that the protonated forms
of the thiols react more quickly with the anionic polymeric
To determine whether the reaction mechanism is as-
sociative or dissociative, the entropy of activation is needed
2
MnO , which has a pzc of 1.93. Between pH 4.0 and pH 9.0,
(
42), which can be evaluated from an Eyring plot [ln(k/ T)
the oxidation products were identified as the corresponding
disulfides: cystine (CSSC) and glutathione disulfide (GSSG).
Similar products were obtained in the oxidation of
versus 1/ T ] based on
q
q
different thiols with MnO
2
in organic solvents (27-29) and
ln(k/ T) ) (∆H / RT ) + ln(k/ h) + (∆S / R)
(II)
in the oxidation of thiols with iron oxides in water (22-25).
At pH values below 2, cysteine was completely oxidized to
an electrochemically inactive product that was identified as
cysteic (cysteinesulfonic) acid using ionic chromatography.
The method used to quantify GSSG allows for the simulta-
q
q
where ∆H is the enthalpy of activation, ∆S is the entropy
-
23
of activation, k is the Boltzmann constant (1.381 × 10
J
-
1
-34
K ), and h is the Planck’s constant (6.626 × 10
J s). The
q
slope (∆H / R) provides the enthalpy of activation, and the
q
neous determination of glutathionesulfonic acid (GSO
3
H) as
intercept {ln(k/ h) + (∆S / R)} yields the entropy of activation.
A large negative ∆S indicates an associative reaction whereas
a large positive ∆S indicates a dissociative reaction (41). All
kinetic activation parameters are given in Table 1, which
q
well (40). No signal characteristic of GSO H was observed in
3
q
experiments done at pH 4 and above. Considering the results
for cysteine, a lower pH is also apparently needed to oxidize
q
GSH to GSO
3
H.
shows that both thiols have a large negative ∆S for MnO
2
The dependence of k with temperature was in agreement
reduction and that the reactions are associative and proceed
via inner-sphere redox reactions.
The values of activation energy, enthalpy, and entropy of
activation for a given reaction are usually indicative of some
-
E / RT
a
with the Arrhenius equation (k ) Ae
preexponential factor, R is the gas constant, T is the absolute
temperature, and E is the overall activation energy (Figure
) where A is the
a
3
3 3 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 15, 2003

effects such as van der Waals, dipole-dipole, H-bonding,
ionic interactions between charged species, and also specific
bonding of reactive moieties. The processes that can be
explained via van der Waals, dipole-dipole or other weak
intermolecular forces are occurring by an outer-sphere
mechanisms and are characterized by a very low activation
energy, while specific bonding of reactive moieties occur by
inner-sphere mechanisms such as the one already mentioned
here and require more higher activation energies.
Dissolution can proceed via several parallel pathways that
involve labilization of bridging oxygens by ligands that are
dynamically stable in the inner-coordination sphere of the
detaching manganese. If any of the dissolution reactions were
completely transport (diffusion) controlled, there should be
no pH dependence of the oxidation rate. Despite this, if Mn(II)
released into solution is the rate-determing step, there should
be no difference for the dissolution rate between both ligands.
Then, the rate is controlled either by the binding of the ligand
to the surface or the detachment of the activated surface
complex from the surface. Thus, the dissolution rate is not
controlled by transport of reduced species away from the
surface (Mn(II) release into solution is not the rate-determing
step), and the reactions at the surface (which are responsible
for the activation energy) must be rate-controlling (42).
Therefore, for both ligands, the dissolution would be surface-
controlled.
2
MnO , and (III) and the competition between thiols (with
carboxylate and amino functional groups) and pyrophos-
phate for any Mn(III) formed. Nevertheless, there is clear
evidence in the literature that Mn(III) is an intermediate in
2
the reductive dissolution of solid-phase MnO . Nico and
Zasoski (50, 51) observed an inhibition of the reductive
dissolution of birnessite with Cr(III), sulfide, and hydro-
quinone in experiments made in the presence of pyrophos-
phate. On the basis of their observations, they proposed a
model where it is assumed that any Mn(III) formed on the
oxide surface is complexed by pyrophosphate and this
complexation inhibits the reaction. Also, Mn(III) has been
detected by XPS as an intermediate in the reductive dis-
solution of birnessite with several reductants such as arsenite
(52), Cr(III) (53), oxalate (54), selenite (55), and humic acids
(56). Their results show that as the reaction proceeds Mn(III)
is formed on the oxide surface. In the cases where the
reductant does not stabilize Mn(III) through complexation
[e.g., As(III), Cr(III), Se(IV)], Mn(III) behaves as an interme-
diate because the profile of Mn(III) concentration versus time
has a maximum, i.e., it is formed and consumed in another
reaction step (52, 53, 55).
