Cyclic Octapeptide Research Articles

Water‐Soluble and pH‐Responsive Polymeric Nanotubes from Cyclic Peptide Templates

Water-soluble organic nanotubes were prepared by convergently conjugating polymers of hydroxyethyl acrylate (HEA) and acrylic acid (AA) to self-assembling cyclic octapeptides of alternating D and L chirality. The structure of the self-assembled tubes was characterised in a number of polar solvents, and notably water, by using light scattering, TEM and small angle neutron scattering (SANS) techniques. In addition, the self-assembly into tubes could be controlled by exploiting the pH responsiveness of acrylic acid polymers.

Thermoresponsive cyclic peptide – poly(2-ethyl-2-oxazoline) conjugate nanotubes

We describe the synthesis of self-assembling cyclic octapeptides of alternating D- and L-chirality conjugated to two poly(2-ethyl-2-oxazoline) chains. The resulting conjugates form nanotubes in water, which are reversibly transformed into microparticles as the temperature reaches the cloud point temperature of the conjugate.

Structure elucidation and control of cyclic peptide-derived nanotube assemblies in solution

We describe the solution assembly of polymer–cyclic peptide conjugates into nanotubes, by direct in situ measurements. The conjugates were assembled by exploiting the β-sheet assembly of the alt(D,L) cyclic octapeptide core. The conjugated polymer permits solubilization of the peptide and the resulting nanotubes, thus allowing for the first time the direct study of the assembly mechanism of this system. The resulting materials present unique properties for a wide range of applications. We find that the polymer can act to both shield the peptide core from the solvent and to put strain on the peptide core through steric repulsions. By controlling both the solvent mixture and the length of the polymer, control over the length of the resulting nanotubes can be obtained. In addition, monitoring the assembly with temperature allows the strength of the assembly to be probed, adding evidence for a co-operative mechanism of assembly. Finally, selective deuteration of the polymer component in SANS analyses leads to the precise measurement of the nanotubes core dimensions, and by cross-linking the polymeric shell, the structure of the nanotubes in solution are confirmed by transmission electron microscopy (TEM). This study establishes the fundamentals needed for the design and control of these new materials.

Synthesis, characterization, conformational analysis of a cyclic conjugated octreotate peptide and biological evaluation of 99mTc-HYNIC-His3-Octreotate as novel tracer for the imaging of somatostatin receptor-positive tumors

Peptides are attracting increasing interest in nuclear oncology for targeted tumor diagnosis and therapy. We therefore synthesized new cyclic octapeptides conjugated with HYNIC by Fmoc solid-phase peptide synthesis. These were purified and analyzed by RP-HPLC, MALDI mass, (1)H NMR, (13)C NMR, HSQC, HMBC, COSY and IR spectroscopy. Conformational analysis of the peptides was performed by circular dichroism spectroscopy, in pure water and trifluoroethanol-water (1:1), revealed the presence of strong secondary structural features like β-sheet and random coils. Labeling was performed with (99m)Tc using Tricine and EDDA as coligands by SnCl(2) method to get products with excellent radiochemical purity >99.5 %. Metabolic stability analysis did not show any evidence of breaking of the labeled compounds and formation of free (99m)Tc. Internalization studies were done and IC(50) values were determined in somatostatin receptor-expressing C6 glioma cell line and rat brain cortex membrane, and the results compared with HYNIC-TOC as standard. The IC(50) values of (99m)Tc-HYNIC-His(3)-Octreotate (21 ± 0.93 nM) and (99m)Tc-HYNIC-TOC (2.87 ± 0.41 nM) proved to be comparable. Biodistribution and image study on normal rat under gamma camera showed very high uptake in kidney and urine, indicating kidney as primary organ for metabolism and route of excretion. Biodistribution and image study on rats bearing C6 glioma tumor found high uptake in tumor (1.27 ± 0.15) and pancreas (1.71 ± 0.03). Using these findings, new derivatives can be prepared to develop (99m)Tc radiopharmaceuticals for imaging somatostatin receptor-positive tumors.

