Selenosulfides Tethered to gem-Dimethyl Esters: A Robust and Highly Versatile Framework for H2S Probe Development

Article abstract of DOI:10.1002/chem.201904133

Selenosulfides coupled to gem-dimethyl esters provide an exceptional platform for H₂S probe development. With the sulfur half being nonessential to its high reactivity and selectivity towards H₂S, we highlight the unique flexibility of our design by improving its biocompatibility and tissue specificity through structural modifications of its sulfide moiety.

Full text of DOI:10.1002/chem.201904133

A Journal of
Accepted Article
Title: Selenosulfides Tethered to Gem-Dimethyl Esters: A Robust and
Highly Versatile Framework for H₂S Probe Development.
Authors: S. Israel Suarez, Rynne Ambrose, Madison A. Kalk, and John
C. Lukesh III
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201904133
Link to VoR: http://dx.doi.org/10.1002/chem.201904133
Supported by

10.1002/chem.201904133
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COMMUNICATION
Selenosulfides Tethered to Gem-Dimethyl Esters: A Robust and
Highly Versatile Framework for H2S Probe Development**
S. Israel Suarez, Rynne Ambrose, Madison A. Kalk, and John C. Lukesh III*
Abstract: Selenosulfides coupled to gem-dimethyl esters provide an
exceptional platform for H2S probe development. With the sulfur half
being nonessential to its high reactivity and selectivity towards H2S,
we highlight the unique flexibility of our design by improving its
biocompatibility and tissue specificity through structural modifications
of its sulfide moiety.
Still, with low water solubility and inefficient intracellular
delivery, many H2S-reactive probes have proven unsuccessful in
their ability to function in a biological setting. The extreme
hydrophobic nature of some even required the use of an organic
co-solvent (such as acetonitrile) for in vitro assays or surfactants
for cellular assays.[6] Therefore, the continued development of
reaction-based
fluorescent
probes
with
improved
physicochemical properties is highly desirable and would provide
a significant contribution to the chemical toolkit that is currently
available for studying H2S physiology.
Hydrogen sulfide (H2S), once regarded as merely a toxic and foul-
smelling gas, has recently been recognized as an endogenous
signaling molecule and important physiological mediator within
mammalian systems.[1–3] Although several H2S-regulated
processes have been discovered in recent years, questions
surrounding its metabolism, distribution, and prevalence still
persist. Therefore, the development of new bioanalytical methods
for probing the biological signaling pathways of H2S is needed to
better discern its primary and secondary roles as a signaling
molecule. To this end, reaction-based fluorescent probes have
proven to be invaluable chemical tools for aiding H2S research by
providing a non-destructive method for the real-time detection and
visualization of H2S within biological samples.[4–10] Developing
reaction-based probes that can selectively detect H2S within a
biological setting, however, does pose significant challenges.
With other biological thiols being present in appreciably higher
concentrations (e.g. glutathione), reaction-based probes must
effectively key in on reactivity differences that distinguish H2S
from these other species. With a suppressed pKa (6.8) relative to
that of other biothiols (pKa: ~8–9), H2S resides predominately in
its reactive HS− form under physiological conditions, rendering it
a vastly superior reductant and nucleophile at neutral pH.[11]
Chang and co-workers were the first to develop a turn-on
fluorescent probe that capitalized on this increased reactivity with
their design of an azide-caged rhodamine dye.[12] With the
addition of H2S, reduction of the aryl azide to the aryl amine
afforded a significant increase in fluorescence intensity upon a
30–60 min incubation period. In an alternative approach, Xian and
He were the first to develop reaction-based probes that relied on
cascade processes to release a fluorescent reporter.[13–18] These
probes also displayed good selectivity as they effectively
exploited the double nucleophilic character of H2S—another
distinctive feature in terms of its reactivity.