•
The formation of RS radicals was proposed by Wallace
(28) in his study of thiol oxidation with solid-phase MnO in
2
organic solvents because, in the presence of olefins, sulfur
addition products (addition to the double bonds) were
•
It has been found that comparatively simple rate laws are
obtained if the observed rates are plotted against the
concentrations of the adsorbed species and surface com-
plexes (43, 44). Then, a possible reaction mechanism,
considering Mn(II) and the disulfides as the main products,
should go through the following steps:
formed. These results are indicative of RS radical formation.
These radicals were also detected by ESR in solutions resulting
from the oxidation of CSH and GSH with peroxynitrite or OH
radicals (57). Because of the relative stability of sulfur radicals
•
(58), RS radicals were proposed as intermediates in reaction
mechanisms to explain the oxidation of mercaptocarboxylic
acids with magnetite (24), the oxidation of thioglycolic acid
and glutathione with Mn(III) complexes, and the oxidation
(
i) Formation of a precursor complex on the soluble
IV
polymeric MnO
2
(similar to an oxide surface, tMn SR):
4
+
of thiols with complexes of iron and another metals Ce
,
k1
3+
5+
2+
IV
IV
Co , V , and Cu (30).
tMn OH + RSH { } tMn SR + H O
(1)
2
k-1
•
(
iii) RS radicals dimerize to form the corresponding
disulfide:
Mn(IV) has a d (t2g3) electronic configuration in octahedral
symmetry. Ions with this electronic configuration are inert
and have a slow rate of ligand exchange (45). Therefore, this
precursor complex is an inner-sphere complex (45), which
3
k7
2
RS•
98 RSSR
(7)
q
is consistent with the large negative ∆S (Table 1). The
Sulfhydryl radicals might dimerize while the sulfur atoms
are still attached to the polymeric MnO . In that case, the
distance between both radicals, which would be equivalent
to the Mn-Mn distance on an oxide surface, should be similar
to the bond length of the corresponding disulfide.
The length for a S-S bond in most molecules having two
or more sulfur atoms linked to each other is aproximately 2
Å (59). However, the distances between vicinal Mn atoms in
most manganese(IV) oxides is slightly larger than 4 Å (60);
therefore, dimerization of radicals while attached to poly-
formation of such complexes as the first step has been
proposed in many reductive dissolution reaction schemes,
including the reaction of solid manganese(IV) oxides with
sulfide (21, 46) and with organic reductants (26, 47-49).
2
(
ii) Electron transfer from the thiol to the Mn (in two
steps) followed by release to the solution of sulfhydryl radicals
•
(
RS ):
k2
IV
III
tMn SR
98 tMn SR
(2)
(3)
2 2
meric MnO or a MnO surface is very unlikely to occur.
k3
III
III
•
+
Another possible dimerization mechanism (24) that cannot
be ruled out as possible is that thiols coordinate to the surface
through other complexing groups (e.g., carboxylate in
cysteine). This second group could work as an anchor
allowing the S-S bond to be formed in solution while the
thiol is still attached to a MnO surface or polymeric MnO
2 2
through the second coordinating group. This would also
permit their further oxidation to sulfonates at low pH by
tMn SR + H O
98 tMn OH + RS + H
2
k4
III
III
tMn OH + RSH
98 tMn SR + H O
(4)
2
k5
III
II
tMn SR
98 tMn SR
(5)
(6)
k6
II
II
+
•
tMn SR + H O
98 tMn OH2 + RS
allowing contact with soluble polymeric MnO
in the E for the oxidation of gluthathione over that for cysteine
2
. The increase
2
a
We postulate two separate electron transfers because the
suggests that carboxylate ion complexation with MnO
most likely.
