Synthetic Approaches to Disulfide-free Circular Bovine Pancreatic Trypsin Inhibitor (c-BPTI) Analogues

Several approaches were investigated with the goal to obtain disulfide-free circularized analogues of the 58-residue small protein bovine pancreatic trypsin inhibitor (BPTI). These approaches include (1) a semisynthesis that uses as a starting point naturally occurring BPTI and takes advantage of the native proximity of the C- and N-termini; (2) a synthesis in which a peptide thioester prepared by stepwise Fmoc solid-phase chemistry is cyclized by a solution native chemical ligation step; (3) a stepwise Fmoc solid-phase synthesis of a protected circularly permuted linear sequence, followed by an attempted selective activation and head-to-tail cyclization; and (4) a stepwise Fmoc solid-phase synthesis of the same analogue, but using a different disconnection point, that features backbone amide linker (BAL) anchoring and attempted on-resin cyclization. The first two of these approaches were indeed successful in providing the desired target molecules in excellent purities and respectable yields, and could well be amenable to generalization. It is not yet clear whether or not the latter two approaches could be salvaged by modifications in the details of the chemical procedures applied.Taken in part from the February 2004 Ph.D. thesis of Judit Tulla-Puche. A preliminary report of portions of this work has appeared (Tulla-Puche et al., 2004).Dedicated to the memory of Bruce Merrifield (July 15, 1921--May 14, 2006), mentor and friend, whose conceptualization and development of solid-phase peptide synthesis opened new chapters of the chemical and biological sciences     

Comparison of the solid-phase fragment condensation and phase-change approaches in the synthesis of salmon I calcitonin.

Salmon I calcitonin was synthesized using both phase-change and conventional solid-phase fragment condensation (SPFC) approaches, utilizing the Rink amide linker (Fmoc-amido-2,4-dimethoxybenzyl-4-phenoxyacetic acid) combined with 2-chlorotrityl resin and the Fmoc/tBu(Trt)-based protection scheme. Phase-change synthesis, performed by the selective detachment of the fully protected C-terminal 22-mer peptide-linker from the resin and subsequent condensation in solution with the N-terminal 1-10 fragment, gave a product of slightly less purity (85 vs. 92%) than the corresponding synthesis on the solid-phase. In both cases salmon I calcitonin was easily obtained in high purity.     

Peptidyl molecular imaging contrast agents using a new solid-phase peptide synthesis approach

A versatile method is disclosed for solid-phase peptide synthesis (SPPS) of molecular imaging contrast agents. A DO3A moiety was derivatized to introduce a CBZ-protected amino group and then coupled to a polymeric support. CBZ cleavage with Et2AlCl/thioanisole was optimized for SPPS. Amino acids were then coupled to the aminoDOTA-loaded resin using conventional stepwise Fmoc SPPS to create a product with DOTA coupled to the C-terminus of the peptide. In a second study, the DO3A moiety was coupled to a glycine-loaded polymeric support, and amino acids were then coupled to the amino-DOTA-peptide-loaded resin using SPPS to incorporate DOTA within the peptide sequence. The peptide-(Tm3+-DOTA) amide showed a paramagnetic chemical exchange saturation transfer (PARACEST) effect, which demonstrated the utility of this contrast agent for molecular imaging. These results demonstrate the advantages of exploiting SPPS methodologies through development of unique DOTA derivatives to create peptide-based molecular imaging contrast agents.     

Synthesis of peptide amides by Fmoc-solid-phase peptide synthesis and acid labile anchor groups

The preparation and use of new anchor groups for the synthesis of peptide amides by solid-phase peptide synthesis employing the Fmoc-method is described. Based on the structure of the 4,4'-dimethoxybenzhydryl group (Mbh) handles were developed, which could be cleaved by mild acid treatment to give carboxamides. The syntheses and application of Fmoc-amino-acid-(4-carboxylatomethyloxyphenyl-4'-methoxyphenyl) methyl amide and Fmoc-(4-carboxylatopropyloxyphenyl-4'-methoxyphenyl) methyl amide are described in detail. These handles were coupled to resins and a stepwise elongation of peptide chains proceeded smoothly with N alpha-9-fluorenylmethoxycarbonyl (Fmoc) amino acid derivatives using a carbodiimide/HOBt mediated reaction. The final cleavage of side-chain protecting groups and the release of the C-terminal amide moiety was achieved by the treatment with trifluoroacetic acid, dichloromethane in the presence of scavengers. Various peptides, such as the Leu-enkephalin amide and Leu-Gly-Gly-Gly-Gln-Gly-Lys-Val-Leu-Gly-NH2, which is a good substrate for F XIII, were prepared in high yields and purities.     