Taking inspiration from these earlier designs, we set out to
develop chemical probes that would not only display remarkable
reactivity and selectivity towards H2S, but unprecedented
modularity, such that issues related to insufficient water solubility,
cell permeability, and/or tissue specificity could be easily
addressed. To achieve this desired versatility without comprising
reactivity and selectivity, we conjectured that the employment of
a selenosulfide functionality would be key for accessing a probe
H2S-Sensing Moiety
O
O
S
O
O
O
Se
N
H
AL1
High Biocompatibility
Physicochemical
Modulator
Ac2O, Et3N
DCM
70%
O
S
O
O
O
Se
NH3
1
Biotin, EDCI, DMAP
DMF
50%
H2S-Sensing Moiety
O
O
O
[*]
S. Israel Suarez, Rynne Ambrose, Madison A. Kalk, and Prof. John
C. Lukesh III
Department of Chemistry, Wake Forest University
Wake Downtown Campus
HN
H
S
NH
O
O
O
Se
N
H
S
H
Winston-Salem, NC 27101
E-mail: lukeshjc@wfu.edu
AL2
Physicochemical
Modulator
Cancer Cell-Specificity
[**]
We are grateful for financial support from Wake Forest University
(start-up funds). We thank Prof. S. Bruce King for helpful
discussions and Dr. Heather Brown-Harding for assistance with
microscopy studies. This paper is dedicated to Prof. Ron Raines on
the occasion of his 60-ish birthday.
Scheme 1. Synthesis of versatile, reaction-based fluorescent probes for H2S
detection.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201904133
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O
O
O
H2S
S
SH
O
O
O
Se
N
R
O
O
O
Se
H
1
2
Glutathione
(GSH)
H2S
Cyclization
O
O
S
Se
SG
+
O
O
OH
O
O
O
Se
Fluorescent Reporter
3
4
Scheme 2. Proposed mechanism for H2S detection with AL1.
Figure 1. Fluorescence response (흀ex: 330 nm) of probes (20 M) in PBS buffer
(pH 7.4) and in the presence of various biological thiols and nucleophiles during
a 10 min incubation period: (1) PBS buffer alone; (2) 50 M Na2S; (3) 1 mM
glutathione; (4) 100 M cysteine; (5) 100 M serine; (6) 100 M lysine; (7) 1 mM
glutathione + 50 M Na2S. Plotted as the mean +/− STD from three independent
experiments. (A): Probe AL1 and (B): Probe AL3.
with these desired attributes. Although selenosulfides have been
rarely reported within the chemical literature,[19,20] we reasoned
that reactivity differences between selenium and sulfur could be
exploited such that the selenium half—when tethered to a second
electrophilic partner—would effectively scavenge and detect H2S,
while the sulfur half would provide a suitable handle for mediating
its physicochemical properties. Therefore, we first synthesized
To assess our hypothesis, the selectivity of our design
towards H2S was initially gauged by incubating AL1 (20 M) with
various biological thiols and nucleophiles in PBS buffer (Figure
1A). After reacting for 10 min at room temperature, the resulting
turn-on response was measured using fluorescence
spectroscopy. The results from this assay were indeed stunning.
In the presence of 50 M Na2S (an H2S equivalent at
physiological pH) a greater than 100-fold increase in relative
fluorescence intensity was observed. Yet, in the presence of other
biologically relevant thiols and nucleophiles, no increase in
fluorescence was detected, even when significantly higher
concentrations were used. Moreover, this assay confirmed that
AL1 is not consumed by the presence of other thiols and that its
reactivity towards H2S is retained in their presence (Figure 1A,
Column 7). To confirm that the gem-dimethyl group affords
enhanced differentiation, we also generated AL3 which lacks the
steric shield alpha to the ester carbonyl. The same selectivity
studies with AL3 unequivocally
selenosulfide
1 from 3-bromopivalic acid (see supporting
information) as it provided an amino group for subsequent
conjugation reactions. And in an initial effort to afford high
biocompatibility, we then appended an acetyl group to access
probe AL1 (Scheme 1).
