Cyclization of peptides on a solid support. Application to cyclic analogs of substance P

Glycosylated cell-penetrating peptides and their conjugates to a proapoptotic peptide: preparation by click chemistry and cell viability studies

Cell-penetrating peptides (CPPs), which are usually short basic peptides, are able to cross cell membranes and convey bioactive cargoes inside cells. CPPs have been widely used to deliver inside cells peptides, proteins, and oligonucleotides; however, their entry mechanisms still remain controversial. A major problem concerning CPPs remains their lack of selectivity to target a specific type of cell and/or an intracellular component. We have previously shown that myristoylation of one of these CPPs affected the intracellular distribution of the cargo. We report here on the synthesis of glycosylated analogs of the cell-penetrating peptide (R6/W3): Ac-RRWWRRWRR-NH(2). One, two, or three galactose(s), with or without a spacer, were introduced into the sequence of this nonapeptide via a triazole link, the Huisgen reaction being achieved on a solid support. Four of these glycosylated CPPs were coupled via a disulfide bridge to the proapoptotic KLAK peptide, (KLAKLAKKLAKLAK), which alone does not enter into cells. The effect on cell viability and the uptake efficiency of different glycosylated conjugates were studied on CHO cells and were compared to those of the nonglycosylated conjugates: (R6/W3)S-S-KLAK and penetratinS-S-KLAK. We show that glycosylation significantly increases the cell viability of CHO cells compared to the nonglycosylated conjugates and concomitantly decreases the internalization of the KLAK cargo. These results suggest that glycosylation of CPP may be a key point in targeting specific cells.

ELECTRONIC SUPPLEMENTARY MATERIAL

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Use of the 3-nitro-2-pyridine sulfenyl protecting group to introduce N epsilon-branching at lysine during solid-phase peptide synthesis. I. Application to the synthesis of a peptide template containing two addressable sites

TASPs (template-assembled synthetic peptides) are generated by the covalent attachment of linear peptides to a common peptide backbone, thus generating larger synthetic peptides/proteins with prefolded structure. In this work we present a strategy for the synthesis of a heterotemplate-assembled synthetic peptide containing two addressable sites. This orthogonal protection strategy would allow the selective introduction of different peptide chains via the epsilon-amino functions of template lysines being protected by either fluorenylmethoxycarbonyl (Fmoc) or 3-nitro-2-pyridine sulfenyl (Npys) groups. The N alpha-Boc-N epsilon-Npys-L-lysine required for solid-phase peptide synthesis (SPPS) is not readily available at a reasonable cost. To facilitate the more widespread use of this reagent we have compared the two published procedures for synthesizing this protected amino acid and evaluated the suitability of the products for SPPS. Two resin-bound peptides, a tripeptide Ac-G-K-Npys)-G-resin and an octapeptide template Ac-P1-K2-K3-L4-K5-K6-P7-G8-resin, were synthesized by SPPS. The epsilon-amino functions of lysines K2 & K6 and K3 & K5 of the octapeptide were protected by Fmoc and Npys groups, respectively. Secondly, these peptides were used to evaluate various reagents and reaction conditions for the deprotection of the epsilon-amino function of lysines bearing the N epsilon-Npys protecting group. A procedure for the optimized selective and quantitative deprotection of the Npys group from the epsilon-amino function of lysine in a resin-bound peptide using 2-mercaptopyridine-N-oxide is described.

Synthesis and stability of 3-nitro-2-pyridinesulfenyl chloride (NpysCl)

3-Nitro-2-pyridinesulfenyl chloride (NpysCl) is the starting material for the synthesis of N-, O- and S-Npys-protected amino acids. Two efficient, novel synthetic routes to NpysCl are described. The stability of NpysCl was determined in a variety of solvents, with and without base, to determine the most suitable solvent and base for the synthesis of N-Npys amino acids. The syntheses of Npys-Ala and Boc-Lys(Npys) tert-butylammonium salt are also described.

Novel version of Multiple Antigenic Peptide allowing incorporation on a cysteine functionalized lysine tree

A novel version of Multiple Antigenic Peptide (MAP) is described. This approach consists of the synthesis of a properly functionalized antigen carrier and the incorporation, on request, of one or more activated antigenic peptides. This method was to synthesize a MAP containing eight epitopes of Hsp 70-1 from Plasmodium falciparum. Mice immunised with this MAP gave a specific response while those immunised with the peptide alone did not.

