An efficient Fmoc-SPPS approach for the generation of thioester peptide precursors for use in native chemical ligation

Article abstract of DOI:10.1002/anie.200705471

(Chemical Equation Presented) Intramolecular activation of C-terminal peptides: Mildly activated N-acylurea peptides are readily formed on the solid support following chain assembly, avoiding over-activation of the C-terminus. The utility of these peptide

Full text of DOI:10.1002/anie.200705471

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DOI: 10.1002/anie.200705471
Peptide Ligation
An Efficient Fmoc-SPPS Approach for the Generation of Thioester
Peptide Precursors for Use in Native Chemical Ligation**
Juan B. Blanco-Canosa and Philip E. Dawson*
The straightforward C-terminal modification of peptides
assembled on a solid support remains a significant challenge
in peptide and protein chemistry. In particular, C-terminal
thioester peptides are important intermediates for the
generation of active esters, amides and hydrazides[1,2] and
are an essential component of many synthetic strategies for
protein synthesis.[3] Currently, the most effective approach for
the synthesis of peptidyl thioesters is the in situ neutralization
protocol for tert-butoxycarbonyl solid-phase peptide synthesis
(Boc-SPPS)[4] using thioester linkers.[2,5] However, many
towards aminolysis and the base lability of previous N-
acylurea forming linkers.[13]
Inspired by this work, we have found that o-aminoanilides
1 are stable synthetic intermediates that can be efficiently
transformed into an aromatic N-acylurea moiety 2[14,16]
following chain elongation (Scheme 1). Since this group has
been previously described as an N-acyl-benzimidazoli-
none,[13–16] we propose the use of the abbreviation Nbz to
indicate this leaving group. The resin-bound acylurea peptide
2 can be deprotected and cleaved from the resin with
trifluoroacetic acid (TFA) using standard acid labile linkers
(e.g. Rink or Wang linkers). The resulting mildly activated
laboratories
use
Fmoc-SPPS
(Fmoc = 9-fluor
enylmethyloxycarbonyl) exclusively and such protocols are
favored when synthesizing glyco- and phospho-
peptides. The thioester linkers used for Boc-
SPPS have limited utility for Fmoc-SPPS due to
the requirement for repeated Fmoc removal
under basic conditions. Considerable effort has
been applied to address this challenge[6] includ-
ing optimized Fmoc deprotection cocktails,[7]
thiol labile safety-catch linkers,[8] activation of
protected peptides in solution,[9] and recently
thioesters have been generated using O-to-S[10]
or N-to-S[11] acyl transfer. Despite these notable
advances, the synthesis of thioester peptides by
Fmoc-SPPS remains significantly more chal-
lenging than the synthesis of the corresponding
acid or amide peptide.
Herein, we describe an alternative approach
for the Fmoc synthesis of peptides for use in
native chemical ligation (NCL)[12] that is based
on the formation of a C-terminal N-acylurea
functionality. N-acylureas are mild acylating
agents that have previously been explored in
peptide synthesis by Pascal et al.[13,14] and for
Scheme 1. Synthetic strategy for Fmoc-SPPS of thioester peptides. Following SPPS,
aminoanilide 1 undergoes specific acylation and cyclization to yield the resin-bound
acylurea peptide 2. Following cleavage and deprotection, peptide-Nbz 3 can undergo
thiolysis to yieldpeptide thioester 4, or direct ligation to yield the native peptide 5.
thioamide synthesis by Rapoport et al.[15] and
Zacharie et al.[16] However, the utility of these
groups as acylating agents has been limited due
to the low reactivity of N-acylurea products
peptide-Nbz 3 is stable to the acidic conditions used for
peptide handling and purification. However, in neutral
aqueous buffers, the fully unprotected acylurea peptides
undergo rapid thiolysis, enabling thioester peptide 4 to be
generated before purification or in situ during a native
chemical ligation. Importantly, the linker is a stable amide
during chain assembly and the key activation step utilizes the
most robust reaction in solid phase peptide synthesis: the
acylation of an amine. As a result, the method is compatible
with amino acid side chains and protecting groups commonly
used in peptide synthesis.
[*] Dr. J. B. Blanco-Canosa, Dr. P. E. Dawson
Departments of Chemistry andCell Biology
The Scripps Research Institute
10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax: (+1)858-784-7319
E-mail: dawson@scripps.edu
Homepage: http://www.scripps.edu/cb/dawson/
[**] This work was supportedby the NIH GM059380 (P.E.D.) andthe
Spanish Ministerio de Educación y Ciencia and the Fulbright
Scholar Program (J.B.B-C).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200705471.
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Scheme 2. Synthesis of LYRAG-Nbz. DIEA=N,N-diisopropylethylamine, HBTU=O-(benzotriazol-l-yl)-N,N,N’,N’-tetramethyluronium hexafluoro-
phosphate, HOBt=1-hydroxybenzotriazole, Pbf=2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl, TIS=triisopropylsilane.