Identification of Elongation Factor G as the Conserved Cellular Target of Argyrin B

Argyrins, produced by myxobacteria and actinomycetes, are cyclic octapeptides with antibacterial and antitumor activity. Here, we identify elongation factor G (EF-G) as the cellular target of argyrin B in bacteria, via resistant mutant selection and whole genome sequencing, biophysical binding studies and crystallography. Argyrin B binds a novel allosteric pocket in EF-G, distinct from the known EF-G inhibitor antibiotic fusidic acid, revealing a new mode of protein synthesis inhibition. In eukaryotic cells, argyrin B was found to target mitochondrial elongation factor G1 (EF-G1), the closest homologue of bacterial EF-G. By blocking mitochondrial translation, argyrin B depletes electron transport components and inhibits the growth of yeast and tumor cells. Further supporting direct inhibition of EF-G1, expression of an argyrin B-binding deficient EF-G1 L693Q variant partially rescued argyrin B-sensitivity in tumor cells. In summary, we show that argyrin B is an antibacterial and cytotoxic agent that inhibits the evolutionarily conserved target EF-G, blocking protein synthesis in bacteria and mitochondrial translation in yeast and mammalian cells.

How to blast osteoblasts? Novel dicarba analogues of amylin-(1–8) to treat osteoporosis

When administered in vivo, amylin (1–8) stimulates osteoblast proliferation increasing bone volume and bone strength. The native cyclic octapeptide amylin (1–8) is unstable, however, it provides an attractive framework for the creation of more stable, orally active synthetic analogues using various peptidomimetic techniques. On-resin ring closing metathesis (RCM) on the olefinic side chains of allylglycine residues and lysine moieties functionalized with an allyloxycarbonyl (Alloc) group, was used to prepare novel carba-bridged surrogates of the disulfide bridge between Cys/2 and Cys/7 in amylin-(1–8). Commercially available Nα-Fmoc Nε-Alloc protected lysine was used as a convenient substrate for Grubbs’ ring closing metathesis. Analogues of amylin-(1–8) prepared by cyclization of allylglycine residues that also contained proline residues at either position 4 or 6, or both, were also prepared to investigate the effect of proline as a ‘kink-inducing’ residue on the efficiency of the RCM reaction. Of the nine novel alkene-bridged analogues prepared, five showed promising biological activity in a proliferation study in primary foetal rat osteoblasts at physiological concentrations. Two of these analogues were chosen for further in vivo evaluation.

Synthesis, docking and anticancer activity studies of D-proline-incorporated wainunuamide

D-proline-incorporated wainunuamide — a cyclic octapeptide was synthesized and characterized by FTIR, 1H and 13C NMR and Mass spectral analysis. Molecular docking studies were carried out for the designed cyclic octapeptide and the results showed greater affinity for HPV18-2IOI receptor (HeLa cancer cell line). The synthesized cyclic octapeptide exhibited potent anticancer activity against HeLa cancer cells. D-proline-incorporated wainunuamide — a cyclic octapeptide was synthesized and characterized by spectral analysis. The compound exhibited potent anticancer activity against HeLa cancer cell line.

Dependences of Water Permeation through Cyclic Octa-peptide Nanotubes on Channel Length and Membrane Thickness

Effects of the channel length and membrane thickness on the water permeation through the transmembrane cyclic octa-peptide nanotubes (octa-PNTs) have been studied by molecular dynamics (MD) simulations. The water osmotic permeability (p(f)) through the PNTs of k × (WL)(4)/POPE (1-palmitoyl-2-oleoyl-glycerophosphoethanolamine; k = 6, 7, 8, 9, and 10) was found to decay with the channel length (L) along the axis (~L(-2.0)). Energetic analysis showed that a series of water binding sites exist in these transmembrane PNTs, with the barriers of ~3k(B)T, which elucidates the tendency of p(f) well. Water diffusion permeability (p(d)) exhibits a relationship of ~L(-1.8), which results from the novel 1-2-1-2 structure of water chain in such confined nanolumens. In the range of simulation accuracy, the ratio (p(f)/p(d)) of the water osmotic and diffusion permeability is approximately a constant. MD simulations of water permeation through the transmembrane PNTs of 8 × (WL)(4)/octane with the different octane membrane thickness revealed that the water osmotic and diffusion permeability (p(f) and p(d)) are both independent of the octane membrane thickness, confirmed by the weak and nearly same interactions between the channel water and octane membranes with the different thickness. The results may be helpful for revealing the permeation mechanisms of biological water channels and designing artificial nanochannels.

First total synthesis of antifungal cyclopeptide tunicyclin d by a solid-phase method

The present study deals with the first total synthesis of a new antifungal cyclic octapeptide, tunicyclin D, cyclo[VNIPPWHG], where all the amino acids are L-configuration, by a two-step solid-phase/solution synthesis strategy. The linear octapeptide was assembled by a solid-phase peptide synthesis (SPPS) method. Subsequently cyclization and deprotection were achieved in solution with high efficiency and reproducibility. The final product was purified by preparative RP-HPLC, and its structure was identified by ESI-MS, 1 H NMR, 13 C NMR, and HR-QTOF-MS.

