Formation of peptide disulfide bonds through a trans-dibromido-Pt(IV) complex oxidation reaction: Kinetic and mechanistic analyses

Article abstract of DOI:10.1016/j.molliq.2020.115195

A trans-dibromido-Pt(IV) complex [PtBr₂(en)₂]Br₂ was synthesized and evaluated for the synthesis of peptide disulfide bonds in this work. The reactions between the Pt(IV) complex and dicysteine-containing peptides were carried out in various aqueous solutions. As a result, excellent yields with fast reaction rates were achieved. Methionine residue was intact when the Pt(IV) complex reacted with a dicysteine and methionine-containing peptide. 3,6-Dioxa-1,8-octanedithiol (DODT) was selected as a model compound of the dicysteine-containing peptide. Kinetic studies for the reaction between the Pt(IV) complex and DODT were performed in different pH solutions. It is first-order both in [Pt(IV)] and in [DODT]. A reaction mechanism was proposed accordingly. The kinetic and mechanistic results demonstrated that the reaction rate for the formation of peptide disulfide bond via the Pt(IV) oxidation is increased with the increase of pH of the reaction medium. The protolytic constants of the two thiol groups in peptide also play a critical role in reaction rate. On the other hand, kinetic studies demonstrated that the Pt(IV) complex trans-[PtBr₂(en)₂]²⁺ can be used in an acidic medium for the purpose of synthesis of disulfide bonds.

Full text of DOI:10.1016/j.molliq.2020.115195

Journal of Molecular Liquids 326 (2021) 115195
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Formation of peptide disulfide bonds through a trans-dibromido-Pt(IV)
complex oxidation reaction: Kinetic and mechanistic analyses
⁎
⁎
Dongying Ma, Changying Song, Jingjing Sun, Shigang Shen , Shuying Huo
College of Chemistry and Environmental Science, Key Laboratory of Analytical Science and Technology of Hebei Province, MOE Key Laboratory of Medicinal Chemistry and Molecular Diagnostics,
Hebei University, Baoding 071002, Hebei Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A trans-dibromido-Pt(IV) complex [PtBr2(en)2]Br2 was synthesized and evaluated for the synthesis of peptide di-
sulfide bonds in this work. The reactions between the Pt(IV) complex and dicysteine-containing peptides were
carried out in various aqueous solutions. As a result, excellent yields with fast reaction rates were achieved. Me-
thionine residue was intact when the Pt(IV) complex reacted with a dicysteine and methionine-containing pep-
tide. 3,6-Dioxa-1,8-octanedithiol (DODT) was selected as a model compound of the dicysteine-containing
peptide. Kinetic studies for the reaction between the Pt(IV) complex and DODT were performed in different
pH solutions. It is first-order both in [Pt(IV)] and in [DODT]. A reaction mechanism was proposed accordingly.
The kinetic and mechanistic results demonstrated that the reaction rate for the formation of peptide disulfide
bond via the Pt(IV) oxidation is increased with the increase of pH of the reaction medium. The protolytic con-
stants of the two thiol groups in peptide also play a critical role in reaction rate. On the other hand, kinetic studies
demonstrated that the Pt(IV) complex trans-[PtBr2(en)2]2+ can be used in an acidic medium for the purpose of
synthesis of disulfide bonds.
Received 21 September 2020
Received in revised form 21 December 2020
Accepted 23 December 2020
Available online 29 December 2020
Keywords:
Platinum(IV) complex
Bromide ligand
Peptide
Disulfide bond
Kinetics and mechanism
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
through a homogenous oxidation reaction is generally adopted in in-
dustrial peptide production or in Lab peptide research [7–14].
