Reactifs de couplage peptidique I (1) - l'hexafluorophosphate de benzotriazolyl N-oxytrisdimethylamino phosphonium (B.O.P.)

Solid phase synthesis of peptides containing the non-hydrolysable analog of (O)phosphotyrosine, p(CH2PO3H2)Phe. Application to the synthesis of 344-357 sequences of the beta 2 adrenergic receptor

Studies about phosphorylation-dephosphorylation mechanisms require the development of probes capable of being used in in vitro and in vivo conditions. We show in this work that the chemically and enzymatically stable p(CH2PO3H2)Phe analog of (O)phosphotyrosine can be easily introduced in peptides by the solid-phase method. It has been incorporated in the 344-357 sequence of the beta 2 adrenergic receptor in place of the Tyr residue in position 350 and/or 354 in order to investigate the role of tyrosine phosphorylation in the receptor agonist-induced down-regulation. Since p(CH2PO3H2)Phe is an ionized hydrophilic residue, peptides containing this amino acid do not easily permeate the cellular membranes. Therefore the modified amino acid was introduced in the synthetic pathway in its N-Boc-p(CH2PO3Et2)Phe form, which could be partially or completely deprotected. Coupling steps, including that of the new amino acid, were performed with good yields (approximately 60% total yield) and further deprotections provided both the p(CH2PO3H2)Phe and p(CH2PO3HEt)Phe containing peptides with yields of around 20% each. The structure of the peptides was assessed by NMR, mass spectroscopy and amino acid analysis and the new amino acid was characterized under its phenylthiocarbamyl form (PTC).

Neuropeptide Y. Optimized solid-phase synthesis and conformational analysis in trifluoroethanol

The 36-amino-acid neuropeptide Y (human), which is one of the most potent vasoconstrictors and which exhibits a number of other biological functions, has been synthesized using automated peptide synthesis. The optimized method, using 9-fluorenylmethoxycarbonyl protecting and single-step coupling, yielded the crude product in 90% purity allowing for single-step reversed-phase HPLC purification to greater than 98% purity and a high overall yield (50%). The hormone was characterized by several chromatographic methods, ion-spray mass spectroscopy and Edman degradation. The conformation of human neuropeptide Y was examined by CD, NMR and computer simulations. The CD measurements in trifluoroethanol/water (9:1) show a large percentage of alpha-helix. Variation of concentration, from 0.5 microM increasing up to the 1 mM used for NMR measurements, indicates no evidence for aggregation. In the same solvent system, the NMR line widths were very broad and therefore the resonance assignment was achieved with the exclusive use of two-dimensional NOE spectra. The 248 clearly distinguishable NOEs from the NMR study were used in distance geometry calculations and the resulting structures were refined with restrained molecular dynamics. The results indicate an alpha-helix extending from Arg19 to Gln34. For the N-terminal half of the molecule no regular structure was observed.

Inhibition of T cell signaling by immunophilin-ligand complexes correlates with loss of calcineurin phosphatase activity

Calcineurin, a Ca2+, calmodulin-dependent protein phosphatase, was recently found to bind with high affinity to two different immunosuppressant binding proteins (immunophilins) with absolute dependence on the presence of the immunosuppressants FK506 or cyclosporin A (CsA) [Liu et al. (1991) Cell 66, 807-815]. The binding affinities of the immunophilin-drug complexes toward calcineurin and the stoichiometry of the resultant multimeric complexes have now been determined, and structural elements of FK506, CsA, and calcineurin that are critical for mediating their interactions have been identified. Analogues of FK506 (FK520, FK523, 15-O-demethyl-FK520) and CsA (MeBm2t1-CsA and MeAla6-CsA) whose affinities for their cognate immunophilins do not correlate with their immunosuppressive activities have been prepared and evaluated in biochemical and cellular assays. We demonstrate a strong correlation between the ability of these analogues, when bound to their immunophilins, to inhibit the phosphatase activity of calcineurin and their ability to inhibit transcriptional activation by NF-AT, a T cell specific transcription factor that regulates IL-2 gene synthesis in human T cells. In addition, FKBP-FK506 and CyP-CsA do not inhibit members of the PP1, PP2A, and PP2C classes of serine/threonine phosphatases. These data suggest that calcineurin is the relevant cellular target of these immunosuppressive agents and is involved in Ca(2+)-dependent signal transduction pathways in, among others, T cells and mast cells.