Our proposed mechanism for H2S detection by AL1 is
depicted in Scheme 2. We hypothesized that an initial exchange
reaction between H2S and AL1 would quickly generate 2 and N-
acetylcysteamine as an innocuous byproduct (note: cysteamine
is a prescribed therapeutic for the treatment of corneal cystinosis
given its low toxicity and superb antioxidant activity).[21] With
selenium’s heightened electrophilic character, nucleophiles will
typically attack at selenium much more rapidly than at sulfur.[22] In
fact, exchange reactions at selenium have been reported to be
orders of magnitude faster than analogous exchange reactions
where sulfur serves as the electrophilic partner.[23,24] This feature
was thought to ensure that our probe would not be consumed by
other biothiols—a serious limitation of earlier probes—as their
attack at selenium would simply regenerate
a
similar
selenosulfide poised for nucleophilic attack by H2S (4, Scheme
2).[14,18] We predicted that once intermediate 2 was formed,
however, a rapid cyclization event would release the fluorescent
reporter and thus confirm the presence of H2S.
Based on this proposed mechanism, implementation of the
gem-dimethyl group was predicted to serve several distinct
functions. For one, it was expected to operate as a steric shield,
blocking access to the ester carbonyl and shutting down
hydrolysis and the direct nucleophilic attack of other biothiols and
nucleophiles. In turn, this would reduce background fluorescence
and increase the overall selectivity of the probe. Indeed, the
introduction of gem-dimethyl groups is a common technique used
in medicinal chemistry to improve plasma and chemical stability
of biologically active
compounds.[25]
Secondly, while
intermolecular attack at the ester would be diminished, we
theorized that the requisite cyclization event was likely to be both
thermodynamically and kinetically enhanced by the presence of
those same methyl groups as introduction of alkyl substituents is
known to favor intramolecular reactions (i.e. The Thorpe-Ingold
Effect).[26]
Figure 2. Fluorescence response (흀ex: 330 nm) of probes (20 M) under varying
conditions: (A and B): (1) 24-hour incubation period in PBS buffer alone; (2) 24
hr incubation period in PBS buffer followed by the addition of Na2S (50 M). (C
and D): (1): 10 min incubation period in fetal bovine plasma alone; (2): 10 min
incubation period in fetal bovine plasma and in the presence of Na2S (50 M).
Plotted as the mean +/− STD from three independent experiments. AL1: 2A and
2C and AL3: 2B and 2D.
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demonstrate the significance of the gem-dimethyl group as a
noticeable reduction in selectivity was observed in their absence
(Figure 1B).
To further establish the increased durability of AL1 relative
to AL3, we tested the stability of both compounds in PBS buffer
over a 24 h incubation period (Figures 2A and 2B). While AL3
was shown to be highly susceptible to hydrolysis in buffer alone
(Figure 2B, Column 1), AL1 exhibited superb stability during the
24 h period (Figure 2A, Column 1). Moreover, the subsequent
addition of Na2S to AL1 resulted in impressive turn-on
fluorescence (Figure 2A, Column 2). These results indicate that
AL1 is not only resistant to hydrolysis, but that its reactivity
towards H2S is preserved, even after prolonged exposure to
buffer. The stability of AL1 and AL3 was also assessed in fetal
bovine plasma (Figures 2C and 2D). Again, AL1 displayed
remarkable stability as little fluorescence was emitted in the
absence of Na2S (Figure 2C, Column 1). Furthermore, even within
this complex environment, AL1 showed a marked turn-on
response when Na2S was present (Figure 2C, column 2). Probe
AL3, on the other hand, underwent significant decomposition in a
very short period of time indicating that its ester moiety is
significantly more labile, especially in the presence of plasma
proteins (Figure 2D, Column 1).
The efficiency of AL1 in sensing H2S in PBS buffer (pH 7.4)
and at room temperature was also assessed (Figure 3A). Under
these conditions, the resulting fluorescence intensity reached a
maximum in less than two minutes, highlighting that our designed
probe is not only selective but also highly reactive towards H2S—
a necessary feature for detecting analytes which are short-lived.
To determine if AL1 could also be used to detect H2S
quantitatively in buffer, the resulting fluorescence was measured
in response to a series of different concentrations of H2S (Figure
3B). Indeed, the ensuing fluorescence intensity increased in a
linear fashion in the range of 0–20 M H2S and with a detection
limit of 0.05 M.
Figure 4. (A) Visualization of H2S in live HeLa cells with AL1 (12.5 M).