Platinum(IV) complexes possess two axial bromide ligands (trans-
dibromido-Pt(IV)) have attracted much attention due to their promoted
antitumor activity [1–5]. The substitution of the axial ligands by a nucle-
ophilic reagent is generally difficult to occur because of the natural ki-
netics inertness of the Pt(IV) complex. However, a reduction reaction
between the Pt(IV) complex and a thiol-containing compound (cyste-
ine, glutathione, homocysteine) is prone to occur through a bromide-
bridged electron-transfer process. The thiol-containing compound was
oxidized to its corresponding disulfide form [6]. Moreover, trans-
dibromido-Pt(IV) was easily soluble in aqueous solution. Therefore,
this kind of Pt(IV) complexes maybe used as oxidants for synthesis of
peptide disulfide bond in aqueous solution through a homogenous oxi-
dation reaction. Although the homogenous oxidation method suffers
from the relatively low productivity, resulting from low peptide concen-
tration used in order to minimize intermolecular oligomerization. How-
ever, until now, synthesis of peptide disulfide bond in aqueous solution
Pt(IV) complexes such as trans-[PtCl2(CN)4]2−, trans-[PtCl2(en)2]2+
,
trans-[PtCl2(phen)(en)]2+, trans-[PtCl2(bpy)(en)]2+ and trans-[Pt(OH)
2(phen)(en)]2+ have been used as very efficient oxidants for synthesiz-
ing intramolecular disulfide bonds in peptides [15–20]. The redox po-
tential of Pt(IV) complex has no influence on the rate of formation of
peptide disulfide bond by Pt(IV) oxidation. But the thioether group of
methionine side chain in peptide can be oxidized to its sulfoxide form
by the Pt(IV) complex with higher redox potential. Generally, the rate
of formation of peptide disulfide bond by Pt(IV) oxidation was influ-
enced by the equatorial and axial ligands of Pt(IV) complex. Among
the five Pt(IV) complexes, trans-[PtCl2(CN)4]2− can oxidize methionine
residue to methionie sulfoxide derivate making it unsuitable used as an
oxidant for synthesis of disulfide bond in a methionine-containing
peptide [15,16]. The other four Pt(IV) complexes are compatible to
methionine residue. Slow reaction rates were achieved when trans-
[PtCl2(en)2]2+ and trans-[Pt(OH)2(phen)(en)]2+ were used to oxida-
tive synthesis of peptide disulfide in an acidic medium [17–19].
trans-[PtCl2(phen)(en)]2+ and trans-[PtCl2(bpy)(en)]2+ can rapidly
convert dithiol to disulfide bond in peptide in acidic medium [20]. In
⁎
Corresponding authors.
E-mail addresses: shensg@hbu.edu.cn (S. Shen), shuyinghuo@hbu.edu.cn (S. Huo).
https://doi.org/10.1016/j.molliq.2020.115195
0167-7322/© 2020 Elsevier B.V. All rights reserved.

D. Ma, C. Song, J. Sun et al.
Journal of Molecular Liquids 326 (2021) 115195
order to expand the toolbox for the synthesis of peptide disulfide, in this
work, trans-[PtBr2(en)2]2+ as an oxidant was used to synthesize disul-
fide bonds, and the kinetics and mechanism of formation of disulfide
bond was investigated in detail.
electrode (Fisher Scientific, Pittsburgh, PA) was used to measure the
pH values of buffer solutions.
2.3. Formation of disulfide bonds in peptides
Pt(IV) complex (1–2 equiv.) was reacted with dicysteine-containing
peptide in various solvents. The reaction mixtures were analyzed by
LC-6 CE HPLC system equipped with a UV–vis detector at 215 nm with
a 250 mm × 4.6 mm C8 column at a flow rate of 1.0 mL/min. Two solvent
systems consisting of 0.1% TFA in acetonitrile and 0.1% TFA in water
were used for peptide elution with a suitable gradient. It is known
that trans-[PtCl2(en)2]2+ can quantitatively convert the dicysteine-
containing peptide to its disulfide form [16]. Therefore, the disulfide
bridged peptides generated from the trans-[PtCl2(en)2]2+ oxidation
were used to characterize and calculate the yields of the disulfide-
containing peptides generated from the trans-[PtBr2(en)2]2+ oxidation.
2. Experimental section
2.1. Reagents
Dicysteine-containing peptides used in this work were purchased
from Nanjing Peptide Company (Nanjing, China) and used as obtained;
the purities of all peptides are 90%. The peptide content of iRGD
(CRGDKGPDC-NH2) is reported to be 85%. 3,6-Dioxa-1,8-octanedithiol
(DODT) and sodium perchlorate were obtained from Sigma-Aldrich
(St. Louis, MO). Standard buffers of pH 4.00, 7.00 and 10.00 were pur-
chased from Fisher Scientific (Fisher Scientific, Pittsburgh, PA). Acetic
acid, sodium acetate, sodium dihydrogenphosphate, disodium hydrog-
enphophate were, all in analytical grade, purchased from Tianjin Chem-
ical Reagent Company (Tianjin, China) and were used for preparation of
buffer solutions. Pt(IV) complexes [PtBr2(en)2]Br2 and [PtCl2(en)2]Cl2
were synthesized according to a method published in the literature
[21,22]. The UV–Vis spectrum of [PtCl2(en)2]Cl2 is in excellent agree-
ment with that reported earlier for trans-[PtCl2(en)2]2+. [Pt(en)2Br2]
Br2 was characterized by elemental analysis: Calc. for C4H16Br4N4Pt·
0.5H2O: C, 7.46%; H, 2.66%; N, 8.70%. Found: C, 7.23%; H, 2.64%; N, 8.86%.