New convenient synthesis of immunostimulating peptides containing meso-diaminopimelic acid. Syntheses of FK-565 and FK-156

A new improved synthesis of two immunostimulating peptides: FK-156 (D-lactyl-alanyl-gamma-D-glutamyl-(L)-meso-2,6-diaminopimelyl-(L)- glycine) and FK-565 (heptanoyl-gamma-D-glutamyl-(L)-meso-2,6-diaminopimelyl-(L)- D-alanine) is described. A proper differentiation between the two chiral amino acid moieties of diaminopimelic acid was accomplished by selective enzymatic hydrolysis of one methyl ester group of the L-centre of Z2-meso-A2pm(OMe)2 (2). Utilization of a commercially available protease and diester 2 as an enzyme substrate made possible the relatively simple synthesis of a key intermediate 4 and considerably simplified the final deprotection steps. Aminolysis of the N-carboxyanhydride (4) with D-AlaONBzl or GlyONBzl was chosen to obtain the appropriate dipeptides with one free amino group as convenient intermediates for further peptide synthesis. The BOP reagent, used for peptide bond formation, secured good yields and high chemical and chiral purity of the peptides. A modification of alanine deamination procedure leading to a significant increase of D-Lac(OAc) yield is presented.

Quantitative analysis of cyclic beta-turn models

The beta-turn is a frequently found structural unit in the conformation of globular proteins. Although the circular dichroism (CD) spectra of the alpha-helix and beta-pleated sheet are well defined, there remains some ambiguity concerning the pure component CD spectra of the different types of beta-turns. Recently, it has been reported (Hollósi, M., Kövér, K.E., Holly, S., Radics, L., & Fasman, G.D., 1987, Biopolymers 26, 1527-1572; Perczel, A., Hollósi, M., Foxman, B.M., & Fasman, G.D., 1991a, J. Am. Chem. Soc. 113, 9772-9784) that some pseudohexapeptides (e.g., the cyclo[(delta)Ava-Gly-Pro-Aaa-Gly] where Aaa = Ser, Ser(OtBu), or Gly) in many solvents adopt a conformational mixture of type I and the type II beta-turns, although the X-ray-determined conformation was an ideal type I beta-turn. In addition to these pseudohexapeptides, conformational analysis was also carried out on three pseudotetrapeptides and three pseudooctapeptides. The target of the conformation analysis reported herein was to determine whether the ring stress of the above beta-turn models has an influence on their conformational properties. Quantitative nuclear Overhauser effect (NOE) measurements yielded interproton distances. The conformational average distances so obtained were interpreted utilizing molecular dynamics (MD) simulations to yield the conformational percentages. These conformational ratios were correlated with the conformational weights obtained by quantitative CD analysis of the same compounds. The pure component CD curves of type I and type II beta-turns were also obtained, using a recently developed algorithm (Perczel, A., Tusnády, G., Hollósi, M., & Fasman, G.D., 1991b, Protein Eng. 4(6), 669-679). For the first time the results of a CD deconvolution, based on the CD spectra of 14 beta-turn models, were assigned by quantitative NOE results. The NOE experiments confirmed the ratios of the component curves found for the two major beta-turns by CD analysis. These results can now be used to enhance the conformational determination of globular proteins on the basis of their CD spectra.

N-terminal portion of motilin determines its biological activity

This study aimed to identify the portion of the 22 amino acid sequence of motilin responsible for the biological activity of the peptide. The contraction of rabbit duodenal muscle in vitro was measured when exposed to synthetic fragments of motilin corresponding to various sequences of the C- or N-terminal portions of the molecule. Fragments 2-22 or 3-22 (where the initial amino acids of the N-terminal ending were removed) were more than 1000 times less potent than the native molecule 1-22. Fragment 1-9 (where the last 13 amino acids located at the C-terminal side of motilin were removed) was devoid of any contractile capacity, while synthetic fragments whose C-terminal structure extended beyond the 1-9 motilin sequence maintained almost complete biological activity. N-terminal amino acid sequence 1-9 is therefore an essential determinant of the contractile activity of motilin.

