Polydopamine as a Catalyst for Thiol Coupling

Article abstract of DOI:10.1002/cctc.201500643

In biological systems, disulfide bonds are formed efficiently under mild conditions without the release of harmful byproducts. Inspired by nature, we report a biomimetic polydopamine (PDA) catalyst for oxidative thiol coupling. This reaction was accelerated with only a small amount of PDA particles in neutral, weakly alkaline, and even weakly acidic aqueous media at room temperature under an air atmosphere. The catalytic particles were facilely separated and were reused without a decrease in activity. The entire process is totally biofriendly, including the synthesis of the PDA particles. This route is especially useful for the synthesis of pharmaceutical molecules. S-imulating nature: Inspired by disulfide bond formation mechanisms in nature, we demonstrate that polydopamine particles can be used as a promising biomimetic catalyst for the thiol coupling reaction in aqueous media under mild conditions. This approach involves no harmful chemicals and produces no waste products except for water.

Full text of DOI:10.1002/cctc.201500643

DOI: 10.1002/cctc.201500643
Communications
Polydopamine as a Catalyst for Thiol Coupling
Yong Du, Hao-Cheng Yang, Xiao-Ling Xu, Jian Wu, and Zhi-Kang Xu*
[a]
[a]
[b]
[b]
[a]
In biological systems, disulfide bonds are formed efficiently
under mild conditions without the release of harmful byprod-
ucts. Inspired by nature, we report a biomimetic polydopamine
the catalysts, which may limit their suitability, especially in
pharmaceutical applications.
Disulfides are formed in nature in an efficient and controlla-
ble way under mild conditions. For example, disulfide bonds
are accurately generated for protein folding through an
enzyme-catalyzed approach in living cells. The two thiol
groups of the enzyme DsbB are oxidized into a disulfide bond
by ubiquinone in E. coli. Subsequently, ubiquinone is regener-
ated by aerobic oxidation, whereas the DsbB enzyme transfers
the disulfide bond to the DsbA enzyme. Finally, DsbA transfers
(
PDA) catalyst for oxidative thiol coupling. This reaction was
accelerated with only a small amount of PDA particles in neu-
tral, weakly alkaline, and even weakly acidic aqueous media at
room temperature under an air atmosphere. The catalytic par-
ticles were facilely separated and were reused without a de-
crease in activity. The entire process is totally biofriendly, in-
cluding the synthesis of the PDA particles. This route is espe-
cially useful for the synthesis of pharmaceutical molecules.
[
23,24]
its disulfide bond for protein folding in bacterial cells.
An-
other example is that thiols are efficiently oxidized into disul-
fides through a redox process with dopamine quinone motifs
[
4]
It is well known that disulfides are very important chemicals
in mussel foot proteins. In this case, the dopamine quinone
motifs are reduced to control the oxidative cross-linking reac-
tion of the mussel foot proteins. This redox process and the re-
sulting cross-linked polydopamine (PDA) play important roles
[1–6]
that widely exist in biological systems.
They play a crucial
role in maintaining the conformational stability and biological
[
1,2]
activity of proteins
of mussel foot protein.
cals have received much attention as a result of their artificial
and participate in the adhesion process
[4,5]
[4]
Over the past decades, these chemi-
for the unique adhesion of mussels on various surfaces. Lee
[
25]
et al. inspired this mechanism and opened the door to the
[
7,8]
applications in synthesizing pharmaceutical molecules,
con-
simple and versatile modification of material surfaces through
[9–11]
[26–33]
structing functional materials,
building dynamic combina-
the oxidative formation of PDA.
Although there are still
[
12,13]
torial libraries,
and preparing sulfur-containing organic
Numerous methods have been developed to
challenges to clarify the chemical structure of synthetic PDA, it
is generally agreed that dopamine must be oxidized into dopa-
mine quinone and that there are abundant quinone motifs in
[
14,15]
compounds.
form disulfide bonds in both scientific laboratories and indus-
trial factories. Up to now, the most straightforward approach is
[
34–36]
the corresponding polymer.