(iv) Desorption of Mn(II) and generation of a new surface
site:
2
is
simultaneous transfer of two electrons is very unlikely to
happen. It should be noted that no Mn(III) was detected
either by SWV and/ or UV-Vis spectroscopy even in experi-
ments done in the presence of pyrophosphate that stabilizes
Mn(III) (39). This appears due to (I) the fast reaction rate
k8
II
tMn OH2+
IV
2+
+
98 tMn OH + Mn (aq) + H
(8)
2
with soluble polymeric MnO , (II) the large excess of thiol to
VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 3 3 5

2
mize the value of ø , which was taken as a convergence
TABLE 2. Fitting Parameters Used in Figure 4 Corresponding
to Eq 14 (See Text) for Cysteine and Glutathione
criterium. Different initial scenarios with different values for
the fitting parameters were used to verify the uniqueness of
convergence.
parameters
The protonation-deprotonation process for the reaction
with cysteine would take place on the nitrogen atom of the
amino group as previously proposed by Amirbahman et al.
(25) in their study of the reductive dissolution of hydrous
ferric oxide with cysteine. The authors suggested the forma-
tion of surface complexes where the metal is complexed either
by the thiol through the sulfur, or the nitrogen, or the oxygen
of the carboxylate groups.
These surface complexes are the ones involved in the
acid-base equilibrium already mentioned. Assuming that
the residual charge on the metal center is +1, some structures
for complexes of overall charge 0 and -1 can be postulated:
thiol
k2(MnSRH)
k2(MnSR-)
K °
app
-
4
-4
-4
cysteine
glutathione
2060 ( 20 200 ( 2 4.4 × 10 ( 0.5 × 10
-4
222 ( 17
12 ( 1 1.2 × 10 ( 0.2 × 10
Assuming that all the intermediates are in steady state, the
initial reduction rate becomes first-order with respect to both
reactants (the surface species {tMn OH} and the free thiol
concentrations [RSH]) (eq 9). Then the stoichiometric rate
law has the same shape as the experimental rate law (eq I):
IV
k1k2
IV
R )
{tMn OH}[RSH]
(9)
k-1 + k2
In the pH range studied, the rate constant for reduction
of polymeric MnO was observed to decrease with increasing
2
pH (Figure 4). These results can be explained considering
that the species, formed in the first reaction step, are involved
in acid-base equilibria characterized by the corresponding
K° value:
app
IV
IV
-
+
tMn SRH / tMn SR + H
(10)
and
IV
-
+
{
tMn SR }[H ]
In the neutral complex, the amino group is protonated; in
the negatively charged complex, it is deprotonated. There
are two possible structures for the latter complex if S binds
to the Mn, one with the nitrogen coordinated to the Mn
center and the other one with the oxygen coordinated to the
metal center.
K°
)
(11)
app
IV
{
tMn SRH}
Both species can undergo the reaction steps already men-
tioned (eqs 1-8), and their different reactivities cause the
reaction rate to change with pH (49). Amirbahman et al. (25)
successfully used a similar model in their study of the
reductive dissolution of hydrous ferric oxide with cysteine.
Therefore, the rate of step 2 would be
Table 2 and Figure 4 demonstrate that the protonated
species are the most reactive species for both thiols. Similar
results were obtained for the reaction between solid MnO
2
and sulfide (21). Also, the pK° , corresponding to the
app
IV
IV
-
protons on the amino groups, are dramatically shifted to
more acidic values. Coordination of cysteine with Fe(III) on
an hydrous ferric oxide surface causes the amino group
proton to be 3 orders of magnitude more acidic than usual,
given that Fe(III) acts as a Lewis acid (25).