14-membered cyclic opioids related to dermorphin and their partially retro-inverso modified analogues. I. Synthesis and biological activity

As a continuation of our program to study structure-activity relationships of opiate peptides, we report the syntheses and biological activities of a series of 14-membered cyclic dermorphin analogues closely related to enkephalin analogue Tyr-c[D-A2bu-Gly-Phe-Leu] incorporating a phenylalanine at the third position in place of glycine. In addition to two parent dermorphin analogues Tyr-c[D-A2bu-Phe-Phe-(L and D)-Leu], four stereoisomeric retro-inverso modified analogues Tyr-c[D-A2bu-Phe-gPhe-(S and R)-mLeu] with a reversed amide bond between residues four and five, and Tyr-c[D-Glu-Phe-gPhe-(L and D)-rLeu] with two reversed amide bonds between residues four and five, and between residue five and the side chain of residue two have been synthesized. The results from the guinea pig ileum (GPI) and mouse vas deferens (MVD) assays show that all analogues are superactive at either one or both opiate receptors and in general display higher activities as compared to the corresponding enkephalin analogues with a glycine at the third position. Results from the in vitro biological assays and conformational analysis using 1H-NMR spectroscopy (adjoining paper) will provide useful information to understand the role of the Phe3 aromatic side chain in dermorphin, and that of the Phe4 aromatic side chain in enkephalin, on opiate activity since these cyclic dermorphin analogues contain two Phe residues at both the third and fourth positions.

Automated solid-phase peptide synthesis: use of 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate for coupling of tert-butyloxycarbonyl amino acids

2-(1H-Benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) has been adapted for use as a coupling reagent for tert-butyloxycarbonyl (Boc) amino acids in automated solid-phase peptide synthesis. When compared to the existing preformed symmetrical anhydride procedure employing dicyclohexyl-carbodiimide (DCC), the use of TBTU in the presence of 1-hydroxybenzotriazole (HOBt) provides a more efficient coupling procedure for Boc-amino acid derivatives. Overall cycle times using TBTU/HOBt coupling reagents (30 min) compare favorably to those of the DCC-mediated procedure (approx 65 min). Dimethylformamide can be used as the sole solvent for both activation and coupling reactions. Implementation of TBTU/HOBt coupling conditions does not require replumbing of any lines of the Applied Biosystems Model 430A instrument and necessitates changes to only three reagent bottle positions. The variable coupling efficiencies of Boc-asparagine following activation with TBTU/HOBt (as low as 89%) can be overcome by protection of the amide function of Boc-asparagine with the 9-xanthyl group. Examples of the synthesis and characterization of a number of peptides ranging in length from 13 to 29 residues are given.

An assay for acidic peptide substrates of protein kinases

The assay of acidic peptides as substrates for protein kinases has not been as easy to perform as testing basic peptides or polypeptides. We have developed a simple, rapid, and cost-effective procedure that allows the design and testing of potential peptide substrates without the constraints imposed by the phosphocellulose filter paper method (the need to incorporate positively charged residues into the peptide sequence). The technique combines the chelation of 32Pi by acid molybdate with PEI-cellulose chromatography. In this way the migration of 32P-labeled Pi, ATP, and protein are impeded while phosphopeptide is eluted in 1.5 ml from a 0.25-ml disposable column. In order to validate the assay we used two angiotensin II analogues as peptide substrates for the protein tyrosine kinase pp60c-src. The assay results using the new procedure were compared to those of the phosphocellulose filter paper technique. We also demonstrated the use of this method to test linear and cyclic peptides that could not be assayed with the phosphocellulose paper technique. This assay will aid those who are attempting to determine the substrate specificity of protein kinases.