These mechanisms have been
[1,6,16–22]
[37]
the oxidative coupling of thiols.
In organic media, thiols
summarized by d’Ischia et al. Therefore, it is reasonable for
us to develop a biomimetic route for the synthesis of disulfides
by using the quinone motifs in PDA.
are oxidized into disulfides by oxygen from the air, catalyzed
[
1,16]
[17]
by heavy metal ions,
metal nanoparticles, or other organ-
Furthermore, this coupling has also been ach-
ieved by the redox reaction of thiols with oxidants such as
[
18,19]
ic catalysts.
We herein report a PDA-based heterogeneous catalyst for
the synthesis of disulfides by aqueous thiol coupling. Our
method is totally biofriendly and no waste or harmful chemi-
cals are produced except for water. The PDA catalyst particles
can be easily separated from the reaction media and reused
with no marked decrease in catalytic activity. Besides, our cata-
lyst can be used in weakly alkaline, neutral, and even weakly
acidic aqueous media at room temperature.
[
20]
[21]
iodine and graphene oxide. In aqueous media, aerobic ox-
[
22]
idative coupling can be easily conducted in alkaline solution.
Nevertheless, this method is not suitable to thiols that are un-
stable in alkaline media, for instance, basic fibroblast growth
[
6]
factor. Other approaches have been developed in neutral or
acidic media. Typical examples include coupling thiols by using
[
6]
organic oxidants and metal-ion-catalyzed oxidation in the
It is well known that dopamine self-polymerizes into PDA
particles in weakly alkaline aqueous media, oxidized by oxygen
dissolved in solution. PDA particles were synthesized in Tris
buffer of pH 8.5 in air. SEM and TEM images indicate that the
particles have diameters ranging from 100 to 500 nm and form
aggregates (Figures S1a and S2 in the Supporting Information).
However, these particles can be dispersed well in phosphate-
buffered solution and can be completely removed from the so-
lution by passing through a 0.22 mm cellulose acetate mem-
brane (Figure S3).
[9,22]
presence of oxygen.
However, these methods suffer from
difficulties in the removal of harmful impurities and reuse of
[
a] Y. Du, H.-C. Yang, Prof. Dr. Z.-K. Xu
MOE Key Laboratory of Macromolecular Synthesis and Functionalization
Department of Polymer Science and Engineering
Zhejiang University
Hangzhou, 310027 (P.R. China)
E-mail: xuzk@zju.edu.cn
[
b] X.-L. Xu, Prof. Dr. J. Wu
Department of Chemistry
Zhejiang University
The thiol coupling reactions were catalyzed by PDA particles
at room temperature in air. Various thiols (0.15 mmol) were
Hangzhou 310027 (P.R. China)
studied in phosphate-buffered D O solutions (0.2m) at pH 6.0,
7.0 and 8.0. This medium is convenient to identify the coupling
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500643.
2
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Communications
product and the conversion. It is reasonable to regard that the
degree of ionization of the thiols is similar to that in corre-
sponding phosphate-buffered H O solutions. For example, 1-
2
mercaptoglycerol (MGL) was coupled to form bis(2,3-dihydrox-
ypropyl)disulfide by the catalysis of PDA particles. The synthe-
sized bis(2,3-dihydroxypropyl)disulfide was isolated by liquid-
1
phase chromatography and was identified by using H NMR
and FTIR spectroscopy and MS (Figures S5 and S6) to confirm
that only the thiol-coupling reaction occurred. The results
show that the coupling conversion is significantly increased by
adding only a small amount of PDA particles with a reaction
time of 24 h (Table 1, entry 1 vs. 2). Besides, our results also in-
dicate that the coupling time can be decreased from 24 to 8 h
simply by increasing the amount of PDA particles (Table 1, en-
tries 1, 3, 4). Thus, the reaction rate can be easily tuned by the
dose of PDA particles.
Table 1. Coupling reaction of 0.15 mm 1-mercaptoglycerol in 1 mL
buffered D O solution under various conditions.
