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Citation: Niño-Ramírez, V.A.;
Insuasty-Cepeda, D.S.;
Rivera-Monroy, Z.J.; Maldonado, M.
Evidence of Isomerization in the
Michael-Type Thiol-Maleimide
Addition: Click Reaction between
L-Cysteine and 6-Maleimidehexanoic
Acid. Molecules 2022,27, 5064.
https://doi.org/10.3390/
molecules27165064
Academic Editors: Sergey Timofeev
and Brenno A.D. Neto
Received: 23 June 2022
Accepted: 4 August 2022
Published: 9 August 2022
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4.0/).
molecules
Article
Evidence of Isomerization in the Michael-Type
Thiol-Maleimide Addition: Click Reaction between L-Cysteine
and 6-Maleimidehexanoic Acid
Víctor Alfonso Niño-Ramírez, Diego Sebastián Insuasty-Cepeda , Zuly Jenny Rivera-Monroy *
and Mauricio Maldonado *
Chemistry Department, Universidad Nacional de Colombia, Bogotá, Carrera 45 No 26-85, Building 451,
Office 409, Bogotá11321, Colombia
*Correspondence: zjriveram@unal.edu.co (Z.J.R.-M.); mmaldonadov@unal.edu.co (M.M.);
Tel.: +57-1-3165000 (ext. 14436) (M.M.)
Abstract:
The reaction between L-cysteine (Cys) and 6-maleimidohexanoic acid (Mhx) in an aqueous
medium at different levels of pH was analyzed via RP-HPLC, finding the presence of two reaction
products throughout the evaluated pH range. By means of solid-phase extraction (SPE), it was
possible to separate the products and obtain isolated profiles enriched up to 80%. The products were
analyzed individually through mass spectrometry, DAD-HPLC, NMR
1
H,
13
C, and two-dimensional
evidence of isomerization between the hydrogen atoms of the
α
-amino and the thiol group present in
the cysteine. Thus, it was concluded that the products obtained corresponded to a mixture of the
isomer Cys-S-Mhx, where the adduct is formed by a thioether bond, and the isomer Cys-NH-Mhx, in
which the union is driven by the amino group. We consider that the phenomenon of isomerization is
an important finding, since it has not previously been reported for this reaction.
Keywords: click reaction; Michael addition; thiol-maleimide addition; isomerization
1. Introduction
The Michael-type thiol-maleimide addition reaction is one of the most versatile in
biochemistry, organic synthesis, and bioconjugation, thanks to its rapid kinetics, quantita-
tive conversion, selectivity, high yields, and lack of by-products. This reaction is classified
within the thiol-ene type reactions, where the maleimide group in particular exhibits great
reactivity compared to other ene groups, such as vinyl sulfones, acrylates, acrylamides,
methacrylates, etc., which have poor reactivity given the structure and groups adjacent
to the active ene, and lower reaction kinetics compared to maleimide [
1
–
3
]. This addition
reaction is very versatile; depending on the physicochemical and structural characteristics
of the starting molecules functionalized with free thiol or maleimide groups, it can be used
with aqueous or organic solvents, both protic and aprotic, allowing a very wide range of
options for solubilizing the different functionalized precursors. Additionally, since there
are two reaction mechanisms catalyzed by bases or by nucleophiles, it is possible to explore
the thiol-maleimide addition reaction in the absence of a catalyst, or by initiators, although
it is typically used with tertiary amines [1,4,5].
The use of this click reaction can be found in various examples, including: (i) In-
corporation of peptide motifs that allow the construction of antibody–drug conjugates
(ADCs) [
6
,
7
]; (ii) Coupling of Pt(IV) coordination compounds with maleimide and lin-
ear peptide molecules [
8
,
9
]; (iii) Design and synthesis of nanoparticles such as molecular
cages or nanoparticle drug delivery systems [
10
,
11
]; (iv) Construction of dendrimeric
molecules [
12
]; (v) Obtaining oligonucleotides-peptide [
13
]; (vi) Functionalization of a
monolithic organic support, poly(GMA-co-EDMA), with a maleimide hexanoic (Mhx)
group that allows the incorporation of a peptide containing a residue of cysteine at the
N-terminal, a reaction that was found to be selective under mild conditions [14].
Molecules 2022,27, 5064. https://doi.org/10.3390/molecules27165064 https://www.mdpi.com/journal/molecules
Molecules 2022,27, 5064 2 of 10
Despite the benefits of this reaction, it has been reported that the thiosuccinimide
resulting from the thiol-maleimide reaction may be hydrolytically unstable through a retro-
Michael reaction (Figure 1), [
2
,
6
,
15
]. In particular, although the presence of retro-Michael
reactions can be seen as a disadvantage, it makes it possible to vary the stability of the
succinimide thioether adduct. Several studies have shown how the reversible reaction and
N-substituting groups can have an effect on stability, and therefore allow proper tuning
and release according to environmental conditions, especially in the case of ADCs [
16
].