2.4. Kinetic experiments
Kinetic traces were recorded on the Applied Photophysics SX-20
stopped-flow spectrometer. [PtBr2(en)2]Br2 was dissolved in a solution
containing 0.90 M NaClO4, 0.09 M NaBr, and 0.01 M HClO4 for preparing
a 1.0 mM stock solution. This stock solution was used daily in fresh. The
solution container was wrapped with aluminum foil and was stored in a
refrigerator. For kinetic measurements, solutions of the Pt(IV) complex
and DODT were prepared, respectively, by adding an appropriate
amount of the Pt(IV) stock solution and of DODT to a specific buffer.
All buffer solutions contained 0.10 M NaBr and 2.0 mM EDTA. Sodium
perchlorate was used to adjust the ion strength of buffers to 1.0 M.
The obtained solutions were flushed for 10 min with nitrogen, and
then were loaded on the stopped-flow machine. Equal volumes of the
Pt(IV) and DODT solutions were mixed automatically on the machine
for recording the kinetic traces under pseudo-first-order conditions
with DODT being at least 10-fold excess.
2.2. Instruments
Peptides were analyzed on a LC-6 CE high performance liquid chro-
matography (HPLC) system (Shimadzu, Kyoto, Japan). UV–Vis spectra
were recorded on a TU-1900 spectrophotometer (Beijing Puxi, Inc., Bei-
jing, China) using 1.00 cm quartz cells. Elemental analysis for C, H, and
N was performed on an Elementar instrument (Vario Micro cube,
Germany). 1H nuclear magnetic resonance (NMR) spectra were re-
corded on a Bruker AVANCE III 600 MHz digital NMR spectrometer
(Bruker Daltonics Inc., Billerica, MA, USA). Kinetic measurements were
performed on an Applied Photophysics SX-20 stopped-flow spectrome-
ter (Applied Photophysics Ltd., Leatherhead, U.K.) equipped with a ther-
mostat (BG-chiller E10, Beijing Biotech Inc., Beijing). Accumet Basic
AB15 Plus pH meter equipped with an Accumet combination pH
2.5. Stoichiometry and products analysis
The stoichiometry was determined by reaction of 1.0 mM Pt(IV)
complex with various concentrations of iRGD (0.1 ≤ [iRGD] ≤ 2.0 mM)
in H2O. The peak area of disulfide bridged iRGD peptide generated
Fig. 1. HPLC chromatograms of a) the reaction between trans-[PtBr2(en)2]2+ (1.0 mM) and peptide 1 (0.8 mM) in H2O; b) the reaction between trans-[PtCl2(en)2]2+ (1.0 mM) and peptide
1 (0.8 mM) in H2O.
2

D. Ma, C. Song, J. Sun et al.
Journal of Molecular Liquids 326 (2021) 115195
Table 1
Synthesis of disulfide bonds in peptides by trans-[PtBr2(en)2]2+ oxidation in H2O at room temperature a
.
No.
Peptide
Sequence of peptide
Reaction time (min)
Yield b (%)
Purity c (%)
1
2
3
4
5
6
7
8
peptide 1
peptide 2
oxytocin
arginine vasopressin
somatostatin
brain binding peptide
iRGD
crustacean cardioactive peptide
phenypressin
vasotocin
CGYCHKLHQMK-NH2
CGYCHKLHQGK-NH2
CYINQCPLG-NH2
CYFQNCPRG-NH2
AGCKNFFWKTFTSC-OH
CLSSRLDAC-NH2
CRGDKGPDC-NH2
PFCNAFTGC-NH2
CFFQNCPRG-NH2
CYIQNCPKG-OH
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
99
99
95
97
99
96
99
98
99
99
99
99
99
85
95
96
91
98
92
90
98
98
9
10
11
atriopeptin II
SSCFGGRIDRIGAQSGLGCNSFR-OH
Reaction conditions: [PtBr2(en)22+] = 1.0 mM, [dicysteine-containing peptide] = 0.5–1.0 mM. b Yields of disulfide-containing peptides were calculated according to the peak areas of
a
disulfide bridged peptides generated from trans-[PtCl2(en)2]2+ oxidation reactions. c Purities of disulfide-containing peptides were calculated by the areas of the chromatographic prod-
ucts peaks recorded at 215 nm; Peak areas of Pt(IV)/Pt(II) are not included in the calculation.
form the reaction was determined by HPLC. Excess of trans-[PtBr2(en)
]
2
2+ (0.6 mM) was reacted with iRGD peptide (0.5 mM) in D2O, the re-
action mixture was analyzed by NMR spectra for characterization of the
product of trans-[PtBr2(en)2]2+
.
