Polypeptide synthesis using an expressed peptide as a building block for condensation with a peptide thioester: application to the synthesis of phosphorylated p21Max protein(1-101).

An expressed peptide proved to be useful as a building block for the synthesis of a polypeptide via the thioester method. A partially protected peptide segment, for use as a C-terminal building block, could be prepared from a recombinant protein; its N-terminal amino acid residue was transaminated to an alpha-oxoacyl group, the side-chain amino groups were then protected with t-butoxycarbonyl (Boc) groups, and. finally, the alpha-oxoacyl group was removed. On the other hand, an O-phosphoserine-containing peptide thioester was synthesized via a solid-phase method using Boc chemistry. These building blocks were then condensed in the presence of silver ions and an active ester component. During the condensation, epimerization at the condensation site could be suppressed by the use of N,N-dimthylformamide (DMF) as a solvent. Using this strategy, a phosphorylated partial peptide of the p21Max protein, [Ser(PO3H2)2.11]-p21Max(1-101), was successfully synthesized.     

Progress in the preparation of peptide aldehydes via polymer supported IBX oxidation and scavenging by threonyl resin.

Peptide aldehydes are of interest due to their inhibitory properties toward numerous classes of proteolytic enzymes such as caspases or the proteasome. A novel access to peptide aldehydes is described using a combination of solid phase peptide synthesis with polymer-assisted solution phase synthesis based on the oxidation of peptide alcohols with a mild and selective polymer-bound IBX derivative. The oxidation is followed by selective purification via scavenging the peptide aldehyde in a capture-release procedure using threonine attached to an aminomethyl resin. Peptide aldehydes are obtained in excellent purity and satisfying yield. The optical integrity of the C-terminal residue is conserved in a high degree. The procedures are compatible with the use of common side-chain protecting groups. The potential for using the method in parallel approaches is very advantageous. A small collection of new and known peptide aldehydes has been tested for inhibitory activity against caspases 1 and 3.     

On the synthesis of orexin A: a novel one-step procedure to obtain peptides with two intramolecular disulphide bonds.

An efficient strategy for the synthesis of orexin A, a recently discovered neuropeptide with two intramolecular disulphide bonds, was developed. Four different methods for the synthesis of peptides containing two disulphide bonds were compared and optimized with respect to reaction time, purity of the crude peptide and yield of the purified peptide. A new one-step cyclization method in solution is presented for fast, easy and high yield synthesis of orexin A, based on iodine oxidation in acetic acid/water and S-acetamidomethyl (S-Acm) and S-trityl (S-Trt) for side-chain protection of cysteine. Disulphide formation without selective side-chain protection leads to the formation of different mono- and bicyclic configurations of orexin A. These data stress the requirement of selective cysteine side-chain protection in the synthesis of orexin A.     

Synthetic strategy for side chain mono-N-alkylation of Fmoc-amino acids promoted by molecular sieves

A new synthetic strategy to alkylate amino groups under mild conditions has been developed. It utilizes only 4 ? molecular sieves as base in order to promote the N-alkylation reaction, in presence of the appropriate alkyl halide. The methodology was validated by the simple and efficient side-chain N-alkylation of o-Ns-protected Fmoc-amino acid. One of them was introduced as building block into a peptide sequence, thus allowing the preparation of site-specific alkylated peptide molecules.     

Facile synthesis of orthogonally protected amino acid building blocks for combinatorial N-backbone cyclic peptide chemistry.

Protected Nalpha-(aminoallyloxycarbonyl) and Nalpha-(carboxyallyl) derivatives of all natural amino acids (except proline), and their chiral inverters, were synthesized using facile and efficient methods and were then used in the synthesis of Nalpha-backbone cyclic peptides. Synthetic pathways for the preparation of the amino acid building units included alkylation, reductive amination and Michael addition using alkylhalides, aldehydes and alpha,beta-unsaturated carbonyl compounds, and the corresponding amino acids. The resulting amino acid prounits were then subjected to Fmoc protection affording optically pure amino acid building units. The appropriate synthetic pathway for each amino acid was chosen according to the nature of the side-chain, resulting in fully orthogonal trifunctional building units for the solid-phase peptide synthesis of small cyclic analogs of peptide loops (SCAPELs). Nalpha-amino groups of building units were protected by Fmoc, functional side-chains were protected by t-Bu/Boc/Trt and N-alkylamino or N-alkylcarboxyl were protected by Alloc or Allyl, respectively. This facile method allows easy production of a large variety of amino acid building units in a short time, and is successfully employed in combinatorial chemistry as well as in large-scale solid-phase peptide synthesis. These building units have significant advantage in the synthesis of peptido-related drugs.     

Fmoc-based chemical synthesis and selective binding to supercoiled DNA of the p53 C-terminal segment and its phosphorylated and acetylated derivatives.

