Synthesis, structure, and biological aspects of cyclopeptides related to marine phakellistatins 7-9

Phakellistatins 7, 8 and 9, three cyclic decapeptides naturally occurring in marine sponges of the genus Phakellia and characterized by the distinctive presence of Pro-Pro tracts, pose a non-trivial synthetic challenge, despite only containing coded amino acid residues. Their chemical synthesis was approached using a combination of solid and solution-phase techniques. As expected, our synthetic efforts yielded, for each cyclopeptide, a mixture of geometric isomers, owing to their cis-trans isomerism at Pro peptide linkages. A further complication arose because their synthesis yielded, together with the desired monomeric cyclopeptides, cyclodimeric species. In the case of phakellistatin 7 (originally determined as cis-Pro², cis-Pro⁸) our synthetic product was chemically and spectrally identical to the natural one, whereas none of the different isomeric products obtained for both phakellistatins 8 and 9 resulted to be fully equivalent (with respect to Pro geometries) to their natural counterparts. Finally, all synthetic cyclopeptides were submitted to biological assays and, as noted before for other members of the ‘proline rich’ family, synthetic compounds did not fully reproduce the biological properties (in terms of in vitro cytotoxicity against a panel of cancer cell lines) originally found for the natural products.     

Synthesis of novel proline-thiazole based cyclic hexa- and octapeptides

Novel proline-thiazole based cyclopeptides were produced by cyclooligomerisation of an L-proline thiazole amino acid HCl in the presence of pentafluorophenyl diphenylphosphinate (FDPP) or diphenyl phosphorazidate (DPPA).     

3D-structure of cycloreticulin C and glabrin A, cyclopeptides from the seeds of Annona reticulata

Two cyclopeptides, the cycloheptapeptide cycloreticulin C, cyclo(Pro¹-Gly²-Gln³-Pro⁴-Pro⁵-Tyr⁶-Val⁷) (1), and the cyclohexapeptide glabrin A, cyclo(Pro¹-Gly²-Leu³-Val⁴-Ile⁵-Tyr⁶) (2), have been isolated from the methanol extract of the seeds of Annona reticulata. Their structures were elucidated on the basis of the MS/MS fragmentation using a Q-TOF mass spectrometer equipped with an ESI source, chemical degradation and extensive 2D-NMR. The solution conformation of cycloreticulin C involves two β-turns, one of type II with trans-Pro¹ and Gly² at the corners, another of type VIa with trans-Pro⁴ and cis-Pro⁵ at the corners, and followed by a β-bulge at the Tyr⁶-Val⁷ level. The solid state and solution conformations of glabrin A have been analyzed by X-ray and 2D-NMR studies, respectively, and are characterized by the presence of two β-turns, the first of type VIa and the second intermediary between types I and III at the solid state and a γ-turn in solution.     

Perthamides C-F, potent human antipsoriatic cyclopeptides

Two new cyclopeptides, perthamides E and F were isolated from the polar extracts of the sponge Theonella swinhoei. The new structures, featuring an unprecedented β-amino acid unit (AHMOA), were determined by interpretation of NMR and MS data. The absolute configuration of the AHMOA residue was proposed on the basis of quantum chemical calculation of NMR chemical shifts. Perthamides were proved to inhibit TNF-α and IL-8 release in primary human keratinocytes cells and therefore could represent potentially leads for the treatment of psoriasis.     

Pashinintide A,the first plant cyclopeptide from Rosaceae,included a sucrose,suggests a new natural receptor for saccharide

Pashinintide A (1), a new cyclic hexapeptide, included a saccharose, and a new cyclic heptapeptide named pashinintide B (2) were isolated from the branches and leaves of Pyrus pashia Buch.-Ham. ex D. Don. 1 and 2 present the first two plant cyclopeptides from Rosaceae. And the stable complex of cyclic hexapeptide and sucrose in 1 suggests that cyclic hexapeptide might be a new natural receptor for saccharide.     