Imaging: ex: 405 nm, em: 452 nm. a and b: No Na2S added (control); c and d:
Addition of Na2S (50 M). Scale bar is set to 20 m. (B) Visualization of H2S in
live HeLa cells with AL4 (5 M). Imaging: ex: 561 nm, em: 587 nm. e and f: No
Na2S added (control); g and h: Addition of Na2S (25 M). Scale bar is set to 20
m.
cells was observed. To show that our framework was highly
adaptable, we also generated AL4. After determining that AL4
displayed similar selectivity and reactivity towards H2S (Figures
S1–S3), we also assessed its ability to visualize H2S in live cells.
Probe AL4 utilizes resorufin as its latent fluorescent reporter
which has the added benefit of longer excitation and emission
wavelengths, further boosting its potential in tissue imaging
applications. Under similar conditions, a strong fluorescence was
again observed within HeLa cells upon treatment with AL4 (5 M)
and Na2S (25 M) (Figure 4B).
We next set to out to showcase the high versatility of our
design by demonstrating that we could target selenosulfide 1 to
specific tissue or cell types by simply modifying its sulfide linker.
As a proof of concept, we generated AL2, a cancer cell-specific
Probe AL1 was also shown to be useful in imaging H2S in
live cells using confocal microscopy. Cultured HeLa cells were
incubated with AL1 (12.5 M) for 45 min and then washed to
remove any excess probe. Under these conditions we did not
observe any fluorescent cells, signifying that AL1 was indeed
stable within a cellular environment (Figure 4A). However, upon
treatment with 50 M Na2S, a strong blue fluorescence within
Figure 3. (A) Time-dependent fluorescence emission (흀ex: 330 nm) of AL1 (20
M) in the presence of Na2S (50 M). (B) Fluorescence Intensity (흀ex: 330 nm)
of AL1 (40 M) in response to increasing Na2S concentrations. Plotted as the
mean +/− STD from three independent experiments.
Figure 5. a and b: Fluorescence images of live HeLa cells treated with AL2 (10
M); c and d: Fluorescence images of live HeLa cells treated with AL2 (10 M)
and Na2S (50 M); e and f: Fluorescence images of live HeLa cells pre-treated
with biotin (2 mM) followed by treatments with AL2 (10 M) and Na2S (50 M).
Imaging: ex: 405 nm, em: 452 nm. Scale bar is set to 20 m.
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H2S probe development. In addition to their high reactivity and
selectivity towards H2S, their remarkable versatility allows for their
physicochemical properties to be easily tuned through structural
modifications of their sulfide handle. Given the distinct chemical
differences of both sulfur and selenium, we foresee the use of
selenosulfides as a new strategy for tuning the physicochemical
and biological properties of molecular cargo.
Experimental Section
Supporting information and the ORCID identification number(s) for the
authors of this article can be found under:
Keywords: H2S • selenosulfides • chemical probes • selenium
Figure 6. Fluorescence images of Live HEK293 cells. a and b: Treatment with
AL2 (10 M) alone; c and d: Treatment with AL2 (10 M) followed by treatment
with Na2S (50 M). Imaging: ex: 405 nm, em: 452 nm. Scale bar is set to 20 m.
chemistry
References
[1] H. Kimura, Amino Acids 2011, 41, 113–121.
[2] M. R. Filipovic, J. Zivanovic, B. Alvarez, R. Banerjee,
Chem. Rev. 2018, 118, 1253–1337.
[3] X. Cao, L. Ding, Z. Xie, Y. Yang, M. Whiteman, P. K.
Moore, J.-S. Bian, Antiox. Redox Signal. 2019, 31, 1–38.
[4] J. Chan, S. C. Dodani, C. J. Chang, Nat. Chem. 2012, 4,
973–984.
[5] V. S. Lin, C. J. Chang, Curr. Opin. Chem. Biol. 2012, 16,
595–601.
[6] B. Peng, M. Xian, in Methods in Enzymology, Elsevier,
2015, pp. 47–62.
[7] W. Xuan, C. Sheng, Y. Cao, W. He, W. Wang, Angew.
Chem. Int. Ed. 2012, 51, 2282–2284.
[8] V. S. Lin, W. Chen, M. Xian, C. J. Chang, Chem. Soc. Rev.
2015, 44, 4596–4618.
[9] V. S. Lin, A. R. Lippert, C. J. Chang, in Methods in
Enzymology, Elsevier, 2015, pp. 63–80.