ChemInform Abstract: Stylissamide X (I), a New Proline‐Rich Cyclic Octapeptide as an Inhibitor of Cell Migration, from an Indonesian Marine Sponge of Stylissa sp.

Abstractisolation, structure elucidation and screening of inhibitory activity against migration of HeLa cells

Stylissamide X, a new proline-rich cyclic octapeptide as an inhibitor of cell migration, from an Indonesian marine sponge of Stylissa sp.

A new proline-rich cyclic octapeptide named stylissamide X (1) was isolated from an Indonesian marine sponge of Stylissa sp. as an inhibitor of cell migration from the guidance of wound-healing assay. The chemical structure of stylissamide X (1) was determined on the basis of spectroscopic analysis, and stereostructure of the amino acids were deduced by Marfey’s method. Compound 1 showed inhibitory activity against migration of HeLa cells in the ranges of 0.1–10μM concentration through both wound-healing assay and chemotaxicell chamber assay, while the cell viability was maintained more than 75% up to 10μM concentration of 1.

The cyclopeptides with the multi-His motif as ligands for copper(II)

The coordination properties of cyclic octapeptides with multi-His motif: c(His-Gly-His-Xaa-His-Gly-His-Xaa) where Xaa=Asp or Lys, were investigated. The binding abilities of this peptides towards Cu(II) ions were studied by using different analytic methods as: potentiometry, spectroscopy and mass spectrometry. The obtained results show that the studied peptides in physiological related pH prefer formation of the species with the {4NIm} binding mode. The efficiency of Cu(II) binding depends on additional side chain groups Asp or Lys. Additionally the analysis of results for His containing cyclopeptides with different numbers of amino acid residues in cyclopeptide ring e.g. four, eight shows that in higher pH in both cases the binding by four amide nitrogens is not observed in the case of α-amino acid peptides.

Highly Efficient and Directional Homo‐ and Heterodimeric Energy Transfer Materials Based on Fluorescently Derivatized α,γ‐Cyclic Octapeptides

Cyclic octapeptides composed of α-amino acids alternated with cis-3-aminocycloalkanecarboxylic acids, self-assemble as drumlike dimers through β-sheet-like, backbone-to-backbone hydrogen bonding. Heterodimerization appears to be significantly more favored than homodimerization, and this represents a novel approach for the design and fabrication of highly stable heterodimeric assemblies. A multicomponent equilibrium network based on fluorescently derivatized self-assembling α,γ-cyclic octapeptides has been successfully used to form light-harvesting/light-converting ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.

Syntheses and activities of backbone‐side chain cyclic octapeptide ligands with N‐functionalized phosphotyrosine for the N‐terminal SH2‐domain of the protein tyrosine phosphatase SHP‐1

The protein tyrosine phosphatase SHP-1 plays an important role in many physiological and pathophysiological processes. This phosphatase is activated through binding of ligands to its SH2-domains, mainly to the N-terminal one. Based on a theoretical docking model, backbone-to-side chain cyclized octapeptides were designed as ligands. Assembly of such modelled structures required the synthesis of N-functionalized tyrosine derivatives and their incorporation into the sequence. Because of difficulties encountered in the condensation of N-protected amino acids to the N-alkylated tyrosine-peptide we synthesized and used preformed dipeptide building units. As all attempts to obtain phosphorylated dipeptide units failed, the syntheses had to be performed with a free phenolic function. Use of different N-alkyl or cycloalkyl residues in the N-functionalized side chains allowed to investigate the effect of ring size, flexibility and hydrophobicity of formed lactam bridges on stimulatory activity. All tested linear and cyclic octapeptides stimulate the phosphatase activity of SHP-1. Stimulatory activities of cyclic ligands increase with the chain length of the lactam bridges resulting in increased flexibility and better entropic preformation of the binding conformation. The strong activity of some cyclic octapeptides supports the modelled structure.

Synthesis of Self-assembling Cyclic Peptide-polymer Conjugates using Click Chemistry

Self-assembling cyclic peptide-polymer conjugates were prepared by ‘clicking’ polymers (prepared by RAFT polymerization) to an azide functionalized d-alt-l cyclic octapeptide via the Huisgen 1,3-dipolar cycloaddition reaction. Due to the high graft density, the efficiency of the click chemistry conjugation reaction was found to be highly dependent on the size of the polymer. At relatively low molecular weights, as many as four polymer chains could be grafted to each 8 residue cyclic peptide ring. Evidence for the self assembly of the conjugates into peptide-polymer nanotubes was observed by TEM and IR.