In addition, other researchers have pointed out that the formation of a single thioether
succinimide product may occur, while still others suggest the possible presence of two
diastereoisomeres. Specifically, the diastereoisomers correspond to the addition of two
adducts with stereocenters R and S into the thiol group on the
α
,
β
-unsaturated bond.
However, if the ring of the resulting thiosuccinimidil is opened by hydrolysis, mixtures
of succinamic acid are generated, although the advantage is that a retro Michael thiol-
maleimide reaction is no longer possible, obtaining a completely stable adduct [
3
,
17
].
Additionally, as noted above, a third subsequent reaction can be caused by the addition of
thiol-maleimide, forming a new adduct with a six-member cycle called thiazine [
9
]. Figure 1
summarizes the reported behavior of this addition reaction.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
that allows the incorporation of a peptide containing a residue of cysteine at the N-termi-
nal, a reaction that was found to be selective under mild conditions [14].
Despite the benefits of this reaction, it has been reported that the thiosuccinimide
resulting from the thiol-maleimide reaction may be hydrolytically unstable through a
retro-Michael reaction (Figure 1), [2,6,15]. In particular, although the presence of retro-
Michael reactions can be seen as a disadvantage, it makes it possible to vary the stability
of the succinimide thioether adduct. Several studies have shown how the reversible reac-
tion and N-substituting groups can have an effect on stability, and therefore allow proper
tuning and release according to environmental conditions, especially in the case of ADCs
[16]. In addition, other researchers have pointed out that the formation of a single thi-
oether succinimide product may occur, while still others suggest the possible presence of
two diastereoisomeres. Specifically, the diastereoisomers correspond to the addition of
two adducts with stereocenters R and S into the thiol group on the α, β-unsaturated bond.
However, if the ring of the resulting thiosuccinimidil is opened by hydrolysis, mixtures
of succinamic acid are generated, although the advantage is that a retro Michael thiol-
maleimide reaction is no longer possible, obtaining a completely stable adduct [3,17]. Ad-
ditionally, as noted above, a third subsequent reaction can be caused by the addition of
thiol-maleimide, forming a new adduct with a six-member cycle called thiazine [9]. Figure
1 summarizes the reported behavior of this addition reaction.
Figure 1. Michael thiol-maleimide addition reaction, rearrangements and products formed.
Considering the great applicability and versatility of this reaction, and its relevance
to the field of biochemistry and particularly the construction of conjugate systems, we
used this reaction to join the 6-maleimidehexanoic acid to a peptide which contained a N-
terminal cysteine. This reaction was monitored via RP-HPLC, and it was observed that
the chromatographic profile presented two signals corresponded to addition reaction
products [14]. In this context, the thiol-maleimide addition was studied in the present in-
vestigation, using as a model the reaction between L-Cysteine (Cys) and 6-maleimide hex-
anoic acid (Mhx) under different experimental conditions. These precursors were selected
due to our interest in using the thiol-maleimido Michael addition reaction for the con-
struction of different conjugates between peptide sequences containing the L-cysteine or
6-maleimidohexanoic motif, and non-peptidyl molecules.
2. Results and Discussion
As mentioned above, the high reactivity of maleimides is caused by their behavior as
an electrophile, due to the α,β-unsaturation and the presence of two carbonyl groups,
which allows them to react quickly with nucleophiles such as thiolate groups, obtaining
Figure 1. Michael thiol-maleimide addition reaction, rearrangements and products formed.
Considering the great applicability and versatility of this reaction, and its relevance
to the field of biochemistry and particularly the construction of conjugate systems, we
used this reaction to join the 6-maleimidehexanoic acid to a peptide which contained a
N-terminal cysteine. This reaction was monitored via RP-HPLC, and it was observed
that the chromatographic profile presented two signals corresponded to addition reaction
products [
14
]. In this context, the thiol-maleimide addition was studied in the present
investigation, using as a model the reaction between L-Cysteine (Cys) and 6-maleimide
hexanoic acid (Mhx) under different experimental conditions. These precursors were
selected due to our interest in using the thiol-maleimido Michael addition reaction for the
construction of different conjugates between peptide sequences containing the L-cysteine
or 6-maleimidohexanoic motif, and non-peptidyl molecules.
