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DOI: 10.1002/chem.201604391
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Chemical Biology
Rethinking Cysteine Protective Groups: S-Alkylsulfonyl-l-
Cysteines for Chemoselective Disulfide Formation
Abstract: The ability to reversibly cross-link proteins and
peptides grants the amino acid cysteine its unique role in
nature as well as in peptide chemistry. We report a novel
class of S-alkylsulfonyl-l-cysteines and N-carboxy anhydrides
(NCA) thereof for peptide synthesis. The S-alkylsulfonyl
group is stable against amines and thus enables its use
under Fmoc chemistry conditions and the controlled poly-
merization of the corresponding NCAs yielding well-defined
homo- as well as block co-polymers. Yet, thiols react immedi-
ately with the S-alkylsulfonyl group forming asymmetric di-
sulfides. Therefore, we introduce the first reactive cysteine
derivative for efficient and chemoselective disulfide forma-
tion in synthetic polypeptides, thus bypassing additional
protective group cleavage steps.
Introduction
of disulfide bonds appears to be highly desirable in the devel-
opment of vaccines, drug or gene delivery systems.[21–23]
Until now, disulfide formation is accomplished either by oxi-
dation of thiols,[24–26] which is time consuming, incomplete and
does not allow the formation of asymmetric disulfides, or by
the formation of reactive thiols (chlorinated thiol and nitroso-
thiols),[27,28] disulfides (2- and 4-nitrophenyl (Nps),[29,30] 2- and 4-
pyridyl[31,32] and 3-nitro-2-pydidyl (Npys)[33,34]) or S-phenylsulfo-
nates in the final polypeptide.[35,36] Activated thiols or disul-
fides, however, share a drawback, consisting of their limited
stability against hydrolysis and aminolysis which prohibits their
use in peptide synthesis and NCA polymerization. Therefore,
a protective group for thiols, which combines stability during
peptide synthesis with the ability to form disulfide bonds by
a chemoselective reaction, appears highly desirable.[18,19] We
have recently reported that S-phenylsulfonyl cysteines show
better, but insufficient, stability towards amines compared to
activated disulfides in NCA polymerization.[37] The critical chal-
lenge is to achieve the delicate balance between stability
against amine and hydroxyl nucleophiles and reactivity to-
wards thiols.
The incorporation of cysteines into peptides by solid phase
peptide synthesis (SPPS) or during polymerization of a-amino
acid N-carboxyanhydrides (NCAs) commonly relies on the pro-
tection of the thiol group in order to avoid side reactions. Nu-
merous protective groups enable peptide synthesis or NCA
polymerization (e.g., benzyl, trityl or acetamidomethyl thioeth-
ers along with tert-butyl disulfides),[1] but in order to retain the
free thiol group for further peptide modification, a cleavage
step is mandatory.
For rapid and large-scale synthesis of synthetic polypeptides
in the field of NCA polymerization,[2–8] a variety of reactive
amino acid derivatives as well as the corresponding NCAs have
been established and implemented in functional polypeptides,
providing a remarkable chemical diversity using post polymeri-
zation modification reactions.[9,10] This chemical toolbox covers
multiple functional groups for chemoselective modification, for
example, alkylation,[11] azide–alkyne,[12] thiol-ene and -yne reac-
tions.[13–15] However, none of them can be applied to direct
chemoselective disulfide bond formation. Regarding the conju-
gation of synthetic peptides to proteins or synthetic poly-
mers,[16,17] a protective group for thiols combining stability
during peptide synthesis with the ability to form disulfide
bonds by a chemoselective reaction[18,19] appears favorable. Di-
sulfide bond formation is particularly attractive for a variety of
biological and medical applications since disulfides are stable
extracellularly but cleavable inside cells.[20] Although this char-
acteristic may not be required in all cases, the bioreversibility
Results and Discussion
In this work we introduce the use of a novel class of S-(alkylsul-
fonyl)-l-cysteines (Figure 1) to address this issue. The S-alkylsul-
[a] O. Schꢀfer,+ Dr. D. Huesmann,+ C. Muhl, Dr. M. Barz
Institute of Organic Chemistry
Johannes Gutenberg University Mainz 55099 Mainz (Germany)
[+] These authors contributed equally to this work.
Supporting information and ORCID from the author for this article are
Figure 1. S-(Ethylsulfonyl)-l-cysteine (Cys(SO2Et), 2a) and S-(isopropylsulfon-
yl)-l-cysteine (Cys(SO2iPr), 2b).
Chem. Eur. J. 2016, 22, 1 – 8
1
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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