Synthesis of peptide amides using Fmoc-based solid-phase procedures on 4-methylbenzhydrylamine resins

pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center Within a Peptide Scaffold

A disulfide-bridged peptide containing two Ni2+ binding sites based on the nickel superoxide dismutase protein, {Ni2(SODmds)}, has been prepared. At physiological pH (7.4) it was found that the metal sites are mononuclear with a square planar NOS2 coordination environment with the two sulfur-based ligands derived from cysteinate residues, the nitrogen ligand derived from the amide backbone and a water ligand. Furthermore, S K-edge X-ray absorption spectroscopy indicated that the two cysteinate sulfur atoms ligated to nickel are each protonated. Elevation of the pH to 9.6 results in the deprotonation of the cysteinate sulfur atoms, and yields a binuclear, cysteinate bridged Ni22+ center with each nickel contained in a distorted square planar geometry. At both pH = 7.4 and 9.6 the nickel sites are moderately air sensitive, yielding intractable oxidation products. However, at pH = 9.6 {Ni2(SODmds)} reacts with O2 at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate the reduced reactivity at pH = 7.4 is a result of a reduction in S(3p) character and deactivation of the nucleophilic frontier molecular orbitals upon cysteinate sulfur protonation.

t-boc synthesis of huwentoxin-i through native chemical ligation incorporating a trifluoromethanesulfonic acid cleavage strategy

Tert-butyloxycarbonyl (t-Boc)-based native chemical ligation (NCL) techniques commonly employ hydrogen fluoride (HF) to create the thioester fragment required for the ligation process. Our research aimed to assess the replacement of HF with Trifluoromethanesulfonic acid (TFMSA). Here we examined a 33 amino acid test peptide, Huwentoxin-I (HwTx-I) as a novel candidate for our TFMSA cleavage protocol. Structurally HwTx-I has an X-Cys(16) -Cys(17) -X sequence mid-region, which makes it an ideal candidate for NCL. Experiments determined that the best yields (16.8%) obtained for 50 mg of a thioester support resin were achieved with a TFMSA volume of 100 μL with a 0.5-h incubation on ice, followed by 2.0 h at room temperature. RP-HPLC/UV and mass spectra indicated the appropriate parent mass and retention of the cleaved HwTx-I N-terminal thioester fragment (Ala(1) -Cys(16) ), which was used in preparation for NCL. The resulting chemically ligated HwTx-I was oxidized/folded, purified, and then assessed for pharmacological target selectivity. Native-like HwTx-I produced by this method yielded an EC50 value of 340.5 ± 26.8 nM for Nav 1.2 and an EC50 value of 504.1 ± 81.3 nM for Nav 1.3, this being similar to previous literature results using native material. This article represents the first NCL based synthesis of this potent sodium channel blocker. Our illustrated approach removes potential restrictions in the advancement of NCL as a common peptide laboratory technique with minimal investment, and removes the hazards associated with HF usage. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 737-745, 2016.

Deglycosylation of glycoproteins with trifluoromethanesulphonic acid: elucidation of molecular structure and function

The alteration of proteins by post-translational modifications, including phosphorylation, sulphation, processing by proteolysis, lipid attachment and glycosylation, gives rise to a broad range of molecules that can have an identical underlying protein core. An understanding of glycosylation of proteins is important in clarifying the nature of the numerous variants observed and in determining the biological roles of these modifications. Deglycosylation with TFMS (trifluoromethanesulphonic acid) [Edge, Faltynek, Hof, Reichert, and Weber, (1981) Anal. Biochem. 118, 131-137] has been used extensively to remove carbohydrate from glycoproteins, while leaving the protein backbone intact. Glycosylated proteins from animals, plants, fungi and bacteria have been deglycosylated with TFMS, and the most extensively studied types of carbohydrate chains in mammals, the N-linked, O-linked and glycosaminoglycan chains, are all removed by this procedure. The method is based on the finding that linkages between sugars are sensitive to cleavage by TFMS, whereas the peptide bond is stable and is not broken, even with prolonged deglycosylation. The relative susceptibility of individual sugars in glycosidic linkage varies with the substituents at C-2 and the occurrence of amido and acetyl groups, but even the most stable sugars are removed under conditions that are sufficiently mild to prevent scission of peptide bonds. The post-translational modifications of proteins have been shown to be required for diverse biological functions, and selective procedures to remove these modifications play an important role in the elucidation of protein structure and function.