2
Scheme 1. Proposed mechanism of the PDA-catalyzed thiol coupling
[a]
reaction.
Entry
Catalyst
pH
Reaction
time [h]
Conversion
[%]
[mg]
1
2
3
4
5
6
7
8
9
4.5
0
9.0
18.0
4.5
0
4.5
0
ꢀ7.0
ꢀ7.0
ꢀ7.0
ꢀ7.0
ꢀ6.0
ꢀ6.0
ꢀ8.0
ꢀ8.0
ꢀ6.0
ꢀ7.0
24
24
12
8
48
48
12
12
24
24
96
<5
94
94
63
15
97
5.9
90
trace
13.5
4.5
[b]
1
0
[
(
a] Conversion was determined by NMR spectroscopy. [b] Dopamine
4.5 mg) was used as the catalyst, under the same conditions as those
listed in entry 1.
We suggest that the coupling reaction of thiols catalyzed by
PDA should follow a mechanism similar to that of mussel ad-
Figure 1. The z potential of the PDA particles after sequentially catalyzing
the oxidative coupling of MPS, MGL, and MPS.
[
4,5]
[23,24]
hesion
and that of disulfide bond formation in DsbB
in
nature. Scheme 1 shows that the proposed mechanism con-
sists of three steps: one, nucleophilic attack of the quinone
groups on the PDA particles (state 1) by the first thiolate anion
to form a thiol–dopamine adduct (state 2), as illustrated by
À23.0 mV. This value decreases to À43.9 mV after catalyzing
the coupling of MPS, which indicates that MPS links the PDA
particles, shown as state 4 in Scheme 1. However, after catalyz-
ing the coupling of MGL the value increases to À24.6 mV,
which is similar to that of the initial PDA particles. The reason
is that the MPS motifs tethered on the PDA particles are re-
placed by MGL groups. In detail, MGL thiolate anions attack
the catalyst in state 2, which can be reversibly transformed
from state 4 (in Scheme 1) to form disulfides and phenol-state
dopamine. Afterwards, other MGL thiolate anions attach to the
catalyst during the reaction. The z potential of these PDA par-
ticles finally decreases to À48.6 mV after catalyzing the cou-
pling of MPS for the second time. This is similar to what occurs
to the PDA particles after catalyzing the coupling of MPS for
the first time, which suggests that the MGL motifs tethered on
[
38]
previous work;
two, attack of the thioether adduct by
a second thiolate anion to form a disulfide and phenol-state
dopamine (state 3); three, oxidization of phenol-state dopa-
mine into quinone-state dopamine by oxygen dissolved in the
solution. The quinone groups on the PDA particles were identi-
fied by X-ray photoelectron spectroscopy (XPS) (Figure S4),
[39]
which is in accordance with the previous report. To further
understand this mechanism, PDA particles were used to se-
quentially catalyze the individual oxidative couplings of
sodium 3-mercapto-1-propanesulfonate (MPS), MGL, and
sodium 3-mercapto-1-propanesulfonate (shown in Figure 1).
We found that the z potential of the initial PDA particles is
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[
25]
the PDA particles are totally replaced by MPS groups. These re-
sults strongly illustrate that the catalytic reaction follows our
suggested mechanism. Dopamine is not able to catalyze the
coupling reaction according to this mechanism, which is in
agreement with our experimental results (Table 1, entry 10). It
is also reasonable to consider that the catalysis process mainly
happens on the surfaces of the PDA particles. This phenomen-
fides. In contrast, in our experiments the reaction medium is
exposed to air during the entire reaction, as oxygen is necessa-
ry for the regeneration of the PDA catalyst. We suggest that
exposure to air and the large surface area of PDA make the
thiol coupling reaction the dominant process. During the cata-
lyzed reaction, the morphology of the PDA particles does not
undergo any clear change relative to that of the original parti-
cles, as observed by SEM (Figure S1b).
[41]
on was also observed by Vaish et al.
We studied the influence of pH on the catalysis properties.
The reaction can be catalyzed in neutral, weakly alkaline, and
weakly acidic aqueous media (Table 1, entries 1, 2, 5–9). How-
ever, the rate of the reaction decreases as the pH decreases.