In our study, the shift in pK°app is even higher, which can
be attributed to the fact that Mn(IV) is more acidic than
Fe(III). In the reaction with glutathione, the group involved
in that acid-base equilibria would be the amino group
corresponding to the glutamic acid. In both cases, the
R ) k2(MnSRH){tMn SRH} + k
2(MnSR-)
{tMn SR } (12)
A species distribution varying with pH is given by the values
of the distribution coefficients (R ) and total concentration
), which are a function of K°app and pH:
i
(C
T
IV
{
tMn SRH} ) C R
T
MnSRH
IV
-
{
tMn SR } ) C R
MnSR-
T
pK° values followed the same behavior than the free thiols
pK . The surface complexes involved in the acid-base
a
app
IV
IV
-
C ) {tMn SRH} + {tMn SR }
(13)
T
equilibria are more labil when increase the strength of the
free thiol. The tendency to form chemical linkages to solid
surface atoms correlates with likelihood of forming com-
parable complexes in solution (61). Likewise, the exchange
of protons by a dissociative or associative processes would
follow a similar tendency.
Therefore, the rate for step 2 can be written as a linear
combination of both distribution factors:
R ) C [k2(MnSRH)RMnSRH + k
2(MnSR-)
-
RMnRS ]
(14)
T
The above equation accounts for the pH dependence of the
overall rate constant k. Figure 4 shows the variation in k for
cysteine and glutathione with pH. The symbols are experi-
mental points, and the lines were plotted as the best fit
2
Unlike the reaction with H S, the speciation of the
oxidation products does not change significantly in the pH
range typical of natural waters as the corresponding disulfides
are the only products detected in this pH range. Sulfonic
acids are formed at much more acidic conditions (pH < 2)
such as those found in salt marshes and other oxidizing
T 2
assuming that C is equal to the total available MnO surface
sites and using three adjustable parameters in eq 14. These
parameters were K° , k2(MnSRH) and k2(MnSR-) accounting for
2
sediments (2); therefore, the reduction of MnO by thiols
app
the differences in reactivity of both surface complexes. Table
may not be a significant source of sulfonates in some natural
systems. Given their widespread occurrence (32, 62), other
mechanisms must also be operational for sulfonate formation
such as sulfite and/ or thiosulfate addition to R,â-unsaturated
2
shows the results for these three parameters for both thiols.
As shown in Figure 4, the proposed model is a good fit to the
experimental data. The fit was done using the non-linear
curve fit tool included in Origin 5.0 software (OriginLab
software). Levenberg-Marquardt routine was used to mini-
2
organic compounds (31, 32). For soluble polymeric MnO ,
3
-1
the rate constants at pH 10 follow the order ksulfide ) 10 M
3
3 3 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 15, 2003

s- (21), kcysteine = 200 M-1 s-1, and kglutathione = 20 M-1 s-1
1
(12) Maw, G. A. In Sulfur in Organic and Inorganic Chemistry;
Senning, A., Ed.; Marcel Dekker: New York, 1972; pp 113-142.
(kcysteine and kglutathione were obtained by extrapolation of the
plots in Figure 4).
(
(
13) Meister, A.; Anderson, M. Annu. Rev. Biochem. 1983, 52, 711.
14) Zinder, S. H.; Brock, T. D. Appl. Environ. Microbiol. 1978, 35,
This rate constants are proportional to the thiol dissocia-
3
44.
tion constants (RSH pK
RdH, CS, and GS respectivily), the pkthiol versus pK
a slope of 0.94 and R ) 0.89. Then the reactivity decreases
in the order HS > CSH > GSH, reflecting the electronic and
steric effects of the thiol substituents on the reaction
considered.
a
values of 7.0, 8.18, and 8.66 with
plot has
(
15) Trudinger, P. A.; Loughlin, R. E. In Comprehensive Biochemistry;
Neuberger, A., et al., Eds.; Elsevier: New York, 1981; pp 165-
257.
a
2
-
(16) Burdige, D. J.; Nealson, K. H. Geomicrobiol. J. 1986, 4, 361.
(
(
17) Yao, W.; Millero, F. J. Mar. Chem. 1996, 52, 1.
18) Peiffer, S.; dos Santos Afonso, M.; Wehrli, B.; G a¨ chter, R. Environ.
Sci. Technol. 1992, 26, 2408.
Although the oxidation of inorganic sulfide with man-
ganese oxides has been studied extensively (16, 21, 46, 63),
oxidation of thiols of environmental importance has not
received much attention. This lack of information hampers
our knowledge of the reaction pathways for several species
(19) dos Santos Afonso, M.; Stumm, W. Langmuir 1992, 8, 1671.