To test the method, we first synthesized the peptide
LYRAG-Nbz (Scheme 2). Starting with Rink-PEG-PS resin
(0.2 mmolgÀ1, ABI; PEG = polyethylene glycol, PS = poly-
styrene, ABI = Applied Biosystems), 3-Fmoc-4-diaminoben-
zoic acid (Fmoc-Dbz) was coupled and the Fmoc group
deprotected with 20% piperidine. Notably, after acylation,
the free amino group in position 3 or 4 becomes hindered/
deactivated and does not need further protection. Subsequent
amino acids were coupled by standard Fmoc-SPPS with the
N-terminal amino acid being introduced as Boc-Leu. Follow-
ing chain assembly, the C-terminus was activated through
acylation with p-nitrophenylchloroformate followed by the
addition of base to promote intramolecular attack of the
anilide to form the resin-bound benzimidazolinone. The
peptide was deprotected and quantitatively cleaved from
the Rink-PEG resin by treatment with TFA.
As shown in Figure 1A, the desired peptide LYRAG-Nbz
was obtained in high purity. No aminoanilide products were
observed indicating that the conversion to the benzimidazo-
linone is quantitative. In this short peptide, the isomeric
acylurea products are resolved by HPLC, indicating a 4:1
ratio. However, treatment of the peptide with 4-mercapto-
phenylacetic acid (MPAA) rapidly yields a single thioester
product (LYRAG-SAr, Figure 1B; Ar= aryl). Importantly,
the thioester conversion occurs at pH 7.0 in under an hour,
minimizing the likelihood of base mediated side reactions in
susceptible peptides. In addition, since the benzimidazolinone
is a poor nucleophile, the equilibrium lies fully towards the
aryl-thioester product. These properties suggest that the Nbz
peptides will be especially useful in the generation of aryl
thioesters used in kinetically controlled and auxiliary-medi-
ated ligation methods.[17]
The rapid conversion of LYRAG-Nbz to LYRAG-SAr
suggested that acylurea peptides could be used directly in
NCL without prior isolation of the thioester precursor. To test
the in situ activation of the LYRAG peptide, we mixed
LYRAG-Nbz (2 mm) with the N-terminal Cys peptide
CRAFS (2.8 mm) at ph 7.0 in 6m Gn·HCl Gn = guanidine,
200 mm Na2HPO4, 200 mm MPAA, 20 mm TCEP·HCl
(TCEP = tris(carboxyethyl)phosphine).[18] As shown in Fig-
Figure 1. Synthesis, thioester exchange andligation of LYRAG-Nbz.
A) HPLC chromatogram of crude LYRAG-Nbz. B) Thiolysis of crude
LYRAG-Nbz in 200 mm MPAA, pH 7.0 after 1 h. C) Ligation of CRAFS
to LYRAG-Nbz after 50 min, pH 7.0.
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ure 1C, the ligation reaction proceeded efficiently, And under
these mild conditions, the only side product stemmed from
trace hydrolysis.[19] At intermediate time points, we found
only trace amounts of the LYRAG-SAr peptide, suggesting
that it was immediately converted into LYRAGCRAFS.
These observations are consistent with aryl thioester forma-
tion being rate determining under these ligation conditions.
To probe the scope of the method, we synthesized a series
of peptides with Ala, Phe, Tyr, Leu, Val, and Pro in the C-
terminal position (Table 1). The loading on the Dbz linker
synthesis of large polypeptides using standard Fmoc-SPPS
protocols. To test the robustness of the Dbz moiety, we
synthesized a functionally diverse 29-amino-acid peptide
derived from rabies virus glycoprotein (Rvg) that has recently
been shown to be an effective carrier for cargo delivery into
cells and through the blood–brain barrier.[22] Synthesis was
performed on a 0.1 mmol scale using 4-fold excess (0.4 mmol)
of amino acid and the activating mixture (HBTU/HOBt/
DIEA (1:1:1.5)). The last residue (Tyr) was introduced as the
side-chain-unprotected Boc-Tyr. After peptide elongation, an
aliquot of peptide-resin (100 mg) was removed and p-nitro-
phenylchloroformate was added in CH2Cl2. The resulting
peptidyl-carbamate was treated with 0.5m DIEA/DMF
(15 min), to afford the peptide-Nbz resin that was then
cleaved with TFA. As shown in Figure 2, the peptide was
Table 1: Peptide-Nbz synthesized by Fmoc-SPPS.
[b]
Entry
Peptide-Nbz
Purity [%][a]
RecoveredYield[%]
1
2
3
4
5
6
7
8
LYRAG-Nbz
LYRGA-Nbz
LARGF-Nbz
LARGY-Nbz
AYRGL-Nbz
LYRAP-Nbz
LYRGV-Nbz
(29aa)Rvg-Nbz
95
90
96
93
92
93
94
57
90
70
88
90
87
71
67
36
[a] Crude purity based on HPLC detection trace at 220 nm. [b] Non-
optimized, the peptide is cleaved quantitatively from the Rink-PEG-PS
resin.
was quantitative for each amino acid (see Supporting
Information). Alternatively, 3-(Fmoc-Ala)-4-diaminobenzoic
acid (Fmoc-Ala-Dbz-OH) was synthesized in order to obtain
the peptide LYRGA-Nbz as a single isomer. We anticipate
that the use of preformed Fmoc-aa-Dbz (aa = amino acid)
linkers will be the most robust method for synthesis of Nbz
peptides using preloaded Fmoc-aa-Dbz-resins in a similar
manner to commercially available Boc-aa-Pam-resins and
Fmoc-aa-Wang-resins.