[10] A. R. Lippert, J. Inorg. Biochem. 2014, 133, 136–142.
[11] M. N. Hughes, M. N. Centelles, K. P. Moore, Free Radic.
Biol. Med. 2009, 47, 1346–1353.
H2S probe, by coupling biotin to its amine handle (Scheme 1).[27–
29] Fluorescence imaging experiments were then conducted using
both a biotin positive (HeLa) and biotin negative (HEK293) cell
line to confirm its selective targeting and delivery.[27] As seen in
Figure 5, when HeLa cells were treated with AL2 alone, minimal
fluorescence was witnessed within cells. However, upon the
addition of Na2S, a strong blue fluorescent signal was emitted
from within, confirming both the intracellular delivery of AL2 and
its high selectivity towards H2S. To confirm that AL2 was targeting
biotin cell surface receptors, in a separate experiment, HeLa cells
were pre-treated with biotin prior to the addition of AL2. Under
these conditions, the subsequent addition of Na2S did not produce
any fluorescence within cells, denoting that the intracellular
delivery of AL2 was in fact mediated via biotin transporters
(Figures 5e and 5f). The selectivity of AL2 was further established
using HEK293 (noncancerous human embryonic kidney) cells.
Under identical conditions, negligible fluorescence was observed,
even upon the addition of exogenous Na2S, indicating that the
cellular uptake of AL2 within HEK293 cells (biotin negative) was
much less efficient (Figure 6).
With many forms of cancer having increased expression of
H2S-producing enzymes, cancer-cell specific probes such as AL2
may prove useful in elucidating the roles of H2S in cancer initiation
and progression.[30–35] Additionally, we note that this technology
may also find utility as a diagnostic for certain cancers where both
a specific cell surface receptor (e.g. biotin, folate, B12, etc.) and
H2S are highly over-expressed as the demonstrated tissue
specificity of our H2S-selective probe provides a convenient dual-
biomarker for turn-on fluorescence.[36,37] Finally, given our
proposed mechanism for H2S detection (Scheme 2), and the fact
that 3 is cleanly generated as a byproduct (see supporting
information), scaffolds similar to AL2 may be advantageous for
the specific capture and removal of H2S within cancerous cells
and tissue. Moreover, with high expression levels of H2S being
linked to several diseases, its precise down-regulation may prove
beneficial,[38] highlighting the potential for selenosulfides, and
frameworks akin to AL1 and AL2, for serving as useful
theranostics.
[12] A. R. Lippert, E. J. New, C. J. Chang, J. Am. Chem. Soc.
2011, 133, 10078–10080.
[13] Y. Qian, J. Karpus, O. Kabil, S.-Y. Zhang, H.-L. Zhu, R.
Banerjee, J. Zhao, C. He, Nat. Commun. 2011, 2, 495.
[14] C. Liu, J. Pan, S. Li, Y. Zhao, L. Y. Wu, C. E. Berkman, A.
R. Whorton, M. Xian, Angew. Chem. Int. Ed. 2011, 50,
10327–10329.
[15] C. Liu, B. Peng, S. Li, C.-M. Park, A. R. Whorton, M. Xian,
Org. Lett. 2012, 14, 2184–2187.
[16] B. Peng, W. Chen, C. Liu, E. W. Rosser, A. Pacheco, Y.
Zhao, H. C. Aguilar, M. Xian, Chem. - Eur. J. 2014, 20,
1010–1016.
[17] Z. Huang, S. Ding, D. Yu, F. Huang, G. Feng, Chem.
Commun. 2014, 50, 9185–9187.
[18] B. Peng, C. Zhang, E. Marutani, A. Pacheco, W. Chen, F.
Ichinose, M. Xian, Org. Lett. 2015, 17, 1541–1544.
[19] W. Chen, S. Xu, J. J. Day, D. Wang, M. Xian, Angew.
Chem. Int. Ed. 2017, 56, 16611–16615.
[20] C. C. Tjin, K. D. Otley, T. D. Baguley, P. Kurup, J. Xu, A. C.
Nairn, P. J. Lombroso, J. A. Ellman, ACS Cent. Sci. 2017,
3, 1322–1328.