Electrophysiological activity of the C-peptide of the Locusta insulin-related peptide. Effect on the membrane conductance of Locusta neurones in vitro

The C-peptide of Locusta insulin-related peptide, which is a 50 residue peptide originally isolated from the corpora cardiaca of the insect Locusta migratoria and to which we refer as 5-kDa peptide, has been synthesised chemically by the solid-phase method, using a BOC strategy. Since this peptide contains in its sequence a potential monobasic cleavage site, we also synthesised its 1-38 residue-related fragment, named 4-kDa peptide, although we have no hints of its natural occurrence in the corpora cardiaca. Electrophysiological studies have shown that both the 5-kDa and 4-kDa peptides depolarise the membrane and increase the membrane conductance of neurones freshly isolated from the thoracic ganglia of Locusta. Under voltage-clamp conditions, the current underlying these effects was inwardly directed and could be resolved into 2 components. One component, I(5-kDa)1, activated at potentials more hyperpolarised than -50 mV, peaked at about -75 mV and was blocked by the potassium channel blockers cesium and rubidium. The second component, I(5-kDa)2 was activated at potentials more depolarised than -50 mV, increased with depolarisation and was not blocked by cesium and rubidium. The effects of the 5-kDa and 4-kDa peptides on the membrane potential and membrane conductance of Locusta neurones suggest that these peptides may have a physiological role in the central nervous system of insects.

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.

Synthesis and NMR conformational analysis of a beta-turn mimic incorporated into gramicidin S. A general approach to evaluate beta-turn peptidomimetics

The solution structure of a gramicidin S (GS) analog containing a beta-turn mimic [BTD4-5, Lys2.2']GS has been compared to that of native GS. The linear [BTD4-5, Lys2.2']GS was synthesized by solid phase methodology and the cyclized peptide was analyzed by NMR. In the peptide portion of [BTD4-5, Lys2.2']GS, the intramolecular hydrogen bonding pattern, inter-residue NOEs, including a transannular H alpha-H alpha NOE, and JN alpha coupling constants all describe a solution structure which is equivalent to that of native GS. These data confirm that the BTD group is a competent Type II' beta-turn mimic since it does not disrupt the native conformation of GS. It also supports the use of GS as a conformational model in which to test beta-turn mimics.

Analysis of subsite preferences of HIV-1 proteinase using MA/CA junction peptides substituted at the P3-P1' positions

The residues P3, P2, P1, and P1' of a peptide corresponding to the matrix/capsid protein junction in the HIV-1 gag protein (Ser-Gln-Asn-Tyr-Pro-Ile-Val) were systematically replaced and the effect of these single amino acid substitutions on the hydrolysis of each peptide by HIV-1 proteinase was studied. Subsites S1 and S1' of the enzyme showed explicit preference for hydrophobic moieties, but beta-branched amino acids and proline are not tolerated in S1. The S2 subsite shows a preference for small polar and apolar amino acids; it may be occupied by Asn, Asp, Glu, Cys, Ala, or Val, other substitutions, especially by Gln and Ser, prevent hydrolysis of the peptides. In subsite S3 all amino acids except proline can be accommodated.

Cyclization studies with tetra- and pentapeptide sequences corresponding to beta-casomorphins

The tetrapeptide Boc-D-Orn-Phe-D-Pro-Gly-OH and the pentapeptide sequence Boc-Tyr(tBu)-D-Orn-Phe-D-Pro-Gly-OH were used to study the influence of different coupling reagents on the yield and purity of these model peptides. The simple structure prevented racemization and cyclodimerization and facilitated the ring formation. The most favorable effects on yield and purity were obtained in both reactions using diphenyl-phosphoryl azide (DPPA) and norborn-5-ene-2,3-dicarboximidodiphenylphosphate (NDPP), while the cyclizations with the powerful activating reagents benzotriazol-1-yl-oxy-tris(dimethylamino)-phosphonium hexafluorophosphate (BOP) and 2-(1-H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) with the exception of the cyclopentapeptide reaction with HBTU/4-dimethylaminopyridine gave unsatisfactory results.

Conductance measurements in solid phase peptide synthesis. I. Monitoring coupling and deprotection in Fmoc chemistry

Potentiated conductance shows promise for monitoring the progress of both coupling and deprotection reactions in solid phase peptide synthesis using Fmoc chemistry. Sterically hindered base may be added to the aprotic polar solvents used for synthesis, this induces ionisation of acids formed in the coupling and deprotection reactions, thus giving rise to a conductance signal in the solvent. Changes in this signal allow reactions occurring at the resin to be followed with no degradation of coupling efficiency.