3. Results and discussion
The reaction between peptide 1 and trans-[PtBr2(en)2]2+ was
investigated in H2O, the reaction mixture was maintained for
1 min, and then analyzed by HPLC (Fig. 1a). A yield of 99% was ob-
tained for the disulfide bridged peptide 1, while no methionine sulf-
oxide derivate was observed. It is known that trans-[PtCl2(en)2]2+
is an excellent oxidant for the formation of disulfide bond in the
methionine-containing peptide [16]. The reaction of trans-[PtCl2
(en)2]2+ with peptide 1 was also investigated in this work
(Fig. 1b). The yield of disulfide bridged peptide 1 is 98%, and the me-
thionine residue is intact. However, it needs about 4 h to convert the
two thiol groups in cysteine residues to disulfide bond.
In order to demonstrate the efficiency of trans-[PtBr2(en)2]2+ for
disulfide bonds formation, other ten peptides with variable lengths
were selected and reacted with trans-[PtBr2(en)2]2+. The results are
summarized in Table 1. As can be seen, all the reactions are very fast,
and excellent yields were achieved for these peptides, illustrating
that this Pt(IV) complex is a very efficient oxidant for synthesizing
peptide disulfide bond. HPLC chromatograms are displayed in the
supplementary material. On the other hand, formation of disulfide
bond in the antitumor drug somatostatin was also investigated in
aqueous mediums with different hydrogen ion concentration; the
results are summarized in Table 2. These reactions with good yields
are very fast. So, the Pt(IV) complex tolerates a range of hydrogen
ion concentration upon the formation of peptide disulfide bonds.
The reaction stoichiometry was determined by reaction of iRGD
peptide with the Pt(IV) complex in H2O. All the reactions were
aged for 1 min, and then were analyzed by HPLC. Plot of the
Fig. 2. HPLC peak areas of disulfide bridged iRGD at 215 nm for a series of reaction
mixtures in which the concentrations of the iRGD were varied in the region
0.1 ≤ [iRGD] ≤ 2.0 mM and [PtBr2(en)22+] = 1.0 mM was kept constant. Reaction
medium: H2O; room temperature.
peak areas of disulfide bridged iRGD versus [iRGD] is displayed in
Fig. 2.
A stoichiometry was estimated as [PtBr2(en)22+]: [iRGD] = 1: 1.02.
The reduction product of trans-[PtBr2(en)2]2+ was studied by NMR
(Fig. S1). As shown, trans-[PtBr2(en)2]2+ was reduced to [Pt(en)2]2+
Table 2
Formation of disulfide bridged somatostatin by trans-[PtBr2(en)2]2+ oxidation in various
solvents at room temperature.a
No. Peptide
Solvent
[H+]/(mol/L) yield a (%) Reaction time (min)
1
2
3
4
5
somatostatin HAc/NaAc 10–3.65
97
95
96
96
95
<1
<1
<1
<1
<1
TFA
TFA
TFA
TFA
10–4.50
10–6.27
0.01
0.05
a
Reaction conditions: [PtBr2(en)22+] = 1.0 mM, [dicysteine-containing
somatostatin] = 0.5–1.0 mM.
Fig. 3. Plots of kobsd versus [DODT] at 25.0 °C.
3

D. Ma, C. Song, J. Sun et al.
Journal of Molecular Liquids 326 (2021) 115195
with loss of two bromide ligands. Therefore, the reaction between trans-
[PtBr2(en)2]2+ and dicysteine-containing peptide can be described by
Eq. (1):
to the disulfide form of DODT. According to the kinetic results, a mech-
anism including different protolytic species of DODT reduction of trans-
The reaction between trans-[PtBr2(en)2]2+ and dicysteine-containing
peptide is very fast as shown in Table 1. So, we want to study the reac-
tion kinetics using stopped-flow spectrometer. Unfortunately, all the
peptides used in this work have a predominant absorbance at 235 nm,
[PtBr2(en)2]2+ was proposed, and is displayed in Scheme 1.
