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10.1002/ejoc.202000089
European Journal of Organic Chemistry
Journal Name
COMMUNICATION
precipitate began to form. MALDI mass analysis of this precipitate
demonstrated that it was constituted of a complex mixture
containing some relatively high mass molecules (up to at least 1500).
However, the structure of these products, which in principle could be
polythiazolines (from the reactions of nitriles with aminothiols)26 or
polypeptides, (Cys)n (from thiazoline rings hydrolysis), was not clearly
identified.
Could all the steps from acetaldehyde (or glycoaldehyde) to cysteine
occur in a single site? Probably not. However, one may imagine
various conditions in various places, thus separating reagents which
seem incompatible (such as copper II and H2S which would readily
form copper sulfide), following previously reported proposals.27,28
Fig. 2 MALDI spectrum of poly-Cys formed from 12 (extended in
supplementary information).
The Pinner reaction of nitriles with thiols under aqueous conditions
leads to thioesters.29 We have shown that using this reaction,
aminonitriles can be transformed into aminothioesters under
prebiotic conditions.26 Such chemistry could bridge a prebiotic
chemistry dominated by nitriles and the Strecker reaction, and the
thioester word imagined by de Duve.30 Thioesters derived from
cysteine would be very reactive, giving polymers via a native
chemical ligation process.31 In order to test this possibility, we
synthesized the trifluoroacetate salt of thioester 12 (Scheme 4). The
MALDI mass spectrum of the precipitate resulting from its evolution
(near 170 ppm), but also of new thioesters (near 200 ppm). An
explanation to this observation is that when a thioester is formed on
a non-terminal Cys residue (without free NH2 group) the thioester to
amide rearrangement needed in the native ligation process is not
possible and the thioester bond remains. Eventually, this would
explain the formation of branched polymers, rather than single chain
or large ring polymers.
in water at pH
6 (Fig. 2 and Supplementary Information)
We also polymerized a 3/1 mixture of thioester 7 and the iso-butyl
ester of cysteine, a mixture accidentally obtained in a non-specific
synthesis of 7. In this case, the ester function was expected to be less
reactive than the thioester. The MALDI analysis (shown in
Supplementary Information) of the isolated precipitate indicated
that all single polymer contained an O-iBu group, so that ring
structures are impossible. Polymers containing up to at least 25 Cys
residues were observed. However, we could not distinguish between
completely linear structures, H(Cys)nO-iBu, or branched structures
with one or more chains attached to the main peptide strand by a
thioester link.
demonstrated that this precipitate is a mixture of polycysteines26 of
various lengths regularly separated by the mass of a single Cys
residue, 103. The higher measured mass correspond to a Cys45
structure (containing 4 disulfide bridges). Furthermore, the observed
masses indicate the absence of SEt termini. Thus, the oligomers
cannot be single chain polycysteines, but are probably based on
rings. As the formation of very large rings is highly improbable (e.g.
cyclo-(Cys)45), we propose that each polymer consist of at least one
ring on which medium sized chains are branched thanks to thioester
functions. Indeed in the first steps of the oligomerization process,
when species are still soluble, the 13C NMR of the solution showed
the formation of amides
Finally, in order to test the possibility to use the reaction of nitriles
with aminothiols26 to obtain cysteine-rich polypeptides, we
synthesized CysValCN 13 (Scheme 5). Its polymer was analysed both
by MALDI and by ASAP high-resolution mass spectrometry. MALDI
indicated the formation of a complex mixture of polyamide-
polythiazoline structures. Each group of masses in the spectrum
corresponds to a given number of Cys-Val structures (up to at least
23 Cys-Val). The large number of observed masses is due to the fact
that each formed thiazoline junction may or may not have been
hydrolysed (a Cys residue accounts for 103, a thiazoline for 85),
added to the observation of both M+H and M+Na masses. TheMALDI
spectrum also suggested the formation of macrocycles. This was
confirmed by ASAP analysis (Fig. 3), completed with MS-MS
measurements. 14, 15, 16 and 17 (Scheme 6) were identified this
way.
O
O
DMAP, DCC
Trt
Trt
S
OH
Boc
S
S
SH
DCM, 0°C
HN
HN
Boc
TFA, TIPS
98 %
DCM, rt
quantitative
O
HS
TFA
S
NH3
12.TFA
Scheme 4 Synthesis of thioester 12.
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