Synthesis of Cyclogossine B Using a Traceless Pseudoproline Turn-Inducer

The first synthesis of the cyclic octapeptide, cyclogossine B, has been achieved, confirming the reported structure of this natural product. Cyclization of a linear precursor containing a cysteine-derived thiazolidine as a traceless turn-inducer occurred in significantly higher yields than cyclization of the analogous alanine-containing precursor under identical conditions. Deprotection of the thiazolidine followed by desulfurization provided cyclogossine B in good overall yield, indicating that cysteine-derived pseudoprolines can be effectively used as traceless turn-inducers to facilitate the cyclization of small peptides.

Conformational properties and metal ion transport by synthetic cyclic octapeptide cyclo(D-Leu-L-Pro)4

Cyclo(D-LeU-L-PrO)4 was synthesized and investigated on conformation, complexation with cations, and cation transport through liquid membrane. Cyclo(D-Leu-L-Pro)4 was found to take a C2-symmetric conformation in CDCl3 solution. However, the intramolecular transformation of conformation was so rapid on n.m.r. time scale that it appeared to take a C4-symmetric conformation in n.m.r. spectrum. Cyclo(D-Leu-L-Pro)4 formed a complex selectively with Ba2+, and the binding constant was estimated to be similar to that of its diastereomer, cyclo(L-Leu-L-Pro)4. However, the rate of complex formation in solution of CyClO(D-Leu-L-Pro)4 with Ba2+ was much faster than that of cyclo (L-Leu-L-Pro)4. This is because cyclo(D-Leu-L-Pro)4 forms the complex without accompanying the isomerization of peptide bonds. The efficiency of cation transport through a CHCl3 liquid membrane by Cyclo(D-LeU-L-PrO)4 was similar to that by cyclo(L-Leu-L-Pro)4, indicating that the rate-determining step of transport should be the diffusion of the complex across the boundary layer between aqueous phase and organic phase.

Clausenain B, a phenylalanine-rich cyclic octapeptide from Clausena anisum-olens

A new cyclic octapeptide, named clausenain B, was isolated by a multi-step chromatography procedure from Clausena anisum-olens. Its structure was established as cyclo(-Phe¹-Ser-Leu¹-Phe²-Phe4-Gly-Leu²-Phe³-) (1) based on extensive spectroscopic studies and chemical evidence. Clausenain B (1) is a phenylalanine-rich cyclic octapeptide.

Second‐harmonic generation from Langmuir–Blodgett film of cyclic peptide carrying two disperse red 1 on fused quartz surface

Cyclic octapeptide carrying one or two nonlinear optical chromophores, disperse red 1 (DR-1), was synthesized and immobilized on a substrate to attain an active surface for second-harmonic generation (SHG). Each cyclic octapeptide was transferred on a fused quartz substrate by the Langmuir-Blodgett (LB) method to afford a uniform monolayer. Infrared reflection-absorption spectroscopy of the LB monolayer revealed that the cyclic skeleton lay roughly flat on the surface. The SHG intensity from the monolayer of the cyclic peptide with two DR-1 units was stronger than that from that with one DR-1 unit. The difference is discussed in terms of molecular orientation and surface density of the active chromophores. The cyclic peptide is shown here to be an effective scaffold to modify a substrate surface with functional groups of a monolayer with taking stability of the monolayer and orientation of the functional group into consideration.

Synthesis and characterisation of a ruthenocenoyl bioconjugate with the cyclic octapeptide octreotate

The reaction of activated ruthenocene carboxylic acid with the resin-bound peptide octreotate yields, after cleavage and purification by preparative HPLC, the first ruthenocenoyl peptide bioconjugate 1. Octreotate is a chemically stabilized analogue of somatostatine. It is a cyclic octapeptide with a disulfide bond and has been previously used for molecular diagnostics due to the fact that somatostatine receptors are over-expressed by a variety of cancer cells. Conjugate 1 was obtained in good yield and purified by preparative HPLC to >95% purity as judged by analytical HPLC. It has been identified by HPLC, IR and mass spectrometry (ESI and MALDI-TOF). The peptide’s NMR signals are assigned by standard 2D methods. In addition, the 1H NMR spectrum of 1 shows characteristic signals for the metallocene between 5.1 and 4.3 ppm. Compound 1 thus is a new example of tumor-targeted organometallic ruthenium bioconjugates.