Solution phase synthesis of Saccharomyces cerevisiae a-mating factor and its analogs

The solution phase synthesis of the Saccharomyces cerevisiae a-mating factor and nonfarnesylated and nonmethylated a-factor analogs are reported. The a-factor, a lipopeptide with the sequence Tyr-Ile-Ile-Lys-Gly-Val-Phe-Trp-Asp-Pro-Ala-Cys(S-Farnesyl)OCH3 was synthesized by the condensation of the amine terminal protected decapeptide with the carboxyl terminal farnesylated dipeptide using benzotriazol-l-yloxy-tris-(dimethylamino)-phosphonium hexafluorophosphate (BOP reagent) as the coupling agent. The synthesis of the decapeptide involved 5 + 5 fragment coupling with the BOP reagent and the successful application of 9-fluorenylmethyl ester(OFm) and 9-fluorenylmethoxycarbonyl(Fmoc) groups for the protection of Asp and Lys side chains and Tyr alpha-amine and of phenacyl esters (OPa) for alpha-carboxyl protection. The OFm and Fmoc groups tolerated repeated couplings and were completely stable to zinc powder in acetic acid, a condition under which the OPa group was removed. The synthesis of the nonfarnesylated alpha-factor was accomplished by the coupling of the decapeptide with tetrapeptide (Ala-CysOCH3)2 followed by the deprotection of the OFm and Fmoc groups with piperidine and the cleavage of the disulfide bond with zinc powder in acetic acid. The nonmethylated a-factor was prepared by 10 + 2 fragment coupling using OFm protection of the dipeptide carboxyl group followed by removal of all protecting groups with piperidine. Attempts to saponify a-factor were not successful. The synthetic nonfarnesylated and nonmethylated a-mating pheromones were 100-1000 times less active than the a-factor, indicating that although the methyl ester and the farnesyl group are not essential for biological activity, they are necessary for high potency.

Solid-phase synthesis of peptides containing phosphoserine using phosphate tert.-butyl protecting group

In this paper, we report the solid-phase synthesis of peptides containing O-phosphonoserine using BOP as coupling reagent. Commercially available Fmoc amino-acids linked to p-alkoxybenzyl resin were used in the first step and Alloc amino acids in the following. Alloc group was removed by catalytic hydrostannolytic cleavage. Acid-labile side-chain protecting groups (including phosphate residue) were used. Thus, both removal of side-chain protecting groups and cleavage of the phosphopeptide from the resin were achieved in one step by treatment with TFA. Alloc serine was phosphorylated by the phosphoramidite method. This strategy enables the preparation of peptides with selectively phosphorylated residue and overcomes problems due to repetitive treatments with TFA and final cleavage with HF.

Recent advances in solid-phase peptide synthesis and preparation of antibodies to synthetic peptides

Peptides prepared by the solid-phase peptide synthesis (SPPS) approach are used increasingly in biological research, for instance to elicit anti-peptide antibodies that will recognize the intact, cognate protein. Recent advances in SPPS are reviewed, including the use of new coupling reagents, new methods for evaluating peptide purity and new techniques of automated and multiple peptide synthesis. Methods for enhancing peptide immunogenicity are discussed such as the use of adjuvants and liposomes, and of synthetic branched polypeptides as carriers.

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.

Repetitive BOP coupling (REBOP) in solid phase peptide synthesis. Luliberin synthesis as model

The coupling reagent (benzotriazol-1-yloxy)tris-(dimethylamino)phosphonium (BOP) hexafluorophosphate was tested in the synthesis of luliberin (LH-RH) with inexpensive classically protected Boc-amino acids, in slight excess, and benzhydryl amino resin, without any other additive. The good solubility of this reagent and its by-products is of particular interest for automated peptide synthesis. [D-His2]LH-RH was also synthesized and compared with LH-RH by proton nuclear magnetic resonance spectroscopy. As shown by the biological tests and the high performance liquid chromatography study, unprotected pyroGlu and Boc-His can be used without any significant racemization but Boc-His(Boc) was found to be preferable since it gave no detectable racemization and no by-products. The difficult isolation of the minority D-derivative from the crude preparation of LH-RH was resolved by a recycling procedure in reversed phase HPLC.