(20) Herszage, J.; dos Santos Afonso, M. Colloids Surf. A 2000, 168,
6
1.
(
(
21) Herszage, J.; dos Santos Afonso, M. Submitted for publication
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in the sulfur biogeochemical cycle (31). Polymeric MnO
2
may
not represent the most typical MnO phases present in natural
2
(23) Blesa, M. A.; Maroto, A. J. G.; Morando, P. J. J. Chem. Soc., Faraday
Trans. 1 1986, 82, 2345.
systems, but the results obtained with this polymeric phase
are relevant and would contribute to understand the chemical
behavior of the anoxic basins.
Further work is needed with other solid manganese oxide
phases typical of natural systems to further assess the
contribution of the studied process to the biogeochemical
cycles of Mn and S. As expected, several results have similarity
to those obtained in a previous study on the reaction with
(
(
(
24) Borghi, E. B.; Morando, P. J.; Blesa, M. A. Langmuir 1991, 7,
652.
25) Amirbahman, A.; Sigg, L.; von Gunten, U. J. Colloid Interface
Sci. 1997, 194, 194.
1
26) Stone, A. T.; Morgan, J. J. Environ. Sci. Technol. 1984, 18, 617.
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28) Wallace, T. J. J. Org. Chem. 1966, 31, 1217.
29) Parida, K. M.; Samal, A.; Das, N. N. J. Colloid Interface Sci. 1998,
2
H S (21). Redox rate constants increase as pH decreases, and
1
97, 236.
this fact can be explained by considering a model where
reactant species, linked to each other via acid base equilibria,
are formed. The proposed mechanism involves complexation
(
30) Capozzi, G.; Modena, G. In The Chemistry of the Thiol Group;
Patai, S., Ed.; 1974; Vol. 2, Chapter 17, pp 785-839.
(31) Luther, G. W., III; Church, T. M. In Sulphur Cycling on the
Continents; Howarth, R. W., Stewart, J. B., Ivanov, M. V., Eds.;
2
to polymeric MnO (or reductant adsorbing on an oxide
1
1
992 SCOPE; John Wiley & Sons Ltd.: New York, 1992; pp 125-
42.
surface for solids) as a first step because the reaction is an
associative process, followed by several electron transfers
and the release of products to the solution. The use of soluble
(
(
32) Vairavamurthy, A.; Zhou, W.; Eglinton, T.; Manowitz, B. Geochim.
Cosmochim. Acta 1994, 58, 4681.
33) P e´ rez-Benito, J. F.; Arias, C.; Amat, E. J. Colloid Intterface Sci.
1996, 177, 288.
polymeric MnO
2
provides the capability of writing a rate law
q
in both reactants and calculating activation parameters (∆S ,
q
q
(34) Hem, J. D. In Particulates in Water: Characterization, Fate, Effects
and Removal Interfacial and Interspecies Processes; Kavanaugh,
M. C., Leckie, J. O., Eds.; ACS Symposium Series 189; American
Chemical Society: Washington, DC, 1980; pp 45-72.
∆
H , ∆G ) to establish whether the reaction is dissociative or
associative, as was done with the reaction of MnO and nitrite
2
species (64), another associative and inner-sphere redox
reaction.
(
35) Luther, G. W., III; Ruppel, D. T.; Burkhard, C. In Mineral-Water
Interfacial Reactions: Kinetics and Mechanisms; Sparks, D. L.,
Grundl, T., Eds.; ACS Symposium Series 715; American Chemical
Society: Washington, DC, 1999; pp 265-280.
Acknowledgments
The authors acknowledge Universidad de Buenos Aires,
Secretaria de Ciencia y T e´ cnica through Project UBACyT
TW99, and the U.S. National Science Foundation (OCE-
(
36) P e´ rez-Benito, J. F.; Brillas, E.; Pouplana, R. Inorg. Chem. 1989,
2
8, 392.
(37) P e´ rez-Benito, J. F.; Arias, C J. Colloid Interface Sci. 1992, 152,
0
096365) for financial support of this work.
70.
(
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38) Benedict; Stedman Analyst 1970, 95, 296.
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40) Winterbourn, C. C.; Brennan, S. O. Biochem. J. 1997, 326, 87.
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