[21] F. Shams, D. Oladiwura, K. Ramaesh, I. Livingstone,
OPTH 2014, 2077.
[22] H. J. Reich, R. J. Hondal, ACS Chem. Biol. 2016, 11, 821–
841.
In closing, we have successfully shown that selenosulfides
tethered to gem-dimethyl esters are exceptional frameworks for
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10.1002/chem.201904133
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[23] D. Steinmann, T. Nauser, W. H. Koppenol, J. Org. Chem.
2010, 75, 6696–6699.
[32] Q. Cao, L. Zhang, G. Yang, C. Xu, R. Wang, Antiox. Redox
Signal. 2010, 12, 1101–1109.
[24] J. C. Lukesh, B. VanVeller, R. T. Raines, Angew. Chem.
Int. Ed. 2013, 125, 13139–13142.
[33] W. Cai, M. Wang, L. Ju, C. Wang, Y. Zhu, Cell Biol. Int.l
2010, 34, 565–572.
[25] T. T. Talele, J. Med. Chem. 2018, 61, 2166–2210.
[26] M. E. Jung, G. Piizzi, Chem. Rev. 2005, 105, 1735–1766.
[27] W. X. Ren, J. Han, S. Uhm, Y. J. Jang, C. Kang, J.-H. Kim,
J. S. Kim, Chem. Commun. 2015, 51, 10403–10418.
[28] X. Song, B. Dong, X. Kong, C. Wang, N. Zhang, W. Lin,
RSC Adv. 2017, 7, 15817–15822.
[29] M. H. Lee, A. Sharma, M. J. Chang, J. Lee, S. Son, J. L.
Sessler, C. Kang, J. S. Kim, Chem. Soc. Rev. 2018, 47,
28–52.
[34] K. Módis, C. Coletta, A. Asimakopoulou, B. Szczesny, C.
Chao, A. Papapetropoulos, M. R. Hellmich, C. Szabo, Nitric
Oxide 2014, 41, 146–156.
[35] C. Chao, J. R. Zatarain, Y. Ding, C. Coletta, A. A. Mrazek,
N. Druzhyna, P. Johnson, H. Chen, J. L. Hellmich, A.
Asimakopoulou, et al., Mol. Med. 2016, 22, 361–379.
[36] M. F. Hansen, E. Greibe, S. Skovbjerg, S. Rohde, A. C. M.
Kristensen, T. R. Jensen, C. Stentoft, K. H. Kjær, C. S.
Kronborg, P. M. Martensen, Cell. Signal. 2015, 27, 1356–
1368.
[30] C. Szabo, C. Coletta, C. Chao, K. Modis, B. Szczesny, A.
Papapetropoulos, M. R. Hellmich, Proc. Natl. Acad. Sci.
USA 2013, 110, 12474–12479.
[37] C. Zhang, Q.-Z. Zhang, K. Zhang, L.-Y. Li, M. D. Pluth, L.
Yi, Z. Xi, Chem. Sci. 2019, 10, 1945–1952.
[31] D. Wu, W. Si, M. Wang, S. Lv, A. Ji, Y. Li, Nitric Oxide
2015, 50, 38–45.
[38] C. Yang, Y. Wang, E. Marutani, T. Ida, X. Ni, S. Xu, W.
Chen, H. Zhang, T. Akaike, F. Ichinose, et al., Angew.
Chem. Int. Ed. 2019, 58, 10898–10902.
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TOC Graphic
H2S-Sensing Moiety
O
O
Fluorescent
Reporter
S
O
Se
N
t3N
H
E
,
H
O
2
M
c
2S
C
A
Physicochemical
Modulator
Steric
Shield
D
High Biocompatibility
O
O
Fluorescent
Reporter
Fluorescent
Reporter
S
+
S
Se
O
Se
NH3
OH
B
i
o
H2S-Sensing Moiety
t
i
n
,
E
D
C
I
,
D
S
D
O
H2
M
O
O
HN
F
M
A
H
Fluorescent
P
S
NH
Reporter
O
Se
N
H
S
H
Steric
Shield
Physicochemical
Modulator
Cancer Cell-Specificity
This article is protected by copyright. All rights reserved.