Chemical deglycosylation can induce methylation, succinimide formation, and isomerization

Interpretation of deglycosylation studies relies heavily on the absence of modifications to the polypeptide chain. We have found that by using a common chemical deglycosylation technique, one can effect at least three changes in a peptide's structure: methylation, isomerization, and ring formation. It was determined that the conditions of chemical deglycosylation introduce a +14 Da shift in the masses of our model peptides, RKDVY, RKEVY, and horseradish peroxidase. This shift is localized to acidic functional groups and is interpreted as methylation of the free carboxylates in our models. An additional shift in mass of -18 Da is found in the model peptide RKDVY consistent with the loss of water associated with succinimide ring formation in this peptide. Chemical treatment induced isomerization of aspartyl residues to isoaspartyl residues in another model peptide, tetragastrin. These results indicate that one should use caution when interpreting the results of chemical deglycosylation experiments.

The synthesis of beta-peptides containing guanidino groups

The synthesis of the beta-peptide 1 by the postsynthetic modification of the corresponding amino-containing peptide 3 is described. The potential of 1 to act as a template for the ligation of complementary negatively-charged peptides is discussed.

Synthesis and application of peptide immunogens related to the sperm tail protein tpx-1, a member of the CRISP superfamily of proteins

The synthesis of peptides containing 0, 1 and 2 cysteine residues related to the human sperm tail protein, tpx-1, is described. These synthetic peptides, following conjugation to keyhole limpet hemocyanin modified with maleimidobenzoic acid N-hydroxysuccinimide ester, were used as immunogens to generate polyclonal antibodies in female New Zealand white rabbits. The binding characteristics of the derived antipeptide sera were evaluated using indirect and competitive ELISA procedures. Western immunoblot experiments also confirmed that these synthetic peptide immunogens are able to generate high-titer polyclonal antibodies capable of cross-reacting with the mature tpx-1 protein present in crude rat sperm tail/testis preparations as well as in outer dense fiber preparations. Consequently, these synthetic peptides represent promising candidates for investigations into the role of tpx-1 in the immunoregulation of sperm function in the rat and other mammalian models, with the derived antisera also providing an avenue to explore possible sites of expression of tpx-1 proteins in other tissues.

Characterization of epitope regions of thyrotropin beta-subunit recognized by the monoclonal antibodies mAb279 and mAb299: a chimeric peptide approach

This investigation describes the design, synthesis and evaluation of chimeric peptides related to the bovine thyrotropin beta-subunit, bTSHbeta. The structures of these chimeric peptides were derived from investigations with linear peptides and sequence alignment studies, in association with a homology model of TSHbeta developed from the hCG X-ray crystallographic structure. The structures of these chimeric peptides comprised beta-turn regions of loop L1 [bTSHbeta(14-20)] and loop L3 [bTSHbeta(65-72)] held in close proximity by a bis-beta-alanine linker and the disulfide bond bTSHbeta[Cys16-Cys67]. Linear and cyclic chimeric peptides were evaluated in immunochemical assays for their ability to inhibit the binding of radio-iodinated bTSHbeta [125I-bTSHbeta] to the monoclonal antibodies, mAb279 and mAb299. Previously, mAb279 and mAb299 have been shown to recognize epitopes accessible on the surface of TSHbeta that lie in close proximity to the TSH receptor-binding site. The results indicate that these chimeric peptides can specifically inhibit in a dose-dependent manner the binding of 125I-bTSHbeta to mAb299, while having a lesser effect on the binding with mAb279. Based on these results, it can be concluded that the bTSHbeta-epitope recognized by mAb299 involves contributions from amino residues from the beta-turn regions of the L1 and L3 loops of TSHbeta, and that these loop regions flank part of the receptor binding site of the bTSH beta-subunit.

Characterisation by immobilised metal ion affinity chromatographic procedures of the binding behaviour of several synthetic peptides designed to have high affinity for Cu(II) ions

In this investigation, several peptides containing an increasing number of histidine residues have been designed and synthesised. The peptides involved repeat units of either the pentameric EAEHA or the tetrameric HLLH sequence motifs. Adsorption isotherms for these synthetic peptides and hexahistidine (hexa-His) as a control substance were measured under batch equilibrium binding conditions with an immobilised Cu(II)-iminodiacetic acid (IDA) sorbent. The experimental data were analysed in terms of Langmuirean binding behaviour. In common with previous studies with synthetic peptides, these investigations have demonstrate that the sequential organisation of the histidine side chains in these peptides can affect the selectivity of the coordination interactions with borderline metal ions in immobilised metal ion affinity chromatographic systems. The results also confirm that peptides selected on the basis of their potential to form amphipathic secondary structures with their histidine residues presented on one face of the molecule can exhibit equivalent or higher affinity constants towards copper ions than hexa-His, although they contain fewer histidine residues. These findings are thus relevant to the selection of peptides produced inter alia by combinatorial synthetic procedures to have enhanced binding properties for Cu(II) or Ni(II) ions, or intended for use as peptide tags in the fusion handle approach for the affinity chromatographic purification of recombinant proteins.

