Convergent Solid‐Phase Synthesis of N‐Glycopeptides Facilitated by Pseudoprolines at Consensus‐Sequence Ser/Thr Residues

Abstract

Dedicated to Professor Hans Paulsen on the occasion of his 90th birthday N-Glycosylation is an important posttranslational modification of proteins. A carbohydrate is transferred to an asparagine within an Asn-X-Ser/Thr consensus sequence.[1]The study of the biological aspects of N-glycosylation often requires the synthesis of N-glycopeptides, [2] which are accessible by two main approaches. In the sequential mode glycosylamino acid cassettes are used for peptide elongation. After incorporation of larger oligosaccharides the solubility and reactivity of the peptide is affected and side reactions, for example, involving free OH groups [3] complicate further elongation. In the convergent mode (Lansbury aspartylation) [4] the sugar is connected to an aspartate after complete assembly of the peptide (Scheme 1). The main drawback of the convergent mode is the formation of cyclic aspartimides during peptide elongation and sugar coupling; [5] this formation depends on the peptide sequence, [6] and the coupling conditions. [7] We found that a pseudoproline (Ypro) [8] at the consensus-sequence Ser/Thr residue (Asn-X-Ser/Thr(Ypro)) efficiently suppresses the formation of aspartimides in the convergent synthesis of N-glycopeptides on the solid phase.The lack of pure N-glycoproteins for biological studies has stimulated research into their synthesis; these syntheses were carried out mainly by ligation techniques.[9] The required glycopeptides and their thioesters are difficult to obtain as the sugar component interferes with the peptide synthesis. The syntheses of longer glycopeptide thioesters are particularly difficult as they require, for example, additional ligation steps [9f, 10] or segment couplings.[3b] For longer N-glycopeptides the convergent approach provides advantages over the sequential approach, [11] especially as the Lansbury aspartylation [4] has been shown to be efficient also on the solid phase.[12]The undesired aspartimide formation can be avoided by peptide backbone (NH) protection, [13] however, this approach is tedious for residues other than glycine, and causes racemization when coupling backbone-protected dipeptides.[12b] Aspartimide formation during peptide elongation can be reduced by using bulky groups to protect the Asp side chain, [14] for example, the 2-phenylisopropylester (PhiPr); [12a] however, trityl anchors are also cleaved under the reaction conditions for PhiPr removal. Dmab-protected [15] Asp residues are compatible with trityl anchors, but the backbone protection of the neighboring amino acid is required. [13a, 16] To provide complex N-glycopeptide thioesters by solidphase Lansbury aspartylation we compared three Asp-sidechain protecting groups for the Fmoc-SPPS of an interleukin-6 (IL-6) 43-48 hexapeptide. The allyl-protected peptide [17] 6 a showed the highest percentage of aspartimide 7 ai formation (15 %), followed by the Dmab peptide 6 b (8 %), and the PhiPr [12a] peptide 6 c (< 1 %; Scheme 2). As well as the protecting group many factors are known to contribute to aspartim...

Total Synthesis of Glycosylated Proteins

“…When the peptidyl Asp side chain is activated, care should be taken to minimize competitive intramolecular aspartimide formation [29]. To address this problem, we and Unverzagt independently developed a simple solution, involving emplacement of a temporary pseudoproline motif, derived from Ser/Thr, at the n + 2 position (relative to Asp) of the Asn-Xaa-Ser/Thr consensus sequence (Scheme 2) [30, 31]. Adoption of this pseudoproline strategy has enabled the efficient and convergent syntheses of extended glycopeptide fragments, thereby providing enabling progress in the chemical synthesis of homogeneous complex N -glycopeptides [32].…”

“…With the synthetic dodecasaccharide amine in hand, we then built up the corresponding glycopeptide fragments using the Lansbury aspartylation reaction [28] with peptide thioesters α-hGPH[31–58] and α-hGPH[59–81], themselves prepared by Fmoc-based SPPS followed by single amino acid attachment under Sakakibara’s epimerization-free conditions [80] (Scheme 8). After removal of all acid-labile protecting groups with a TFA-based cocktail, the fully deprotected (except Acm-containing cysteines) α-hGPH[82–92] and glycosylated α-hGPH [59–81] and α-hGPH[31–58] were then combined by means of a sequential two-step protocol involving ligation and subsequent cysteine unmasking.…”

“…After removal of all acid-labile protecting groups with a TFA-based cocktail, the fully deprotected (except Acm-containing cysteines) α-hGPH[82–92] and glycosylated α-hGPH [59–81] and α-hGPH[31–58] were then combined by means of a sequential two-step protocol involving ligation and subsequent cysteine unmasking. Lastly, the peptide thioester α-hGPH[1–30] (prepared by SPPS and direct thioesterification of the C-terminal glycine residue) was coupled with α-hGPH[31–92] via NCL to provide the Acm-protected primary sequence of α-hGPH featuring two defined, complex N -linked glycans [81]. …”

Erythropoietin Derived by Chemical Synthesis

“…S4) (22). Finally, and critical to our mission, was the capacity to realize maximally convergent amidation of highly complex oligosaccharide glycosylamines with suitably differentiated aspartates, even in substantially sized polypeptides (23, 24). …”

Homoserine as an Aspartic Acid Precursor for Synthesis of Proteoglycan Glycopeptide Containing Aspartic Acid and a Sulfated Glycan Chain

“…The aspartimide could not be converted back to Asp in high yields. 23 Although the generation of aspartimide in the N-glycan glycopeptide synthesis could be suppressed by using pseudoproline on Ser or threonine (Thr) within the tripeptide sequence Asn-Xaa-Ser/Thr, 24,25 it is not applicable to PG synthesis due to the need for strong acidic condition to deprotect the pseudoproline as well as the infrequencies of Asn-Xaa-Ser/Thr sequences in PG. We have also tested allyl ester as a protective group for Asp, which gave similar outcomes as the benzyl ester.…”