Synthesis of the β-amyloid fragment <Superscript>5</Superscript>RHDSGY<Superscript>10</Superscript> and its isomers

The peptide RHDSGY, a fragment of the human β-amyloid Zn-binding site, and its isomers RH(D-Asp)SGY and RH(β-Asp)SGY have been obtained as amides by means of solid-phase synthesis and analyzed by HPLC and various mass spectrometric methods. The problem of low yield of the RHDSGY peptide and its isomers attributed to 9-fluorenylmethoxycarbonyl (Fmoc)-amino acids and/or formation of such side-products as RH(β-Asp)SGY (or RHDSGY during synthesis of RH(β-Asp)SGY) and RH(Asp-imide) SGY was solved via selection of individual reagents for removal of Fmoc groups from α-amino groups of the growing peptide chain.     

α- 促黑激素的固相合成研究

目的:探索α-促黑激素的合成工艺。方法:采用多肽固相合成法制备α-促黑激素。以Rink amide-MBHA树脂为载体、使用Fmoc保护策略、TBTU、HOBt、DIEA为缩合剂体系,最后用TFA、苯甲硫醚、水、苯酚、乙二硫醇混合液将多肽从树脂上切割下来。结果:合成后的目标多肽产率达64.9%,经过RP-HPLC纯化纯度可达98%,质谱鉴定显示纯化产物与目标多肽理论相对分子质量一致。结论:该方法操作方便,反应结果稳定,为固相合成生产α-促黑激素提供了一种可行的工艺方案。     

Silyl linker-based approach to the solid-phase synthesis of Fmoc glycopeptide thioesters

An efficient solid-phase synthesis of Fmoc (glyco)peptide thioesters is described. Fmoc x Ser x OAll and Fmoc x Thr x OAll bound to resin with a silyl ether linker were deallylated by Pd(0) catalysis and condensed with thiophenol, benzyl mercaptane, and ethyl 3-mercaptopropionate by activation with DCC/HOBt. The thioesters were released from the resin either by treatment with CsF-AcOH or by acidic hydrolysis. The effectiveness of this silyl linker strategy is further demonstrated by the synthesis of more complex (glyco)peptide thioesters 25, 26 and 27 involving N-->C and C-->N peptide elongation.     

Chemical synthesis of NS-1 region of hepatitis C-viral polyprotein fragments: a comparison of PS-BDODMA resin and merrifield resin

Three peptide fragments selected from the NS-1 region of hepatitis C-viral polyprotein (Leu-Ile-Asn-Thr-Asn-Ala-Ser-Trp-His-Ala-Asn-Arg-Thr-Ala-Leu-Ser Asn-Asp Ser-Lys Leu Asn Thr-Gly Ala NH(2), Leu-lle Asn Thr Asn Ala Ser-Trp-His-Ala-Asn-Arg-Thr Ala NH(2) and Leu-Asn-Cys(Acm)-Asn-Asp-Ser-Leu-Asn-Thr-Ala-NH(2)) have been synthesized on PS-BDODMA resin. The synthetic capability of the resin PS-BDODMA resin was compared with Merrifield resin. The peptides were synthesized by the stepwise fluoren-9-yl methoxycarbonyl (Fmoc) solid-phase method. The synthesized peptides were purified by HPLC and the identity of the peptides was established by mass spectrum and amino acid analysis. The synthesis of these peptides illustrates the application of the PS-BDODMA resin for the synthesis of long chain peptides in high yield and homogeneity compared to the Merrifield resin.     