Solid phase synthesis of cyclic peptides: model studies involving i-(i+4) side chain-to-side chain cyclisation

Conditions for the synthesis of i-(i + 4) side chain-to-side chain head-to-tail Lys-->Glu and Glu-->Lys linked cyclic peptides related to hypoglycaemic analogues of human growth hormone hGH [6-13] have been examined. The success of the cyclisation reaction with the corresponding resin-bound, partially protected linear peptides was found to be both reagent as well as sequence dependent, with competing inter-chain oligomerisation predominating in some cases. The results also indicated that protection with the bulky Fmoc group of the amino acid residues immediately adjacent to the side chain-deprotected Lys and Glu residues, which participate in the cyclisation reaction, enhanced the rate of lactam formation.

Comparison of the solution conformations of a human immunodeficiency virus peptidomimetic and its retro-inverso isomer using 1H NMR spectroscopy

The solution conformations of the all L-alpha-peptide 1 and the corresponding retro-all D-alpha-peptide 2, two 20-metric peptides which generate antibodies that cross-react with the gp 120 envelop protein of human immunodeficiency virus-1 (HIV-1), have been investigated by high-field 1H NMR spectroscopy. Complete sequential and inter-residue interaction assignments were made from the 2D NMR spectra acquired at 500 MHz and 600 MHz in 40% deuterotrifluoroethanol (d3-TFE)/H2O at pH 2.3, and in 300 mM sodium dodecyl sulphate (SDS) in 100% D2O or 90% H2O/10% D2O at pH 2.6. Based on analysis of the nuclear Overhauser effect (NOE) and amide exchange data, peptide 1 and its retro-inverso isomer 2 in the polar solvent environment of 40% d3-TFE/H2O at pH 2.3 show very similar topological features. However, in the relatively non-polar 300 mM SDS micellar environment, peptides 1 and 2 exhibit differences in their solution structures in terms of the amide backbone and side-chain orientations. In particular, under the SDS micellar condition, peptide 1 maintains much of the secondary structure observed for this 20-mer peptide in 40% d3-TFE/H2O, pH 2.3, whereas peptide 2 adopts a more extended structure. These NMR results provide the first confirmation that the secondary structure of the all L-a-peptide 1 is maintained in both polar and non-polar environments, whereas the secondary structure and topology of the notionally equivalent retro-inverso isomer depends more on the solvent conditions. These results with the all L-a-peptide 1 and its retro-inverso isomer 2 provide important insight into the conformational influences of the C- and N-end group with L-alpha- and retro-D-alpha-isomer pairs in non-polar environments, and thus have general relevance to the design of bioactive retro-inverso peptidomimetic analogues related to immunogenic or hormonal peptides.

Synthesis and biological activity of novel backbone-bicyclic substance-P analogs containing lactam and disulfide bridges

A biased library of 60 novel backbone-bicyclic Substance P analogs was prepared by the simultaneous multiple peptide synthesis method. The peptides, containing both a lactam and a disulfide ring, were synthesized by combined Boc and Fmoc chemistries, and were cyclized on the resin. Cleavage of the S-benzyl group and oxidation of the sulfhydryl groups was enabled by adaptation of the diphenylsulfoxidetrichloromethylsilane method to solid-phase synthesis. The peptides were screened for NK-1 and NK-3 activity, and were found to be weak agonists.

Backbone cyclization of the C-terminal part of substance P. Part 1: The important role of the sulphur in position 11

Novel backbone-to-side chain and backbone-to-backbone cyclic analogues of substance P (SP) were prepared by solid-phase synthesis and screened for biological activity. An analogue containing a thioetherlactam ring between positions 9 and 11 showed an EC50 value of 20 nM toward the neurokinin 1 (NK-1) and was inactive toward the NK-2 and NK-3 receptors. On the other hand, in a multiple backbone cyclic peptide library of similar analogues, in which the sulphur was excluded from the ring, very low activity was detected. The activity was re-evaluated and was found to be even lower (EC50 = 0.11 mM) than the previously published data. These results indicate that the thioether moiety has a crucial role in receptor activation. The results also show tolerance of the NK-1 receptor, but not NK-2 or NK-3, to cyclization of the C-terminal portion of the SP6-11 hexapeptide.