A rate law as Eq. (4) was deduced according to the proposed
mechanism, where aH stands for the proton activity which corre-
sponds exactly to the pH measurements or is calculated by equa-
tion: pH = − log[H+] + 0.20 for acidic solution [24]. Thus, the
second order rate constant k' is depended on aH and the protolytic
constants of DODT. The Eq. (5) was used to fit the k' - pH data
listed in Table 3.
which is the maximum absorption wavelength of trans-[PtBr2(en)2]2+
.
In our previous work, 3,6-dioxa-1,8-octanedithiol (DODT) was oxidized
to an intramolecule disulfide loop by Pt(IV) complex, and the size of the
loop is close to those of the oxidized active site Cys-Xaa-Yaa-Cys of
thioredoxins. Moreover, the pKa values (pKa1 = 8.7 and pKa2 = 9.6)
for the DODT are about the same to the active site peptide [23]. Most im-
portantly, DODT has no absorption at 235 nm. Therefore, DODT was
chosen as a model compound for the dicysteine-containing peptide
used in this work.
k1a2H þ k2Ka1aH þ k3Ka1Ka2
−d½PtðIVÞꢀ=dt ¼
ð4Þ
ð5Þ
a2H þ Ka1aH þ Ka1Ka2
The reaction between trans-[PtBr2(en)2]2+ and DODT was followed
at 235 nm [6] for recording the kinetic trace. As shown in Fig. S2, the de-
crease in absorbance was simulated by Eq. (2), where At, A0 and A∞ stand
for absorbance at time t, zero and infinity, respectively.
k1a2H þ k2Ka1aH þ k3Ka1Ka2
k0 ¼
a2H þ Ka1aH þ Ka1Ka2
A good fit was achieved as displayed in Fig. 4; the rate constants of
rate determining steps listed in Table 4 were calculated from the fit.
Unfortunately, k1 was undetermined. By comparison, the rate of oxida-
tion of DODT by trans-[PtBr2(en)2]2+ is faster than that of oxidation of
DODT by trans-[PtCl2(CN)4]2−, while the redox potential of trans-
[PtBr2(en)2]2+ (Eo′ = 0.54 V) [25] is lower than that of trans-[PtCl2
(CN)4]2− (Eo′ = 0.926 V) [26]. trans-[PtCl2(CN)4]2− possesses higher
redox potential making it can oxidize the methionine residue in peptide
1 to methionine sulfoxide derivate [27], while trans-[PtBr2(en)2]2+ un-
able to oxidize the methionine residue.
The kinetic results demonstrated that thiolate (deprotonated
form of thiol) are much more reactive than the protonated thiol. As
a result, the reaction rate for the formation of peptide disulfide
bond via Pt(IV) oxidation is increased with the increase of pH of
the reaction medium. Therefore, the protolytic constants of the two
thiol groups in peptide play a critical role in the reaction rate. On
the other hand, kinetic studies demonstrated that the Pt(IV) com-
plex trans-[PtBr2(en)2]2+ can be used in acidic medium for the pur-
pose of synthesis of disulfide bond.
At ¼ ðA0–A∞Þ exp :ð−kobsdtÞ þ A∞
ð2Þ
The good simulation demonstrated that the reaction is first order in
[Pt(IV)]. Pseudo first-order rate constant kobsd was thus derived from
the simulation; the average value of kobsd was from five to seven dupli-
cate runs; standard deviation was usually less than 5%. The influences of
varying [DODT] on the reaction rates were investigated in various
buffers. Values of kobsd are summarized in Tables S1. Plots of kobsd versus
[DODT] are straight lines with no significant intercepts (Fig. 3), indicat-
ing that the reaction is also first-order in [DODT].
An overall second-order rate law is established as Eq. (3), where k'
denotes the observed second-order rate constant. Values of k' were cal-
culated from the plots of kobsd versus [DODT], and are listed in Table 3.
The values of k' listed in Table 3 increase several orders of magnitude
when the reaction medium is changed from acidic solution to slightly
acidic ones. The large increase of k' reflects that the deprotonated
forms thiolates are much more reactive than the protonated thiols.