Figure 2. Rvg-Nbz HPLC andESI mass (insert) of crude synthetic
product. Mass observed: [M+H] 3426.0 Da, (calcd: [M+H]
3426.8 Da). Rvg sequence: YTIWMPENPRPGTPCDIFTNSRGKRASNG-
Nbz.
obtained in high purity (57% by HPLC integration) and a
yield of 36% was recovered after preparative HPLC. As with
the LYRAG-Nbz model peptide, the Rvg-Nbz peptide
efficiently exchanged with thiols to yield activated aryl
thioesters (Supporting Information, pages S32–S34).[23] In
addition, 2.0 mm of Rvg peptide (3.6 mg, 1.0 ” 10À3 mmol) was
ligated in situ with a second polypeptide, Tsr[24] (30 aa, 3.8 mg,
1.2 ” 10À3 mmol, 2.4 mm) at pH 7.0 in 6m Gn·HCl, 200 mm
Na2HPO4, 200 mm MPAA, 20 mm TCEP·HCl to yield a 59
amino acid polypeptide (4.7 mg, 70%, Figure 3). After 2 h, no
acylurea peptide remained. Significantly, these peptides
contain the diverse functionality typical of complex peptides
and proteins.
These results suggest that N-acylurea activation will have
broad utility in organic synthesis, including peptide and
protein chemistry. These mildly activated peptides undergo
rapid thiolysis to yield thioester peptides under standard NCL
conditions. Importantly, activation of the Dbz moiety is
compatible with both unhindered (Gly) and hindered (Val) C-
terminal amino acids and is resistant to epimerization. In
addition, the Dbz group is stable to standard Fmoc-SPPS
coupling and deprotection protocols as demonstrated by the
efficient synthesis of Rvg, a 29 aa rabies-derived peptide.
Since the method relies exclusively on the acylation of amines
and requires no additional protecting groups, it is compatible
with standard side-chain protection strategies, linkers and
Following chain assembly, the activation of the Dbz group
with p-nitrophenylchloroformate and subsequent acidolytic
cleavage was near quantitative in all cases including the
hindered Val and Pro residues. Importantly, epimerization of
the C-terminal amino acid was less than 2%.[20] One
advantage of the acylurea method is that chain elongation
occurs with a C-terminal amide linkage. As a result, no
diketopiperazine formation was observed, even with the
cyclization-prone LYRAP-Nbz peptide. Indeed, in previous
work we observed significant diketopiperazine formation in
the same peptide when using Boc-SPPS on a thioester resin.[5]
The six peptides were ligated with 2–3 mm CRAFS,
pH 7.0, 258C, reaching completion in under 2 h except for
Val (10 h) and Pro (24 h) residues considered to be poorly
compatible with NCL.[5] On a preparative scale, 2.5 mm
AYRGL-Nbz (7.5 mg, 1.0 ” 10À2 mmol) and 3.0 mm CRAFS
(7.0 mg, 1.2 ” 10À2 mmol) formed the product AYRGL-
CRAFS (10.9 mg, 9.5 ” 10À3 mmol) in 95% yield of isolated
product with no side reactions other than trace hydrolysis.
From these studies, we anticipate that N-acylurea peptides
will be versatile precursors to peptidyl thioesters.[2,5,21]
Since a primary utility of thioester peptides is in the
synthesis of proteins and complex bioconjugates, it is essential
for the o-aminoanilide linker to be compatible with the
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activated Nbz peptides should widen the application of
chemoselective ligation methods for the synthesis of complex
post-translationally modified proteins and other macromole-
cules.
Received: November 30, 2007
Revised: May 27, 2008
Published online: July 24, 2008
Keywords: aminoanilides · native chemical ligation · peptides ·
.
solid-phase synthesis · thioester
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Figure 3. Ligation of Rvg-Nbz andTsr (N-terminal Cys, 30 aa). HPLC
traces are shown at 0 and2 h. RvgTsr, mass observed: [ M+H]
6367.0 Da, (calculated: 6367.3 Da).
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[19]In the absence of thiol additives (MPAA), significant hydrolysis
of the LYRAG-Nbz was observed (see the Supporting Informa-
tion, pages S30–S32).
[20]As monitored by HPLC in comparison to C-terminally epimer-
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[23]Note, both the Rvg-SAr and the cyclic thiolactone products were
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[24]Tsr is a 30-mer peptide fragment of Thrombospondin-1 anti-
angiogenic protein. T. K. Tiefenbrunn, P. E. Dawson, unpub-
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