The C-terminal domain of p53 comprises a linker, the tetramerization domain and the regulatory domain, and contains at least seven sites of potential post-translational modification. An improved strategy was developed for the synthesis of large peptides that contain phosphorylated amino acids and p53(303-393), a 91-amino acid peptide, and three post-translationally modified derivatives were synthesized through the sequential condensation of three partially protected segments. Peptide thiolesters were prepared using the sulfonamide-based 'safety-catch' resin approach and employing Fmoc-based solid-phase peptide synthesis. At the N-terminus of the middle building block, a photolabile protecting group, 3,4-dimethoxy-6-nitrobenzyloxycarbonyl, was incorporated to differentiate the N-terminal amino group from the side-chain amino groups. Two sequential couplings were accomplished following this protection strategy. The synthetic products, p53(303-393) and its phosphorylated or acetylated derivatives, exhibited the ability to bind specifically to supercoiled DNA, which is one of the characteristics of this domain.     

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.     

Unexpected aspartimide formation during coupling reactions using Asp(OAl) in solid-phase peptide synthesis

Succinimide ring closure is a well-documented side reaction in the synthesis of certain Asp-containing peptides. This side reaction is typically acid- or base-catalyzed, and its occurrence during coupling reactions has not been previously noted. This unforeseen manifestation of aspartimide formation was detected while exploring a new strategy for side-chain to side-chain lactam formation on a solid support to synthesize cyclo[D-Asp2,Dap5]dynorphin A-(1–11) amide. The availability of allyl protecting groups, which provide an additional level of orthogonality in solid-phase peptide synthesis, was very appealing for use in preparing this conformationally constrained analogue. We found that the allyl ester (OAl) was not sufficient protection from this side reaction in this susceptible D-Asp2-Gly3 sequence. Remarkably, the aspartimide formation appeared to occur during the coupling reaction in the absence of base if excess coupling reagent was present.     

Synthesis of peptide aldehydes.

The functionalization of peptides and proteins by aldehyde groups has become the subject of intensive research since the discovery of the inhibition properties of peptide aldehydes towards various enzymes. Furthermore, peptide aldehydes are of great interest for peptide backbone modification or ligation reactions. This review focuses upon their synthesis, which has been developed following two main strategies. The first strategy consists of prior synthesis of the peptide, followed by the introduction of the aldehyde function. The second possible strategy uses alpha-amino aldehydes as starting materials. After protection of the aldehyde, peptide elongation occurs. At the end of the synthesis, the aldehyde function can be unmasked.     

Solid Phase Synthesis of a Hydroxypyrrolidine Derivative and its Use in Solid Phase Peptide Synthesis as Constrained Statine Mimic

As part of our efforts to design constrained peptide mimics and introduce them in peptide sequences, we set up the synthesis of racemic N-Fmoc protected hydroxypyrrolidine by reduction of the corresponding oxopyrroline. Hydroxypyrrolidines are synthesized using amino acid building block and β-ketoester via a 4-steps solid supported route on Wang resin beads. The hydroxypyrrolidine template can be seen as a constrained mimic of statine. As proof of concept, the pseudopeptide JMV 2776, incorporating this new statine mimic has been synthesized. We replaced the phenyl statine building block in the sequence of known BACE 1/2 inhibitors by 5-benzyl 2-methyl 4-hydroxypyrrolidine, using conventional Fmoc SPPS on Rink amide PS resin. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Post-synthesis incorporation of a lipidic side chain into a peptide on solid support.

A new strategy for the synthesis of lipopeptides has been developed. Using Weinreb (N-methoxy, N-methyl) amide as an aldehyde function precursor on the side chains of Asp or Glu residues, this new strategy avoids the synthesis of a lipidic amino acid residue before its incorporation in the peptide sequence. The aldehyde generated on the solid support can react with ylides leading to unsaturated or saturated side chains or with various nucleophiles to yield non-coded amino acid residues incorporated into the sequence.     

Facile removal of the N-indole-mesitylenesulfonyl protecting group using HF cleavage conditions

Summary Nitrogen indole protection of the -methyltryptophan side-chain residue is important for avoiding undesired side reactions during peptide synthesis. Of great importance is the choice of a side-chain protecting group for orthogonal peptide synthesis and its stability under a variety of chemical conditions required for synthesis of the four isomers of this unusual amino acid. We report here the successful use of the mesitylenesulfonyl (Mts) protecting group for -methyltryptophan in the synthesis of melanotropin and CCK peptide analogues and the ready cleavage of this protecting group under HF conditions.     

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 peptide nucleic acid FRET probes via an orthogonally protected building block for post-synthetic labeling of peptide nucleic acids at the 5-position of uracil

We report the design and synthesis of an orthogonally protected peptide nucleic acid (PNA) building block, Fmoc-PNA-U'-(Dde)-OH, and its use in the construction of PNA FRET probes. This building block allows for the post-synthetic attachment of reporter groups to the amino group attached to the 5-position of uracil (U) following selective deprotection of the Dde group. We illustrate the use of this building block for the synthesis of a series of FAM Cy5 donor acceptor pairs and their ability to detect a target DNA sequence.     