2. Results and Discussion
As mentioned above, the high reactivity of maleimides is caused by their behavior
as an electrophile, due to the
α
,
β
-unsaturation and the presence of two carbonyl groups,
which allows them to react quickly with nucleophiles such as thiolate groups, obtaining
succinimide adducts. Furthermore, the addition of Michael thiol-maleimide is highly
Molecules 2022,27, 5064 3 of 10
specific to thiols at pH 6.5–7.5, and is 1000 times faster than the slow addition that would
occur in the presence of
ε
-amino groups of Lys at pH 7.0, due to the nucleophilia of the
thiolate group and the pKa of the thiol group [
7
]. Therefore, to explore the reactivity,
selectivity, and conditions of the click reaction between cysteine and maleimidehexanoic
acid, the process was initially begun through Michael addition in water at room tempera-
ture, in a manner similar to that described in the literature [
18
–
20
]. After 2 h, the reaction
mixture was analyzed by means of LC-MS (ESI) and RP-HPLC. As shown in Figure 2,
the reaction’s chromatographic profile showed four signals, corresponding to cysteine
(Cys,
tR= 0.92 min
), 6-maleimidohexanoic acid (Mhx, t
R
= 7.56 min), and two products at
tR= 4.6
and 4.7 min, which showed similar signals in their MS spectra, at m/z333.1118 and
333.1117, respectively. For the species [M + H]
+
the calculated exact mass corresponded to
333.1120, and therefore, in both cases the error was less than 1 ppm (Figure 2).
Figure 2 (improved version)
Figure 2.
Analysis of the reaction between cysteine (Cys) and 6-maleimidohexanoic acid (Mhx) in
water, by means of LC-MS (ESI). The TIC showed signals at t
R:
4.6 and 4.7 min that corresponded to
the reaction’s products.
The chromatographic method for the analysis of the reaction by RP-HPLC (DAD) was
improved, and it was possible to separate the two reaction products with a resolution of
two. The RP-HPLC-DAD analysis, in the range of 190 to 270 nm, allowed determination of
the purity of each product. The two peaks had a similar UV spectrum. Product 1 had a peak
purity of 99.7%, and Product 2 had a peak purity of 99.8% (Figure 3A, in water). Then the
reaction was carried out by varying the reaction conditions, such as solvent type and pH
(Figure 3), and the reaction mixtures were analyzed using the optimized RP-HPLC method.
As can be seen, the reaction was favored in protic solvents. When methanol or
water were used, the reaction was quantitative and occurred in time spans of less than
5 min. The Mhx peak disappeared completely, and the two reaction products previously
described were formed (peaks 1 and 2). When the polarity of the solvent began to decrease,
as in the case of ethanol, the appearance of reaction products was observed at a rather
low proportion, and the Mhx peak was observed (peak 3). It is possible that solvation
phenomena or the effect of the polarity of the solvent reduced the yield of the reaction.
Finally, in the case of acetonitrile (ACN), the reaction did not take place; no reaction
products were seen and only the Mhx peak appeared in the chromatogram. The reaction in
Molecules 2022,27, 5064 4 of 10
the aqueous phase was evaluated by varying the pH from 2.5 to 6.8, and in all cases, there
was the rapid appearance of the two previously observed reaction products. This result
suggests that the reaction took place over a wide pH spectrum.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
Figure 3. RP-HPLC monitoring of the reaction between Cys and maleimidehexanoic acid (Mhx). (A)
Reaction in different solvents (H2O, MeOH, EtOH, and ACN). (B) Reaction in aqueous solution at
different pH values: 2.5, 4.0, 5.0 and 6.8. Peaks 1 and 2 correspond to the reaction products.
As can be seen, the reaction was favored in protic solvents. When methanol or water
were used, the reaction was quantitative and occurred in time spans of less than 5 min.
The Mhx peak disappeared completely, and the two reaction products previously de-
scribed were formed (peaks 1 and 2). When the polarity of the solvent began to decrease,
as in the case of ethanol, the appearance of reaction products was observed at a rather low
proportion, and the Mhx peak was observed (peak 3). It is possible that solvation phe-
nomena or the effect of the polarity of the solvent reduced the yield of the reaction. Finally,
in the case of acetonitrile (ACN), the reaction did not take place; no reaction products were
seen and only the Mhx peak appeared in the chromatogram. The reaction in the aqueous
phase was evaluated by varying the pH from 2.5 to 6.8, and in all cases, there was the
rapid appearance of the two previously observed reaction products. This result suggests
that the reaction took place over a wide pH spectrum.
The unexpected formation of two products by means of the methodology adapted
from the literature contrasts with results obtained previously [3,16,21], in which it was
reported that thiomethylation was the only reaction to occur. The formation of two prod-
ucts with the same mass, as evidenced in the chromatogram, is an interesting result. We
considered two possible explanations, viz., the formation of a diasteroisomer mixture or
of a tautomer mixture. To establish which of the two reaction routes was used, we pro-
ceeded to separate the two products via the RP-SPE technique [22] (see the experimental
section). The isolated products exhibited a great tendency to be hygroscopic; for this rea-
son, their characterization was only carried out in solution.
