Solid-phase synthesis of DOTA-peptides

A general synthetic route to two DOTA-linked N-Fmoc amino acids (DOTA-F and DOTA-K) is described that allows insertion of DOTA at any endo-position within a peptide sequence. Three model pentapeptides were prepared to test the general utility of these derivatives in solid-phase peptide synthesis. Both DOTA derivatives reacted smoothly by means of standard HBTU activation chemistry to the point of insertion of the DOTA amino acid, but extension of the peptide chain beyond the DOTA-amino acid insertion required the use of pre-activated C-pentafluorophenyl ester N-alpha-Fmoc amino acids. Three Gal-80 binding peptides (12-mers) were then prepared by using this methodology with DOTA positioned either at the N terminus or at one of two different internal positions;the binding of the resulting GdDOTA-12-mers to Gal-80 were compared. The methodology described here allows versatile, controlled introduction of DOTA into any location within a peptide sequence. This provides a potential method for the screening of libraries of DOTA-linked peptides for optimal targeting properties.     

A convenient solid phase synthesis of S-palmitoyl transmembrane peptides

S-Palmitoylated peptides are important tools as models for integral membrane proteins to study peptide-lipid interactions. Herein, we report a convenient solid phase synthesis of S-palmitoyl transmembrane peptides. The highly acid labile S-(4-methoxytrityl) group is preferred over the S-(tert-butylsulfanyl) group for protection of the cysteine side chain since the latter gives rise to quantitative desulfurization during on-resin deprotection. The resulting free thiol function is modified with palmitic acid via a carbodiimide-mediated coupling and the title compounds are obtained in good yields and purity.     

Solid-phase synthesis of lipidated peptides

Characteristic partial structures of lipidated proteins embodying different lipid groups as well as additional fluorescent tags or a maleimide for coupling to proteins can be synthesized readily by means of a new solid-phase technique employing the oxidative cleavage of the hydrazide linker as well as on-resin farnesylation and palmitoylation after appropriate deprotection of cysteine thiol groups as the key steps.     

Synthesis of functionalized rab GTPases by a combination of solution- or solid-phase lipopeptide synthesis with expressed protein ligation

Prenylated proteins with non-native functionalities are generally very difficult to obtain by recombinant or enzymatic means. The semisynthesis of preparative amounts of prenylated Rab guanosine triphosphatases (GTPases) from recombinant proteins and synthetic prenylated peptides depends largely on the availability of functionalised prenylated peptides corresponding to the proteins' native structure or modifications thereof. Here, we describe and compare solution-phase and solid-phase strategies for the generation of peptides corresponding to the prenylated C terminus of Rab7 GTPase. The solid-phase with utilisation of a hydrazide linker emerges as the more favourable approach. It allows a fast and practical synthesis of pure peptides and gives a high degree of flexibility in their modification. To facilitate the analysis of semisynthetic proteins, the synthesised peptides were equipped with a fluorescent group. Using the described approach, we introduced fluorophores at several different positions of the Rab7 C terminus. The position of the incorporated fluorescent groups in the peptides did not influence the protein-ligation reaction, as the generated peptides could be ligated onto thioester-tagged Rab7. However, it was found that the positioning of the fluorescent group had an influence on the functionality of the Rab7 proteins; analysis of the interaction of the semisynthetic Rab7 proteins with REP (Rab escort protein) and GDI (guanosine diphosphate dissociation inhibitor) molecules revealed that modification of the peptide side chains or of the C-terminal isoprenoid did not significantly interfere with complex formation. However, functionalisation of the C terminus was found to have an adverse effect on complex formation and stability, possibly reflecting low structural flexibility of the Rab GDI/REP molecules in the vicinity of the lipid-binding site.     

Solid-phase synthesis of dihydrovirginiamycin S1, a streptogramin B antibiotic

We describe the first solid-phase synthesis of dihydrovirginiamycin S(1), a member of the streptogramin B family of antibiotics, which are nonribosomal-peptide natural products produced by Streptomyces. These compounds, along with the synergistic group A components, are "last line of defense" antimicrobial agents for the treatment of life-threatening infections such as vancomycin-resistant enterococci. The synthesis features an on-resin cyclization and is designed to allow production of streptogramin B analogues with diversification at positions 1', 1, 2, 3, 4, and 6. Several synthetic challenges known to hinder the synthesis of this class of compounds were solved, including sensitivity to acids and bases, and epimerization and rearrangements, through the judicious choice of deprotection conditions, coupling conditions, and synthetic strategy. This work should enable a better understanding of structure-activity relationships in the streptogramin B compounds, possible identification of analogues that bypass known resistance mechanisms, and perhaps the identification of analogues with novel biological activities.     