This is because the catalyzed coupling needs a thiolate ion as
We subjected a variety of commercially available water-solu-
ble thiols to the catalyzed thiol-coupling reaction. We changed
the coupling conditions for different thiols to distinguish the
reaction rate of the catalyzed reaction and the noncatalyzed
one. The PDA particles facilitated the coupling of all the select-
ed thiols, as shown in Figure 3, whereas the reaction rate was
[
4,5,23,24]
an intermediate,
and the concentration of this ion in-
creases in weakly alkaline solution.
We also evaluated the reusability of the PDA particles as
a catalyst for the coupling reactions. Typical results are shown
in Figure 2 with the formation of bis(2,3-dihydroxypropyl)disul-
fide as an example. The mass loss of the catalyst after each run
Figure 3. Catalyzed coupling reactions of various thiols. In each reaction,
0.15 mm thiol was used as the reactant and 4.5 mg PDA particles was used
as the catalyst. They were incubated in 1 mL buffered D O solution at room
2
temperature in air for a certain period of time. The pH value of the solution
and the reaction time are shown.
slightly different. Thiols containing hydroxy groups, carboxylic
groups, or sulfonate groups showed similar results during the
coupling reaction. The slight rate increase for 2-aminoethane-
thiol can be ascribed to a fast noncatalyzed reaction. The low
reaction rate of the coupling of 2-mercaptoacetic acid is the
result of the difficulty to form the thiolate ion.
Figure 2. Reusability of the PDA particles as a catalyst for the coupling reac-
tion of thiols. All reactions were performed by using 0.15 mm MGL as the re-
actant and PDA particles as the catalyst in buffered D
2
O solution (pHꢀ7.0)
at room temperature in air for 24 h. After each run, the catalyst was re-
moved by centrifugation, freeze-dried, and weighed. The amount of solution
for the next run was determined according to the weight of the catalyst left
after the former run. The PDA dose was 4.5 mg for the first run.
We also tested several commonly used organic solvents to
study whether our catalyst was suitable for reaction in organic
media. However, the PDA particles might be partially dissolved
in polar organic solvents, such as DMSO, DMF, and methanol,
as the solution turned brown after the PDA particles were dis-
persed and then separated from these solvents. Besides, this
reaction can also not be performed in nonpolar organic sol-
vents, such as hexane, as the PDA particles cannot be easily
dispersed in these solvents. Therefore, we suggest that it is
very difficult to find a proper organic solvent in which this re-
action can be performed.
is approximately 10% because the PDA particles are easily dis-
persed in water and some of them are inevitably removed
along with water during each washing step. The PDA particles
can be reused over at least five runs after separation from the
reaction media by centrifugation without any detectable de-
crease in the catalytic activity. A thiol molecule can attach to
the PDA particles to form a CÀS bond through a Michael addi-
[
25,38–40]
tion reaction
(shown as state 4 in Scheme 1). However,
In conclusion, we demonstrated the use of a biomimetic
PDA catalyst for disulfide synthesis by thiol coupling under
mild conditions. The reaction can be efficiently performed in
a neutral, weakly alkaline, or weakly acidic environment. We
suggest that the catalysis follows a mechanism similar to that
of mussel adhesion and disulfide formation in DsbB in nature
by using dopamine quinone motifs as the active center. This
method shows great potential in preparing disulfides, especial-
state 4 can reversibly transform into state 2 according to our
suggested mechanism. Thus, the thiolated PDA in state 2 still
has the ability to catalyze the coupling reaction. Besides, thiols
tethered on the PDA particles can finally couple with other
thiols in the solution to form disulfides. Notably, in a previous
study that used thiols to functionalize polydopamine, the reac-
tion medium was deoxidized to avoid the formation of disul-
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Keywords: biomimetic catalysts · heterogeneous catalysis ·
polydopamine · sulfur · thiol coupling
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Received: June 8, 2015
Revised: July 25, 2015
Published online on September 23, 2015
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