The mixture and the purified products were analyzed by means of NMR spectros-
copy. The analysis of the 1H-NMR spectrum of the unpurified mixture showed two prod-
ucts corresponding to the isomeric mixture that presented well-resolved signals in the
aliphatic region, which allowed easy assignment of the two isomers and even their molar
ratios in the crude product (50/50) (Figure 4). After separating the isomers, the NMR anal-
ysis of each of the products obtained was carried out.
The 1H-NMR spectra, in Methanol-d6, of the more polar compound (peak 1 in Figure
3) showed three signals, at 3.14, 3.70, and 4.23 ppm, corresponding to the protons in the
maleimide ring. The signals corresponding to the cysteine residue were seen at 4.02 for
the alpha proton and at 3.25 and 2.55 for the diasterotopic protons. Additionally, the 1H-
NMR spectrum displayed characteristic signals of hexanoic acid proton residue at 1.34,
1.60, 2.29, and 3.51 ppm. The 13C-NMR spectrum showed thirteen signals, which were
unambiguously assigned through 1D- and 2D-NMR experiments, including the HMQC
and HMBC spectra. The signals for carbon atoms linked to hydrogen atoms were assigned
based on the observed correlations in the HMQC spectrum. The signals that did not show
any correlation in the HMQC spectrum were those that were not linked to protons. Thus,
(A) SOLVENT ANALYSIS (B) pH ANALYSIS
Minutes
pH: 2.5
pH: 4.0
pH: 5.0
pH: 6.8
1 2
0.0 1.0 2.0 3.0 4.0 5.0 6.0
H2O
Minutes
12
MeOH
EtOH
ACN
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Figure 3.
RP-HPLC monitoring of the reaction between Cys and maleimidehexanoic acid (Mhx).
(
A
) Reaction in different solvents (H
2
O, MeOH, EtOH, and ACN). (
B
) Reaction in aqueous solution
at different pH values: 2.5, 4.0, 5.0 and 6.8. Peaks 1 and 2 correspond to the reaction products.
The unexpected formation of two products by means of the methodology adapted from
the literature contrasts with results obtained previously [
3
,
16
,
21
], in which it was reported
that thiomethylation was the only reaction to occur. The formation of two products with
the same mass, as evidenced in the chromatogram, is an interesting result. We considered
two possible explanations, viz., the formation of a diasteroisomer mixture or of a tautomer
mixture. To establish which of the two reaction routes was used, we proceeded to separate
the two products via the RP-SPE technique [
22
] (see the experimental section). The isolated
products exhibited a great tendency to be hygroscopic; for this reason, their characterization
was only carried out in solution.
The mixture and the purified products were analyzed by means of NMR spectroscopy.
The analysis of the
1
H-NMR spectrum of the unpurified mixture showed two products
corresponding to the isomeric mixture that presented well-resolved signals in the aliphatic
region, which allowed easy assignment of the two isomers and even their molar ratios in
the crude product (50/50) (Figure 4). After separating the isomers, the NMR analysis of
each of the products obtained was carried out.
The
1
H-NMR spectra, in Methanol-d
6
, of the more polar compound (peak 1 in Figure 3)
showed three signals, at 3.14, 3.70, and 4.23 ppm, corresponding to the protons in the
maleimide ring. The signals corresponding to the cysteine residue were seen at 4.02 for
the alpha proton and at 3.25 and 2.55 for the diasterotopic protons. Additionally, the
1
H-NMR spectrum displayed characteristic signals of hexanoic acid proton residue at 1.34,
1.60, 2.29, and 3.51 ppm. The
13
C-NMR spectrum showed thirteen signals, which were
unambiguously assigned through 1D- and 2D-NMR experiments, including the HMQC
and HMBC spectra. The signals for carbon atoms linked to hydrogen atoms were assigned
based on the observed correlations in the HMQC spectrum. The signals that did not show
any correlation in the HMQC spectrum were those that were not linked to protons. Thus, all
the patterns were consistent with the expected Michael thiol-maleimide addition product,
Compound
1
(Cys-S-Mhx, Figure 4). On the other hand, the spectrum of the second
product, Compound
2
, displayed essentially the same signals for the cysteine residue and
for the hexanoic acid residue; however, the signals for the protons of the maleimide ring
showed certain differences, particularly in their chemical shifts. In this way, three different
proton moieties for the maleimide ring, with two signals at 3.29 and 3.48 ppm assigned
to the methylen group in the maleimide ring, and one signal at 4.40 ppm corresponding
to the methyne proton in the maleimide ring, indicated that the ring was attached to the
Molecules 2022,27, 5064 5 of 10
nitrogen of the cysteine residue. Like Compound 1, the
13
C-NMR spectrum of the second
product showed thirteen signals, which were unambiguously assigned through