Cyclization of disulfide-containing peptides in solid-phase synthesis

Disulfide-containing peptides may be obtained in good yields and purities when oxidations are carried out on peptide chains anchored to polymeric supports used for solid-phase synthesis. Such approaches take advantage of the pseudo-dilution phenomenon which favors intramolecular processes. A variety of procedures have been demonstrated using the related model peptides Ac-Cys-Pro-D Val-Cys-NH2 and Ac-Pen-Pro-D Val-Cys-NH2 (which both readily assume a type II beta-turn conformation that becomes stabilized by a 14-membered disulfide-containing intramolecular ring), and oxytocin (conformationally mobile 20-membered disulfide ring). Both Boc and Fmoc were used for N alpha-amino protection, the beta-thiols of cysteine or penicillamine were blocked by S-acetamidomethyl (Acm), S-9-fluorenylmethyl (Fm), or S-trityl (Trt), and compatible anchoring linkages included HF-labile 4-methylbenzhydrylamide (MBHA), TFA-labile tris (alkoxy)benzylamide (PAL), and photolabile o-nitrobenzylamide (Nonb). Assemblies of linear sequences proceeded smoothly, and polymer-supported oxidations were carried out in a variety of ways either directly or after deblocking to the resin-bound dithiol. Chains were released from the support without substantial damage to the disulfide bridges, and overall yields were as high as 60-90%.

Synthesis of cyclic peptides on solid support. Application to analogs of hemagglutinin of influenza virus

In order to mimic a well-known loop structure (site A) of the hemagglutinin of influenza virus, a series of cyclic peptides derived from the region 139-147 were synthesized. The lactam analogs cyclised between the N-terminus Cys 139 and the beta-carboxyl of aspartic acid 148 (small loop) or the epsilon-NH2 of lysine 148 via succinimidyl linker (large loop) were synthesized by the solid phase method. Cyclisation was directly performed on the solid support prior to final cleavage of the peptide. We describe two protection schemes which allow us to obtain different loop sizes derived from the same sequence. Eight of the analogs contained relatively large ring structures (up to 38 membered). For protection of the side chain of aspartic acid in combination with N-alpha-Fmoc protection, the cyclohexyl ester was more satisfactory than the benzyl ester with respect to imide formation. When the rate of cyclodimerisation, as a function of resin substitution, was compared to the rate of cyclic monomer formation, it was found that dimerisation was proportional to the charge of the resin. Furthermore, a comparison of the recently reported BOP reagent over the classical DIPC/HOBt method for the cyclisation reaction shows that in our case the reaction proceeded more rapidly by the BOP procedure although it gave a less pure crude product.

Thiolysis of the 3-nitro-2-pyridinesulfenyl (Npys) protecting group. An approach towards a general deprotection scheme in peptide synthesis

The hydroxylic side-chain functional groups of serine, threonine, hydroxproline and tyrosine, the alpha and epsilon-amino moieties of lysine and the thiol group of cysteine were masked by the 3-nitro-2-pyridinesulfenyl (Npys) protecting group. Deprotection was mildly affected by thiolysis with either 2-mercaptopyridine and 2-mercaptomethyl imidazole (O- and N-Npys) or with 3-mercaptoacetic acid and 2-mercaptoethanol (S-Npys). Thiolysis was monitored spectrophotometrically and was completed in a rather short time. Incorporation of the Npys group into a whole and single thiolyzable deprotection scheme is suggested.

Solid phase peptide synthesis utilizing 9-fluorenylmethoxycarbonyl amino acids

9-Fluorenylmethoxycarbonyl (Fmoc) amino acids were first used for solid phase peptide synthesis a little more than a decade ago. Since that time, Fmoc solid phase peptide synthesis methodology has been greatly enhanced by the introduction of a variety of solid supports, linkages, and side chain protecting groups, as well as by increased understanding of solvation conditions. These advances have led to many impressive syntheses, such as those of biologically active and isotopically labeled peptides and small proteins. The great variety of conditions under which Fmoc solid phase peptide synthesis may be carried out represents a truly "orthogonal" scheme, and thus offers many unique opportunities for bioorganic chemistry.