Unexpected aspartimide formation during coupling reactions using Asp(OAl) in solid-phase peptide synthesis

Summary Succinimide ring closure is a well-documented side reaction in the synthesis of certain Asp-containing peptides. This side reaction is typically acid-or base-catalyzed, and its occurrence during coupling reactions has not been previously noted. This unforeseen manifestation of aspartimide formation was detected while exploring a new strategy for side-chain to side-chain lactam formation on a solid support to synthesizecyclo[D-Asp², Dap⁵]dynorphin A-(1-11) amide. The availability of allyl protecting groups, which provide an additional level of orthogonality in solid-phase peptide synthesis, was very appealing for use in preparing this conformationally constrained analogue. We found that the allyl ester (OAl) was not sufficient protection from this side reaction in this susceptible D-Asp²-Gly³ sequence. Remarkably, the aspartimide formation appeared to occur during the coupling reaction in the absence of base if excess coupling reagent was present.     

Synthesis of different types of dipeptide building units containing N- or C-terminal arginine for the assembly of backbone cyclic peptides.

Different types of dipeptide building units containing N- or C-terminal arginine were prepared for synthesis of the backbone cyclic analogues of the peptide hormone bradykinin (BK: Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg). For cyclization in the N-terminal sequence N-carboxyalkyl and N-aminoalkyl functionalized dipeptide building units were synthesized. In order to avoid lactam formation during the condensation of the N-terminal arginine to the N-alkylated amino acids at position 2, the guanidino function has to be deprotected. The best results were obtained by coupling Z-Arg(Z)2-OH with TFFH/collidine in DCM. Another dipeptide building unit with an acylated reduced peptide bond containing C-terminal arginine was prepared to synthesize BK-analogues with backbone cyclization in the C-terminus. To achieve complete condensation to the resin and to avoid side reactions during activation of the arginine residue, this dipeptide unit was formed on a hydroxycrotonic acid linker. HYCRAM technology was applied using the Boc-Arg(Alloc)2-OH derivative and the Fmoc group to protect the aminoalkyl function. The reduced peptide bond was prepared by reductive alkylation of the arginine derivative with the Boc-protected amino aldehyde, derived from Boc-Phe-OH. The best results for condensation of the branching chain to the reduced peptide bond were obtained using mixed anhydrides. Both types of dipeptide building units can be used in solid-phase synthesis in the same manner as amino acid derivatives.     

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.     

Synthesis of an Fmoc-threonine bearing core-2 glycan: A building block for PSGL-1 via Fmoc-assisted solid-phase peptide synthesis

Selectins (L, E, and P) are vascular endothelial molecules that play an important role in the recruitment of leukocytes to inflamed tissue. In this regard, P-Selectin glycoprotein-1 (PSGL-1) has been identified as a ligand for P-Selectin. PSGL-1 binds to P-Selectin through the interaction of core-2 O-glycan expressing sialyl Lewis^x oligosaccharide and the three tyrosine sulfate residues. Herein, we report the synthesis of threonine-linked core-2 O-glycan as an amino acid building block for the synthesis of PSGL-1. This building block was further incorporated in the Fmoc-assisted solid-phase peptide synthesis to provide a portion of the PSGL-1 glycopeptide.     

Solid-phase peptide synthesis without side-chain hydroxyl protection of threonine.

A peptide containing four threonine residues was synthesised by the solid-phase method using fluorenyl-methoxycarbonylamino acid reactive esters or coupling by preactivation with 1-hydroxybenzotriazole and Castro's reagent. In two separate experiments the synthesis was carried out with or without protection of the side-chain hydroxyl group of threonine as the tert.-butyl ether. Comparison of the crude peptides after deprotection and detachment from the synthesis resin suggests that side-chain protection of threonine is unnecessary under the synthetic conditions employed.     

A concise preparation of the fluorescent amino acid l-(7-hydroxycoumarin-4-yl) ethylglycine and extension of its utility in solid phase peptide synthesis

Incorporation of the unnatural amino acid l-(7-hydroxycoumarin-4-yl)ethylglycine (7-HC) is a powerful and reliable approach for the preparation of fluorescently labeled proteins. The growing popularity of this valuable amino acid prompted us to pursue an improved protocol for its synthetic preparation. The optimized procedure here described provides ready access to multi-gram quantities of 7-HC. Also reported is an extension of the utility of 7-HC in the generation of a protected building block suitable for use in solid phase peptide synthesis. The building block was successfully incorporated at various positions in a series of model peptides, including analogues of the cell penetrating HIV-Tat peptide, further illustrating the utility of this unique amino acid.