Successful synthesis of a glial-specific blood–brain barrier shuttle peptide following a fragment condensation approach on a solid-phase resin

Successful manual synthesis of the TD2.2 peptide acting as a blood–brain barrier shuttle was achieved. TD2.2 was successfully synthesised by sequential condensation of four protected peptide fragments on solid-phase settings, after several unsuccessful attempts using the stepwise approach. These fragments were chosen to minimise the number of demanding amino acids (in terms of coupling, Fmoc removal) in each fragment that are expected to hamper the overall synthetic process. Thus, the hydrophobic amino acids as well as Arg(Pbf) were strategically spread over multiple fragments rather than having them congested in one fragment. This study shows how a peptide that shows big challenges in the synthesis using the common stepwise elongation methodology can be synthesised with an acceptable purity. It also emphasises that choosing the right fragment with certain amino acid constituents is key for a successful synthesis. It is worth highlighting that lower amounts of reagents were required to synthesise the final peptide with an identical purity to that obtained by the automatic synthesiser.     

Problem of aspartimide formation in Fmoc-based solid-phase peptide synthesis using Dmab group to protect side chain of aspartic acid.

The sequence-dependent, acid- or base-catalysed aspartimide formation is one of the most serious side reactions in solid-phase synthesis of peptides containing aspartic acid. In the present work, we investigated the susceptibility of 4-(N-[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl]amino)benzyl (Dmab), an aspartic acid beta-carboxy side-chain protecting group, for aspartimide formation. As a model, 15-amino acid-residue galanin fragment analogue containing the Asp-Ala motif was used during Fmoc-based solid-phase synthesis. Our study showed a strong tendency of Dmab-protected peptide to form aspartimide with unusual high efficiency. Furthermore, to investigate the susceptibility of Asp-Ala motif for aspartimide formation during the synthesis using Asp(ODmab), a 5-amino acid-residue galanin fragment LGPDA, different types of resin linkers, variety of Fmoc-deprotection conditions and coupling methods were applied.     

Solid-phase synthesis of reactive peptide crosslinker by selective deprotection

An effective and simple strategy for preparing peptide crosslinkers is described. An MMP-13 degradable peptide QPQGLAK-NH(2) was prepared on the solid-phase using Fmoc chemistry. The peptide crosslinker was synthesized on-bead by the coupling reaction between acrylic acid and the amine groups of glutamine and lysine residues. The synthetic procedure employed the acid-labile Fmoc-Lys (Mtt)-OH and base-labile Fmoc-AA-OH derivatives. Selective deprotection, of -Mtt and -Fmoc groups on-bead, freed the amine end-groups on glutamine and lysine residues for coupling reaction with acrylic acid while maintaining the peptide attached to the resin. Subsequent cleavage from the resin yielded a peptide crosslinker with two unsaturated acrylate end-groups with high yield and purity. This method can be generally employed for the synthesis of a wide range of peptides with one or more reactive groups for grafting in the fabrication of biomimetic scaffolds in tissue engineering applications.     

Solid-phase synthesis of neurokinin A antagonists. Comparison of the Boc and Fmoc methods.

During the preparation of the NK-2 selective tachykinin antagonist MEN 10208 (Thr-Asp-Tyr-D-Trp-Val-D-Trp-D-Trp-Arg-NH2) and its analogs by the solid-phase method employing the Boc strategy routinely used in our laboratory, we encountered difficulties in the coupling of hydrophobic amino acids D-Trp and Val. To study the coupling problems several syntheses of MEN 10208 and analogs were carried out with different activation strategies. These syntheses yielded considerable amounts of deletion sequences even though a negative Kaiser test was obtained after each coupling. Inaccessibility of the free amino group of the growing peptide due to steric hindrance of the hydrophobic residues during coupling, and for the ninhydrin complex during the Kaiser test, may account, at least in part, for the unsatisfactory synthetics results and for the false-negative ninhydrin tests. Repetition of each synthesis with the Fmoc strategy on a newly developed DOD resin for peptide amides using the DCC/HOBt chemistry gave superior results in terms of the yield and purity of the crude peptides. Therefore, the Fmoc strategy appears to offer advantages over the Boc method for the preparation of these peptides containing hydrophobic amino acids.     