1
D- and
2
D-NMR experiments, including the HMQC and HMBC spectra. In this way, the signal
at 52.89 ppm allowed confirmation of the connectivity of the cysteine with the maleimide
ring by means of nitrogen; additionally, this signal was not observed in the
13
C-NMR
spectrum of Compound
1
. The complete signal assignment and the observed correlations
for Compound 2are shown in Table 1.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
all the patterns were consistent with the expected Michael thiol-maleimide addition prod-
uct, Compound 1 (Cys-S-Mhx, Figure 4). On the other hand, the spectrum of the second
product, Compound 2, displayed essentially the same signals for the cysteine residue and
for the hexanoic acid residue; however, the signals for the protons of the maleimide ring
showed certain differences, particularly in their chemical shifts. In this way, three differ-
ent proton moieties for the maleimide ring, with two signals at 3.29 and 3.48 ppm assigned
to the methylen group in the maleimide ring, and one signal at 4.40 ppm corresponding
to the methyne proton in the maleimide ring, indicated that the ring was attached to the
nitrogen of the cysteine residue. Like Compound 1, the 13C-NMR spectrum of the second
product showed thirteen signals, which were unambiguously assigned through 1D- and
2D-NMR experiments, including the HMQC and HMBC spectra. In this way, the signal at
52.89 ppm allowed confirmation of the connectivity of the cysteine with the maleimide
ring by means of nitrogen; additionally, this signal was not observed in the 13C-NMR spec-
trum of Compound 1. The complete signal assignment and the observed correlations for
Compound 2 are shown in Table 1.
Figure 4. Comparison of NMR spectra in DMSO-d6, for mixtures of products and individual ad-
ducts.
Table 1. Correlations for Compound 2.
Carbon
δ (ppm)
Correlation HMQC
Correlation HMBC
1
175.99
---
1.62, 2.29
2
33.22
2.29
--
3
26.73
1.62
--
4
25.80
1.34
--
5
26.73
1.62
--
Cys-N-Mhx
Cys-S-Mhx
Mix
Figure 4.
Comparison of NMR spectra in DMSO-d
6
, for mixtures of products and individual adducts.
Table 1. Correlations for Compound 2.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
all the patterns were consistent with the expected Michael thiol-maleimide addition
product, Compound 1 (Cys-S-Mhx, Figure 4). On the other hand, the spectrum of the
second product, Compound 2, displayed essentially the same signals for the cysteine
residue and for the hexanoic acid residue; however, the signals for the protons of the
maleimide ring showed certain differences, particularly in their chemical shifts. In this
way, three different proton moieties for the maleimide ring, with two signals at 3.29 and
3.48 ppm assigned to the methylen group in the maleimide ring, and one signal at 4.40
ppm corresponding to the methyne proton in the maleimide ring, indicated that the ring
was attached to the nitrogen of the cysteine residue. Like Compound 1, the 13C-NMR
spectrum of the second product showed thirteen signals, which were unambiguously
assigned through 1D- and 2D-NMR experiments, including the HMQC and HMBC
spectra. In this way, the signal at 52.89 ppm allowed confirmation of the connectivity of
the cysteine with the maleimide ring by means of nitrogen; additionally, this signal was
not observed in the 13C-NMR spectrum of Compound 1. The complete signal assignment
and the observed correlations for Compound 2 are shown in Table 1.
Figure 4. Comparison of NMR spectra in DMSO-d6, for mixtures of products and individual
adducts.
Table 1. Correlations for Compound 2.
Carbon δ (ppm) Correlation HMQC Correlation HMBC
1 175.99 --- 1.62, 2.29
2 33.22 2.29 --
3 26.73 1.62 --
4 25.80 1.34 --
5 26.73 1.62 --
Cys-N-Mhx
Cys-S-Mhx
Mix
N
O
O
H
N
OH
O
SH
HO
O1
23
4
5
6
7
8
9
10
11
12
13
Carbon δ(ppm) Correlation HMQC Correlation HMBC
1 175.99 — 1.62, 2.29
2 33.22 2.29 –
3 26.73 1.62 –
4 25.80 1.34 –
5 26.73 1.62 –
6 38.46 3.51 –
7 178.41 — 3.51
8 174.95 — 2.56, 3.29, 3.51
9 32.99 3.29, 3.48 –
10 52.89 4.40 –
11 35.27 2.56, 3.28 –
12 41.09 4.03 2.56, 3.29
13 168.98 — –
Based on this information from NMR and from the mass spectra, the products formed
corresponded to a mixture of tautomers. The formation of the two isomers can be explained
as follows. Initially, the reaction of the Michael addition, which took place by means of
Molecules 2022,27, 5064 6 of 10
nucleophilic attack of the thiol group of the cysteine on the maleimide ring, generated
Compound
1
. Then, the presence of an amino group in the cysteine residue favored an
intramolecular nucleophilic substitution of the thioether by the amino group, forming
a cyclic intermediary, which finally generated Compound
2
, Cys-NH-Mhx, as shown in
Figure 5, as described in previous papers [3,17].