Solid-phase synthesis of transition-metal complexes

This overview highlights recent progress in the field of selective construction of linear, oligonuclear transition-metal complexes by using solid-phase synthesis procedures. Two general protocols have been identified: formation of coordinative bonds between metal centres and bridging ligands and formation of covalent bonds between preformed kinetically inert transition-metal-containing building blocks in the chain growth step. Currently available suitable building blocks for the second approach are based on ferrocene units, bis(terpyridine)-ruthenium(II) moieties or metal porphyrins.     

Solid-phase synthesis of homodetic cyclic peptides from Fmoc-MeDbz-resin

Cyclic homodetic peptides are very appealing for medicinal chemistry programs. In addition to the high efficiency and selectivity inherently associated with peptides, a cyclic structure totally formed by amide bonds increases their stability under physiological conditions. Here Fmoc-MeDbz-resin was studied for the preparation of these peptides. Our results demonstrate the usefulness of this strategy for the preparation of cyclic “head-to-side chain” peptides through cyclative cleavage (simultaneous cyclization and release from the resin). In contrast, for the synthesis of the “head-to-tail” counterparts, the cyclization-cleavage should be carried out in the presence of thiophenol.     

Stepwise solid phase synthesis of uridylylated viral genome-linked peptides using uridylylated amino acid building blocks

The uridylylated amino acid building blocks 2-cyanoethyl-(N^α-9-fluorenylmethoxy-carbonyl-tyrosin-4-yl)-(2′,3′-di-O-acetyluridin-5′-yl) phosphate and 2-chlorophenyl-(N^α-fluorenyl-methoxycarbonyl-serin-3-yl)-(2′,3′-di-O-acetyluridin-5′-yl) phosphate have been used successfully in an on-line SPPS of the VPgpU from the polio, coxsackie and cowpea mosaic virus.     

Solid-phase oligosaccharide synthesis of a small library of N-glycans

Solid-phase oligosaccharide synthesis is based on a hydroxymethylbenzyl benzoate spacer linker which is connected to the Merrifield resin (1 P). Glycosylation was performed with O-glycosyl trichloroacetimidates of glucosamine, mannose, and galactose permitting chain extension (2e, 5e), branching (4b, 7b, 8b), and chain termination (3t, 6t, 9t) with the use of O-benzyl, O-benzoyl, and N-dimethylmaleoyl as permanent and O-fluorenylmethoxycarbonyl (Fmoc) and O-phenoxyacetyl (PA) as temporary protecting groups. The steps required on solid phase are i) glycosylation under TMSOTf catalysis, ii) selective cleavage of the temporary protecting groups, Fmoc with NEt3 and PA with 0.5 equivalents of NaOMe in CH2Cl2/MeOH, and iii) product cleavage from the resin with 4.0 equivalents of NaOMe in CH2Cl2/MeOH and following O-acetylation for convenient product isolation. Thus a highly successful synthesis of a small library of seventeen N-glycan structures was made possible comprising the N-glycan pentasaccharide core structure 53 and two further chain extended hexa- and heptasaccharide N-glycans with a glucosamine or a lactosamine residue, respectively, which is attached to one of the mannose residues of the core structure (56 and 59).     

Solid-phase synthesis of peptide and glycopeptide thioesters through side-chain-anchoring strategies

An efficient new strategy for the synthesis of peptide and glycopeptide thioesters is described. The method relies on the side-chain immobilization of a variety of Fmoc-amino acids, protected at their C-termini, on solid supports. Once anchored, peptides were constructed using solid-phase peptide synthesis according to the Fmoc protocol. After unmasking the C-terminal carboxylate, either thiols or amino acid thioesters were coupled to afford, after cleavage, peptide and glycopeptide thioesters in high yields. Using this method a significant proportion of the proteinogenic amino acids could be incorporated as C-terminal amino acid residues, therefore providing access to a large number of potential targets that can serve as acyl donors in subsequent ligation reactions. The utility of this methodology was exemplified in the synthesis of a 28 amino acid glycopeptide thioester, which was further elaborated to an N-terminal fragment of the glycoprotein erythropoietin (EPO) by native chemical ligation.