Use of the Npys thiol protection in solid phase peptide synthesis. Application to direct peptide-protein conjugation through cysteine residues

The protection of the thiol function of cysteine with the 3-nitro-2-pyridylsulfenyl (Npys) group has been successfully applied in the solid phase synthesis of nine peptides. A reexamination of the chemical stability of the protecting group has shown that, while Npys is essentially suitable for standard Boc/benzyl synthesis conditions, it is inadequate for the Fmoc strategy. Its proven stability to "high" HF acidolysis can not be extended to "low-high" conditions without significant thiol deprotection. On the other hand, the Npys group is quite compatible with standard photolytical cleavage conditions. The stability of Npys to HF and its thiol-activating character allow its application in peptide-carrier protein conjugation reactions by specific coupling through cysteine residues in the peptide.

Analysis of tachykinin-binding site interactions using NMR and energy calculation data of potent cyclic analogues of substance P

The three-dimensional structures of [Cys3,6,Tyr8]-, [Gly2,Cys3,6,Tyr8]- and [DCys3,Cys6]substance P, designed as conformational analogues of substance P, have been studied by 1H-NMR (500 MHz) in different solvents and by energy calculations. As previously observed for substance P and physalaemin, two tachykinins acting via the NK-1 receptor, [Cys3,6,Tyr8]substance P presents an alpha-helical structure of the 4----8 sequence in methanol. This structure is stabilized by a beta-turn III via the formation of three hydrogen bonds involving the Cys-6, Phe-7 and Tyr-8 NH groups. In contrast to substance P, two of these hydrogen bonds are still present in dimethyl sulfoxide and in water the Cys-6 NH hydrogen bond is the only one remaining, such that a beta-turn structure inside the ring can be envisaged. In close agreement with the NMR data, the energy calculations lead to three types of folding for the core of [Cys3,6,Tyr8]substance P: a beta-turn III, a less stable beta-turn I (delta E = 3 kcal), and a beta-turn II (delta E = 4.6 kcal). The structure of Gly-Leu-Met-NH2 is strongly affected by changing the hydrophobicity of the medium. The most stable calculated conformation is the helix; however, numerous unrelated structures are destabilized by about 2-3 kcal/mol. These data are analyzed and discussed in connection with the high potency of [Cys3,6,Tyr8]substance P for both the NK-1 and NK-3 binding sites; that is the internal region of tachykinins (non-homologous amino acids) might present a similar three-dimensional structure when bound to the receptors (which may be at the origin of some lack of selectivity), whereas paradoxically the selectivity may be due to the common C-terminal sequence.

Interaction of tachykinins with their receptors studied with cyclic analogues of substance P and neurokinin B

The activities of two groups of cyclic agonists of substance P (SP) have been studied. The disulfide bridge constraints have been designed on the basis of conformational studies on SP and physalaemin indicating an alpha-helical structure for the core of these two tachykinins (group I) and a folding of the C-terminal carboxamide towards the side chains of the glutamines 5 and 6 (group II). Only peptides simulating the alpha-helix present substantial potencies. [Cys3,6]SP is as active as SP in inhibiting 125I-labeled Bolton and Hunter SP-specific binding on rat brain synaptosomes and on dog carotid bioassay, two assays specific for the neurokinin 1 receptor. Moreover, [Cys3,6]SP is as potent as neurokinin B in inhibiting 125I-labeled Bolton and Hunter eledoisin-specific binding on rat cortical synaptosomes as well as in stimulating rat portal vein, two tests specific for the neurokinin 3 receptor. Interestingly, in contrast to neurokinin B, [Cys3,6]SP is a weak agonist of the neurokinin 2 receptor subtype, as evidenced by its binding potency in inhibiting 3H-labeled neurokinin A-specific binding on rat duodenum and in inducing the contractions of the rabbit pulmonary artery, a neurokinin 2-type bioassay. To increase the specificity of the cyclic analogue [Cys3,6]SP positions 8 and 9 were modified. [Cys3,6, Tyr8, Ala9]SP is slightly less selective than SP for the neurokinin 1 receptor subtype. [Cys2,5]neurokinin B constitutes a selective cyclic agonist for the neurokinin 3 receptor. The very weak potencies of the peptides from group II indicate that a certain degree of flexibility in the C-terminal moiety is required. Collectively, these results suggest that the neurokinin 1 and neurokinin 3 tachykinin receptors may recognize a similar three-dimensional structure of the core of the tachykinins. Different orientations of the common C-terminal tripeptide may be related to the selectivity for the different receptor subtypes.