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
6
38.46
3.51
--
7
178.41
---
3.51
8
174.95
---
2.56, 3.29, 3.51
9
32.99
3.29, 3.48
--
10
52.89
4.40
--
11
35.27
2.56, 3.28
--
12
41.09
4.03
2.56, 3.29
13
168.98
---
--
Based on this information from NMR and from the mass spectra, the products formed
corresponded to a mixture of tautomers. The formation of the two isomers can be ex-
plained as follows. Initially, the reaction of the Michael addition, which took place by
means of nucleophilic attack of the thiol group of the cysteine on the maleimide ring, gen-
erated Compound 1. Then, the presence of an amino group in the cysteine residue favored
an intramolecular nucleophilic substitution of the thioether by the amino group, forming
a cyclic intermediary, which finally generated Compound 2, Cys-NH-Mhx, as shown in
Figure 5, as described in previous papers [3,17].
Figure 5. Possible formation of the cyclic intermediate between Compounds 1 and 2.
To corroborate the results obtained by NMR and MS that pointed toward the for-
mation of tautomers, a final study was carried out in which the amino acid Fmoc-Cys-OH
was used. If the reaction products correspond to tautomers, when the Fmoc-Cys-OH re-
acts with the Mhx, the amino group is blocked by the Fmoc group, so tautomerism is not
possible and a single reaction product should appear. This reaction was carried out in an
aqueous medium and was monitored by means of RP-HPLC. As can be seen (Figure 6,
peak 3), at 6.75 min a single product was observable. This result strengthens the hypoth-
esis that the products corresponded to tautomers.
Figure 5. Possible formation of the cyclic intermediate between Compounds 1and 2.
To corroborate the results obtained by NMR and MS that pointed toward the formation
of tautomers, a final study was carried out in which the amino acid Fmoc-Cys-OH was
used. If the reaction products correspond to tautomers, when the Fmoc-Cys-OH reacts with
the Mhx, the amino group is blocked by the Fmoc group, so tautomerism is not possible
and a single reaction product should appear. This reaction was carried out in an aqueous
medium and was monitored by means of RP-HPLC. As can be seen (Figure 6, peak 3), at
6.75 min a single product was observable. This result strengthens the hypothesis that the
products corresponded to tautomers.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
Figure 6. Fmoc-Cys-OH and Mhx reaction. Monitoring by RP-HPLC: panels (A) and (B) correspond
to starting material, panel (C) shows the reaction mixture.
Finally, to establish if this behavior was evidenced with other substrates, we decided
to observe the reaction between L-cysteine and N-(2-aminoethyl)maleimide under the
same experimental conditions (H2O/MeOH 80:20). As can be seen in Figure 7, two peaks
with very similar retention times were observed in the chromatogram, allowing the con-
clusion that the reaction proceeded to form the same types of products.
Figure 7. Chromatogram of the reaction mixture between L-cysteine and N-(2-aminoethyl)malei-
mide in H2O/MeOH 80:20.
Minutes
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3
Mhx
A
B
C
Minutes
0 1 2 3 4 5 6 7 8 9 10
mAU
0
100
200
300
400
500
1.78
2.05
Figure 6.
Fmoc-Cys-OH and Mhx reaction. Monitoring by RP-HPLC: panels (
A
,
B
) correspond to
starting material, panel (C) shows the reaction mixture.
Molecules 2022,27, 5064 7 of 10
Finally, to establish if this behavior was evidenced with other substrates, we decided
to observe the reaction between L-cysteine and N-(2-aminoethyl)maleimide under the same
experimental conditions (H
2
O/MeOH 80:20). As can be seen in Figure 7, two peaks with
very similar retention times were observed in the chromatogram, allowing the conclusion
that the reaction proceeded to form the same types of products.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 5
Figure 6. Fmoc-Cys-OH and Mhx reaction. Monitoring by RP-HPLC: panels (A) and (B) correspond
to starting material, panel (C) shows the reaction mixture.
Finally, to establish if this behavior was evidenced with other substrates, we decided
to observe the reaction between L-cysteine and N-(2-aminoethyl)maleimide under the
same experimental conditions (H2O/MeOH 80:20). As can be seen in Figure 7, two peaks
with very similar retention times were observed in the chromatogram, allowing the con-
clusion that the reaction proceeded to form the same types of products.