−d½PtðIVÞꢀ=dt ¼ k0½PtðIVÞꢀ½DODTꢀ
ð3Þ
By comparison, the reaction of trans-[PtCl2(en)2]2+ with dicysteine-
containing somatostatin was also studied in 10.0 mM HCl solution. It
needs about 15 h to complete the reaction, while only 1 min is need
for trans-[PtBr2(en)2]2+ oxidation (See Table 2). Therefore, the rate for
formation of peptide disulfide by trans-[PtBr2(en)2]2+ oxidation is dra-
matically faster than that of formation of peptide disulfide by trans-
[PtCl2(en)2]2+ oxidation in acidic solution. However, the redox poten-
tial (Eo′ = 0.54 V [25]) of [PtBr2(en)22+]/[Pt(en)22+] is almost equal to
that of [PtCl2(en)22+]/[Pt(en)22+] (Eo′ = 0.58 V [25]). Therefore, the
redox potential of Pt(IV) complex has no influence on the rate of disul-
fide bond formation. This was also observed in the reduction of Pt(IV)
complexes with axially coordinated-chloride/bromide ligands by cyste-
ine or selenomethionine in our previous works [28,29]. In these reac-
tions, a parallel reaction mechanism was proposed, and a chloride/
bromide bridge mediated inner sphere electron transfer was occurred
in these reactions. Generally, redox potential plays a dominant role in
the outer sphere electron transfer process and not in the inner sphere
electron transfer. Thus, the formation of peptide disulfide bond by
trans-[PtBr2(en)2]2+ oxidation was also occurred through the inner
sphere electron transfer (As shown in Scheme 1). Moreover, this obser-
vation strongly emphasizes that the axially coordinated-bromide is a
better bridging ligand.
The oxidation product of DODT was characterized by mass spectrum
(Fig. S3); the value of m/z was determined as 181.03506 corresponding
Table 3
Values of k' at 25.0 °C and 1.0 M ionic strength.
[H+]/M
pH
k'/M−1 s−1
0.2
0.1
1.27
4.03
7.93
14.1
20.9
46.8
94.5
743
(1.95
(4.24
(7.46
(1.67
(2.95
(6.44
(1.93
0.03
0.11
0.14
0.3
0.4
1.0
0.05
0.03
0.02
0.01
0.005
2.7
12
3.18
3.51
3.85
4.12
4.51
4.74
5.11
5.55
0.04) × 103
0.03) × 103
0.05) × 103
0.02) × 104
0.02) × 104
0.06) × 104
0.02) × 105
4

D. Ma, C. Song, J. Sun et al.
Journal of Molecular Liquids 326 (2021) 115195
Scheme 1. The proposed mechanism for the formation of disulfide bond via trans-[PtBr2(en)2]2+ oxidation.
4. Conclusion
Pt(IV) complex trans-[PtBr2(en)2]2+ was synthesized and was suc-
cessfully used to prepare disulfide bonds in peptides. Excellent yields
with fast reaction rates were achieved in acidic or neutral reaction me-
dium. Methionine residue is compatible to the Pt(IV) complex oxidation
method. Therefore, trans-[PtBr2(en)2]2+ is a very efficient oxidant for
the purpose of formation of disulfide bonds. The kinetics and mecha-
nism for the reaction between trans-[PtBr2(en)2]2+ and the model com-
pound DODT was investigated in detail. The results demonstrated that
the rate of formation disulfide is very fast, and is depended on both
pH of reaction medium and pKa of thiol group in peptide. Moreover,
the rate of formation of peptide disulfide is influenced by the axial
ligands of Pt(IV) complex but not its redox potential.
Declaration of Competing Interest
Authors report no conflict of interest in this work.
Acknowledgements
Fig. 4. Observed second-order rate constants, k', as a function of pH at 25.0 °C (data points).
The solid curve represents the best fit of Eq. (5) to the experimental data by a weighed
nonlinear least-squares routine.
Financial support for this work through grants from the National
Natural Science Foundation of China (21406047) and the Natural
Science Foundation of Hebei Province (B2016201014) is gratefully
acknowledged.
Table 4
Values of the rate-determining steps derived from curve-fitting at 25.0 °C and 1.0 M ionic
strength.
Appendix A. Supplementary data
Pt(IV) Complex
km
Value/M−1 s−1
Ref
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.molliq.2020.115195.
trans-[PtCl2(CN)4]2−
k1
k2
k3
k1
k2
k3
38
(2.32
_ a
2
[23]
0.01) × 107
trans-[PtBr2(en)2]2+
_ b
(5.37
(6.71
this work
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