Removal of acid-labile protecting or anchoring groups in the presence of polyfluorinated alcohol: Application to solid-phase peptide synthesis

We describe herein a new method for cleaving from resin and removing acid-labile protecting groups in solid-phase peptide synthesis in the presence of a polyfluorinated alcohol (either trifluoroethanol, TFE, or hexafluoroisopropanol, HFIP). It was shown that 0.1 M HCl in hexafluoroisopropanol or trifluoroethanol removes the acid-labile protecting groups commonly used in Fmoc SPPS for the protection of amino acid side-chains, such as t-butyl ester and ether, Boc, trityl, and Pbf groups including the most acid-resistant p-hydroxymethylphenoxyacetyl group (HMPA), p-benzyloxy benzyl ester (Wang resin), Rink amide, and peptide amide linker (PAL). The addition of 5–10% of a hydrogen-bonding solvent was shown to considerably retard or even fully inhibit the reaction. However, nonhydrogen-bonding solvents, such as dichloromethane, do not slow down the reaction.     

A simple oxazolidine linker for solid-phase synthesis of peptide aldehydes

A very simple and cheap linker has been used for solid-phase synthesis of peptide aldehydes. Protected amino acid aldehydes are immobilized on 2-Cl(trt) resin as oxazolidine formation via diethanolamine. After classical Fmoc SPPS, treatment of the resin with AcOH/DCM/H(2)O (8:1:1) affords peptide aldehydes in high yield and purity.     

Preparation of protected peptide amides using the Fmoc chemical protocol. Comparison of resins for solid phase synthesis.

Different resins were examined for their potential use in the solid phase synthesis of protected peptide amides using the 9-fluorenylmethoxycarbonyl (Fmoc) chemical protocol. The model protected peptide amide BocTyr-Gly-Gly-Phe-Leu-Arg(Pmc)NH2 (1) was synthesized on both the acid-labile 4-(2',4'-dimethoxyphenyl-Fmoc-aminomethyl)phenoxy resin (Rink amide resin) (2) and on resins containing the base-labile linker 4-hydroxymethylbenzoic acid. Of the resins examined only the methylbenzhydrylamine resin containing the 4-hydroxymethylbenzoic acid linkage, which was cleaved by ammonolysis in isopropanol, gave the model peptide 1 in good overall yield (53% including functionalization). Thus the synthesis of protected peptide amides by solid phase synthesis using Fmoc-protected amino acids with t-butyl-type side chain protecting groups is feasible. The choice of peptide-resin linkage and its cleavage conditions, however, are critical to the success of such syntheses. The potential application of this synthetic strategy to the preparation of novel peptide amides is discussed.     

An efficient, asymmetric solid-phase synthesis of benzothiadiazine-substituted tetramic acids: potent inhibitors of the hepatitis C virus RNA-dependent RNA polymerase

An efficient, asymmetric solid-phase synthesis of benzothiadiazine-substituted tetramic acids is reported. Starting from commercially available chiral Fmoc-protected alpha-amino acids loaded onto Wang resin, Fmoc removal, reductive amination followed by amide bond formation, and base-catalyzed cyclization with simultaneous cleavage from the resin provided the desired products. Compounds described are potent inhibitors of the hepatitis C virus RNA-dependent RNA polymerase.     

To Rink or Not to Rink Amide Link, that is the Question to Address for More Economical and Environmentally Sound Solid-Phase Peptide Synthesis