Figure 7. Chromatogram of the reaction mixture between L-cysteine and N-(2-aminoethyl)malei-
mide in H2O/MeOH 80:20.
Minutes
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3
Mhx
A
B
C
Minutes
0 1 2 3 4 5 6 7 8 9 10
mAU
0
100
200
300
400
500
1.78
2.05
Figure 7.
Chromatogram of the reaction mixture between L-cysteine and N-(2-aminoethyl)maleimide
in H2O/MeOH 80:20.
3. Materials and Methods
3.1. Reagents and Materials
L-Cysteine, 6-maleimidohexanoic acid, trifluoroacetic acid (TFA), acetonitrile (ACN),
methanol (MeOH), and ethanol (EtOH) were purchased from Sigma-Aldrich (St. Louis,
MO, USA). Methanol-d6was obtained from Merck (Darmstadt, Germany).
3.2. General Procedure for the Reaction of Cysteine and 6-Maleimidohexanoic Acid
14.6 mg of L-cysteine and 28.2 mg of 6-maleimidohexanoic acid were mixed in water-
methanol 95:5 at pH 6.8 and stirred for 3 min. The mixture was analyzed via RP-HPLC, the
two products obtained were purified and enriched via SPE, and were analyzed individually
via RP-HPLC, ESI-MS, NMR
1
H,
13
C, and two-dimensional NMR.
1
H-NMR and
13
C-NMR
spectra were recorded at 400 MHz on a Bruker Avance 400 instrument. Chemical shifts
were reported in ppm, using the solvent residual signal. Molar mass was determined
on a Thermo Scientific HPLC-MS, RP–HPLC analyses were performed using an Agilent
1200 Liquid Chromatograph (Agilent, Omaha, NE, USA), and for the DAD study a Thermo
Scientific Ultimate 3000 HPLC was used. The product was characterized via
1
H NMR,
13
C
NMR, MS, and HPLC. The following was obtained:
6-(3-((2-amino-2-carboxyethyl)thio)-2,5-dioxopyrrolidin-1-yl)hexanoic acid (1)
1
H-
NMR, Methanol-d
4
,
δ
(ppm): 1.26 (d, 12H, J= 8Hz, CH3), 4.42 (q, 4H, J= 8Hz, CH),
6.11 (s, 4H, H orto to OH), 6.73 (s, 4H, H meta to OH), 8.53 (s, 8H, OH);
13
C-NMR, Methanol-
d
4
,
δ
(ppm): 20.3; 29.1; 104.0; 124.8; 126.4; 152.7. ESI–TOF/MS analysis showed a signal at
m/z = 333.1118 corresponding to [M + H]+, calculated m/z333.1120
6-(3-((1-carboxy-2-mercaptoethyl)amino)-2,5-dioxopyrrolidin-1-yl)hexanoic acid (2)
1
H-
NMR, DMSO-d
6
,
δ
(ppm): 0.85 (t, 12H, J= 7Hz, CH
3
), 1.19 (m, 8H, CH
2
), 1.28 (m, 16H,
CH
2
), 2.03 (d, 8H, CH
2
) 4.24 (t, 4H, J= 6.8 Hz, CH), 6.17 (s, 4H, H orto to OH), 7.16 (s, 4H,
Hmeta to OH), 8.90 (s, 8H, OH).
13
C-NMR, DMSO-d
6δ
(ppm): 14.1; 22.4; 27.6; 31.6; 33.1;
34.1; 102.5; 123.2; 124.9; 151.8. ESI–TOF/MS analysis showed a signal at m/z = 333.1117
corresponding to [M + H]+, calculated m/z333.1120
Molecules 2022,27, 5064 8 of 10
3.3. Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC) Analysis
For the reaction products between 6-maleimidohexanoic acid and L-cysteine RP-
HPLC analysis, 10
µ
L solution of each evaluated reaction condition was injected. A linear
gradient from 5% to 50% of solvent B (ACN-TFA 0.05%) in solvent A (H
2
O-TFA 0.05%) on
a monolithic Chromolith
®
C-18 column (Merck KGaA, Darmstadt, Germany, 50
×
100 mm)
for 8 min at a wavelength of 210 nm was used. For the DAD study, a Thermo Scientific
Ultimate 3000 DAD-HPLC was used, with 10
µ
L of the Cys–Maleimido reaction mixture
taken in H
2
O and analyzed by means of the linear gradient previously described, using a
Thermo Scientific Eclipse C-18 packed column (100 mm
×
4.5
µ
m). The reaction peaks, at a
length of 190–270 nm, were analyzed by means of DAD.
