Communication
ChemComm
(R2 = H) (Scheme 5). The reaction required warming to 37 1C for 2 h to
ensure complete conversion to the dual modified peptide 22. To
confirm the formation of two cleavable thiomaleimides the conjugate
was treated with 2-mercaptoethanol (100 equiv.) to quantitatively
convert to reduced somatostatin. To demonstrate that this approach
could be employed to access dual functionalised peptides, we con-
structed miniPEG–fluorescein and rhodamine–fluorescein bis-
conjugates (23 and 24). Notably other electrophiles can also be
employed in the second modification step; for example maleimide
was employed affording a dual conjugate in which one of the tags was
cleavable by thiols and the other was not. Trypsin digest of 25
confirmed that the reaction led, as expected, to a mixture of the two
possible regioisomers (see ESI,† Fig. S3).
In conclusion, by varying the leaving groups on a maleimide core,
a range of next generation maleimides can be produced with tuneable
properties. Here a selection of aryloxymaleimides have been demon-
strated to serve as efficient cysteine labelling reagents of attenuated
reactivity. This enables the aryloxymaleimides to be employed in
succinimide bridging, offering a novel strategy for disulfide labelling.
The reversibility of the succinimide bridge in the peptide also enabled
a one-pot dual labelling strategy to be developed.
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7142 | Chem. Commun., 2014, 50, 7139--7142
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