Synthesis and immunosuppressive activities of conformationally restricted cyclosporin lactam analogues

Cyclosporin A (CsA, 1), an immunosuppressive cyclic undecapeptide, is known to adopt predominantly a single conformation in chloroform solution, characterized in part by a type II' beta-turn encompassing Abu-Sar-MeLeu-Val (residues 2-5). In order to evaluate whether this beta-turn is bound by the receptor, we previously had prepared a conformationally restricted beta-turn analogue, (delta-lactam) CsA (2), which was found to retain only weak immunosuppressive activity, a result that could indicate that steric hindrance between receptor and the lactam atoms in 2 diminished activity or that the type II' beta-turn is not a feature in the bioactive conformation of CsA. In an attempt to distinguish between these two possibilities, we have synthesized two new CsA analogues, (gamma-lactam) CsA (3) and (des-N-methyl-lactam) CsA (4), which contain less sterically demanding conformational restrictions. The immunosupressive activity of each analogue (4-13% and 7-17%, respectively, relative to CsA), measured in an assay that determined the inhibition of concanavalin A stimulated thymocytes, is essentially equipotent with that of the delta-lactam. The chemical shifts and temperature dependencies of the protons in analogues 3 and 4 are very similar to the corresponding protons in CsA and in 2, which suggest that the solution conformations of the small lactam analogues are very similar to that of the delta-lactam 2. The synthesis of the lactam components and the corresponding CsA derivatives is described. Reduction in the size of the lactam ring does not lead to enhanced immunosuppressive activity.

Drug targeting: anti-HSV-1 activity of mannosylated polymer-bound 9-(2-phosphonylmethoxyethyl)adenine

The antiviral drug, 9-(2-phosphonylmethoxyethyl) adenine (PMEA) was linked to a synthetic and neutral polymer bearing mannosyl residues to allow its internalization by macrophages via membrane lectins. PMEA bound to the mannosylated polymer was more efficient in vitro than free PMEA in preventing lysis of human macrophages by herpes virus.

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.

BOP reagent for the coupling of pGlu and Boc-His(Tos) in solid phase peptide synthesis

The model peptide TRH was successfully synthesized using benzotriazol-1-yl-oxy-tris(dimethylamino)phosphonium hexafluorophosphate (BOP reagent). The coupling reactions were carried out in N,N-dimethylformamide or N-methylpyrrolidone. These solvents allowed the incorporation of the N-terminal pyroglutamic acid residue into the peptide chain, without using the derivative bearing the N-benzyloxycarbonyl group, which acts as a solubility promoter. A comparative racemization study showed that Boc-His(Tos) can be coupled by means of BOP reagent with less racemization than with DCC when the amount of diisopropylethylamine (DIEA) is kept minimal (same ratio of equivalents as for Boc-His(Tos), i.e. 3 equiv.). However, with the use of a larger amount of DIEA in the coupling mixture (9 equiv.), approximately 3% of epimer was found in the crude product. Our study showed that even under low DIEA conditions, the rate of coupling of the residues with BOP remained comparable to that observed with DCC.

Helix-coil stability constants for the naturally occurring amino acids in water. XXIII. Proline parameters from random poly (hydroxybutylglutamine-co-L-proline)

Water-soluble random copolymers containing L-proline and N5-(4-hydroxybutyl)-L-glutamine were synthesized by copolymerization of the tripeptides H-L-Glu(OBzl)-L-Glu(OBzl)-L-Glu(OBzl)-OH and H-L-Glu(OBzl)-L-Pro-L-Glu(OBzl)-OH, using benzotriazolyl-N-oxy-tris(dimethylamino)-phosphonium hexafluorophosphate as condensing reagent, and subsequent aminolysis of the Bzl ester groups with 4-amino-1-butanol. These copolymers were found to contain significant amounts of N5-(4-hydroxybutyl)-D-glutamine, thus requiring the synthesis of a binary copolymer containing only D- and L-N5-(4-hydroxybutyl)glutamine residues in order to evaluate the possible effects of the D-residues on the conformational properties of poly(hydroxybutylglutamine-co-L-proline). The different copolymers were fractionated, and their thermally induced helix-coil transition curves were obtained in water at neutral pH. When proper corrections were applied for the helix-destabilizing properties of N5-(4-hydroxybutyl)-D-glutamine, the Zimm-Bragg parameters sigma and s for L-proline could be deduced from the melting curves of poly(hydroxybutylglutamine-co-L-proline). The results indicate that L-proline acts as a very strong helix breaker over the entire temperature range from 0 to 60 degrees C.