3.4. RP-SPE Purification
To enrich the reaction products obtained in the reaction, the method previously de-
scribed by Insuasty et al. was used [
22
]. Briefly, an RP-SPE (Supelclean
TM
) column was
activated with 30 mL of methanol, 30 mL of ACN-TFA (0.05%), and 30 mL of H
2
O-TFA
(0.05%). Subsequently, 1 mL of solution containing the mixture obtained in the previous
section was added, and the elution of the products was performed by increasing the quan-
tity of solvent B in the eluent by 12 mL fractions. Analysis was completed by means of
RP-HPLC, and the fractions in which the reaction products were more enriched were frozen
and later lyophilized for storage.
3.5. LC-MS Methodologies
The Cys–Mal mixture reaction samples were prepared according to the previous
general procedure. The mixture reaction was centrifuged at 15,000 rpm for 3 min at
room temperature, the supernatant was diluted 1000 times, and 2
µ
L was analyzed in a
Bruker Impact II LC Q-TOF MS equipped with electrospray ionization (ESI) in positive
mode. The chromatographic conditions were maintained with an Intensity Solo C18 col-
umn (
2.1 ×100 mm
, 1.8
µ
m) (Bruker Daltonik, Billerica, MA, USA), at a temperature of
40
◦
C and a flow rate of 0.250 mL min
−1
. Mobile phase water (A) and acetonitrile (B)
were used, each containing 0.1% formic acid. Gradient elution was 5/5/95/95/5/5%B
at 0/1/11/13/13.1/15 min. ESI source conditions: end plate offset 500 V, capillary
4500 V, nebulizer 1.8 bar, dry gas nitrogen 8.0 L/min, dry temp 220
◦
C. Scan mode Au-
toMS/MS with spectral range
20–1000 m/z
, spectra rate 2 Hz, collision energy 5.0 eV (See
Supplementary Materials).
4. Conclusions
The reaction between Cys and Mhx is a rapid and quantitative reaction that is favored
in protic solvents and occurs over a wide pH range (2.5 to 8), under mild conditions. Our
research unexpectedly found that this reaction generated two reaction products, and by
means of NMR, MS, and HPLC, it was possible to verify that these corresponded to the
tautomers Cys-S-Mhx, where a rearrangement of the S and N atoms occurred. This is an
interesting finding, since the Michael reaction between the thiol group and the maleimide
group is a well-known and widely-used reaction with various biochemical applications,
but up to now the formation of two reaction products corresponding to tautomers had
never been reported.
Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10
.3390/molecules27165064/s1, Figure S1:
1
H-NMR spectrum (400 MHz, MeOD, 303 K) of compound
1
, Figure S2.
13
C-NMR spectrum (400 MHz, MeOD, 303 K) of compound
1
, Figure S3. MS/MS for
TIC of 4.7 min corresponding to compound
1
, Figure S4.
1
H-NMR spectrum (400 MHz, MeOD, 303 K)
of compound
2
, Figure S5.
13
C-NMR spectrum (400 MHz, MeOD, 303 K) of compound
2
,
Figure S6
.
1
H-NMR COSY spectrum (400 MHz, MeOD, 303 K) of compound
2
, Figure S7.
1
H-NMR HSQC
spectrum (400 MHz, MeOD, 303 K) of compound
2
, Figure S8.
1
H-NMR HMBC spectrum (400 MHz,
Molecules 2022,27, 5064 9 of 10
MeOD, 303 K) of compound 2, Figure S9. MS/MS for TIC of 4.6 min corresponding to compound 2,
Figure S10. 1H-NMR spectrum (400 MHz, MeOD, 303 K) of mix products.
Author Contributions:
Conceptualization, M.M. and Z.J.R.-M.; methodology, V.A.N.-R. and D.S.I.-C.;
formal analysis, V.A.N.-R., D.S.I.-C., M.M. and Z.J.R.-M.; investigation, V.A.N.-R. and D.S.I.-C.; data
curation, M.M., V.A.N.-R. and D.S.I.-C.; writing—original draft preparation, V.A.N.-R., D.S.I.-C. and
M.M.; writing—review and editing, Z.J.R.-M. and M.M.; supervision, M.M. and Z.J.R.-M.; project
administration, M.M.. and Z.J.R.-M.; funding acquisition, M.M. and Z.J.R.-M. All authors have read
and agreed to the published version of the manuscript.
Funding:
This research was funded by MINCIENCIAS, Project: “Diseño y obtención de nuevos
agentes antibacterianos basados en dendrímeros péptido-resorcinareno: Una alternativa para com-
batir la resistencia bacteriana”. Grant number RC-846-2019 and to the Universidad Nacional de
Colombia-Sede Bogotáfor the doctoral scholarship “Asistente Docente”.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Not available.
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