A comparative study is presented on the solid-phase peptide synthesis (SPPS) of the acyl carrier protein (ACP 65–74) sequence on a series of Rink amide resins possessing different matrix structures: poly(vinyl alcohol)-graft-poly(ethylene glycol) (PVA-g-PEG, 4), Tentagel-S-RAM (TG, 5), NovaGel (NG, 6), ChemMatrix (CM, 7) and polystyrene-divinylbenzene (PS-DVB, 8). In this comparison, the PEG-containing resins proved significantly better suited for the synthesis of pure ACP target sequence than the conventional PS-DVB solid supports (75–90% versus 52% crude purity). Amongst themselves, the PEG resins 4-7 exhibited similar capacity for providing pure peptide. Selecting PVA-g-PEG resin for a comparison of Rink amide linker versus no linker, the ACP (65–74) sequence was synthesized directly on the PVA-g-PEG resin 1, under identical conditions as employed in the synthesis on resin 4 bearing the Fmoc Rink linker, except for the final cleavage step, which was performed under more environmentally sound conditions using ester displacement with aqueous ammonia. Relative to its Rink amide counterpart 4, PVA-g-PEG resin 1 was cheaper to produce and possessed twice as much loading capacity (0.48 vs. 0.81 mmol/g). Moreover, Rink-less resin 1 gave higher yields of isolated pure peptide (61 vs. 45%) relative to its Fmoc Rink linker counterpart 4. In light of these results, the importance of the linker has been brought into question. As the need for large scale solid-phase peptide synthesis grows with greater demand for peptide products, ideal resins should be inexpensive to produce and employable under environmentally sound conditions to provide pure products. In this light, PVA-g-PEG resin 1 has demonstrated significant promise for economic and “green” SPPS.     

An efficient, convenient solid-phase synthesis of amino acid-modified peptide nucleic acid monomers and oligomers

An efficient and highly versatile method for the synthesis of amino acid-modified peptide nucleic acid (PNA) monomers is described. By using solid-phase Fmoc techniques, such monomers can be assembled readily in a stepwise manner and obtained in high yield with minimal purification. Protected neutral hydrophilic, acidic, and basic amino acids were coupled to 2-chlorotrityl chloride resin. Following Fmoc removal, innovative conditions for the key step, reductive alkylation with N-Fmoc-aminoacetaldehyde, were developed to circumvent problems encountered with previously reported methods. Activation and coupling of pyrimidine and purine nucleobases to the resulting secondary amines afforded amino acid-modified PNA monomers. The mild reaction conditions utilized were compatible with sensitive and labile functional groups, such as tert-butyl ethers and tert-butyl esters. PNA monomers were obtained in 36-42% overall yield and very high purity, after cleavage and purification. Using standard solid-phase Fmoc chemistry, two of these monomers were incorporated with high coupling efficiency into a variety of modified PNA oligomers, including four tetradecamers designed to target bcl-2 mRNA. Such modified oligomers have the potential to enhance water solubility and cell portability, while maintaining hybridization affinity and promoting favorable biodistribution properties.     

Synthesis of cyclic penta- and hexapeptides: A general synthetic strategy on DAS resin

The active part or receptor-binding sequence of peptide hormones can usually be defined by a span of 4–8 amino acids. Cyclic penta- and hexapeptides are excellent model systems for performing conformational and structure-function studies on this class of bioactive molecules. A synthetic scheme has been devised comprising solid-phase Fmoc chemistry followed by resin cleavage, cyclization in solution, and, finally, side-chain deprotection. A new resin, DAS, cleaved under weak acid conditions, is an excellent solid-phase synthesis support, and HBTU or PyBOP are the activation reagents of choice, not only during synthesis, but also for the cyclization reaction. Three cyclic peptides were synthesized using this method, one requiring extensive side-chain protection, and this method has general applicability for any cyclic pentapeptide or hexapeptide, giving good yields and high purity.     

Backbone amide linker (BAL) strategy for Nalpha-9-fluorenylmethoxycarbonyl (Fmoc) solid-phase synthesis of peptide aldehydes.

A rapid and efficient strategy has been developed for the general synthesis of complex peptide aldehydes. N(alpha)-Benzyloxycarbonylamino acids were converted to protected aldehyde building blocks for solid-phase synthesis in four steps and moderate overall yields. The aldehydes were protected as 1,3-dioxolanes except for one case where a dimethyl acetal was used. These protected amino aldehyde monomers were then incorporated onto 5-[(2 or 4)-formyl-3,5-dimethoxyphenoxy]butyryl-resin (BAL-PEG-PS) by reductive amination, following which the penultimate residue was introduced by HATU-mediated acylation. The resultant resin-bound dipeptide unit, anchored by a backbone amide linkage (BAL), was extended further by routine Fmoc chemistry procedures. Several model peptide aldehydes were prepared in good yields and purities. Some epimerization of the C-terminal residue occurred (10% to 25%), due to the intrinsic stereolability conferred by the aldehyde functional group, rather than any drawbacks to the synthesis procedure.