Synthesis, structural aspects and cytotoxicity of the natural cyclopeptides yunnanins A, C and phakellistatins 1, 10

Yunnanins A and C, two cyclic heptapeptides occurring in the roots of Stellaria yunnanensis, and phakellistatins 1 and 10, a hepta- and an octacyclopeptide first isolated from marine sponges of the genus Phakellia, were efficiently synthesized using a combination of solid and solution-phase techniques. Structural analysis on the synthetic members of the yunnanin series showed that the synthetic sample of yunnanin A exhibited a configurational pattern at the Pro peptide linkages identical to the natural product (trans-Pro³, trans-Pro⁵), while yunnanin C was obtained as a complex mixture of geometric/conformational isomers; the major isomer (trans-Pro³) was indistinguishable from the natural cyclopeptide and co-occurred along with lower amounts of a mixture (1:1 ratio) of two different rotamers, both displaying cis geometry at the Pro³ linkage. In the phakellistatin series, the synthetic phakellistatin 1 (determined as cis-Pro¹, cis-Pro³, cis-Pro⁵) was identical to the natural one, while two different isomeric products of phakellistatin 10 could be obtained: a major one (trans-Pro¹, trans-Pro⁴, trans-Pro⁶) showing spectral properties superimposable with the natural metabolite, and a minor geometric isomer of the natural cyclopeptide. Interestingly, the synthetic cyclopeptides, although found to be chemically identical with their natural counterparts, did not display the same biological properties (in vitro cytotoxicity against a panel of cancer cell lines), leaving presently open the question whether or not the potent bioactivity reported in the literature should really be attributed to these natural cyclic peptides.     

Controversy of Peptide Cyclization from Tripeptide

The present investigation reports an attempt to synthesize naturally occurring α-cyclic tripeptide cyclo(Gly-l-Pro-l-Glu) 1, [cyclo(GPE)], previously isolated from the Ruegeria strain of bacteria with marine sponge Suberites domuncula. Three linear precursors, Boc-GPE(OBn)₂, Boc-PE(OBn)G and Boc-E(OBn)GP, were synthesized using a solution phase peptide coupling protocol. Although cyclo(GPE) 1 was our original target, all precursors were dimerized and cyclized at 0 °C with high dilution to form corresponding α-cyclic hexapeptide, cyclo(GPE(OBn))₂ 7, which was then converted to cyclic hexapeptide cyclo(GPE)₂ 2. Cyclization at higher temperature induced racemization and gave cyclic tripeptide cyclo(GP_DE(OBn)) 9. Structure characteristics of the newly synthesized cyclopeptides were determined using ¹H-NMR, ¹³C-NMR and high-resolution mass spectrometry. The chemical shift values of carbonyls of 2 and 7 are larger than 170 ppm, indicating the formation of a cyclic hexapeptide.     

Anomeric spiroannelated 1,4-diazepine 2,5-diones from furano exo-glycals: towards a new class of spironucleosides

The first synthesis of 1,4-diazepine 2,5-dione peptides containing a β-amino acid in which the β carbon is also the anomeric carbon of a furanoid sugar is described. These new anomeric spirosugars obtained with a stereoselective control in the d-gulo, d-manno, d-allo and d-ribo series can be regarded as the first members of a new class of spironucleosides. In the course of our study, two symmetrical tetrameric cyclopeptides comprising two identical sugar β-amino acid and α-amino acid residues were also isolated, these structures could be of interest as new potential host molecules.     

Mechanism of alkaloid cyclopeptide synthesis in the ergot fungus Claviceps purpurea

Background: Previous analyses of the biosynthesis of the alkaloid cyclopeptides from the ergot fungus Claviceps purpurea were hampered by a lack of suitable systems for study in vitro, and this led to conflicting results concerning the mechanism of alkaloid cyclopeptide formation. Recently, D-lysergyl peptide synthetase (LPS) of the ergot fungus Claviceps purpurea, which assembles the non-cyclol precursors of the ergopeptines, has been partially purified and shown to consist of two polypeptide chains of 370 kDa (LPS 1) and 140 kDa (LPS 2); these contain all the sites necessary for the assembly of the D-lysergyl peptide backbone. The mechanism of D-lysergyl peptide synthesis remained unclear, however.Results: We have identified the obligatory peptidic intermediates in d-lysergyl peptide synthesis and the sequential order of their formation. The two LPS subunits catalyze the formation of d-lysergyl mono-, di-, and tripeptides as enzyme-thioester intermediates, the formation of which appears to be irreversible. Peptide synthesis starts when d-lysergic acid binds to the LPS 2 subunit, which most probably occurs after the previous round of synthesis has been completed by the release of the end product from the LPS enzyme.Conclusions: We have shown that the mechanism of d-lysergyl peptide synthesis is an ordered process of successive acyl transfers on a multienzyme complex. This knowledge opens the way for enzymatic and genetic investigations into the formation of novel alkaloid cyclopeptides.     

环肽的合成研究进展

对环肽进行了分类, 并对具代表性的经典型、DKP (Diketopiperazine, 哌嗪二酮)型、醚桥型、烯桥型、单硫及多硫醚型、刚性桥型、仲(叔)胺型、Mannich碱型、联苯型及Freidinger型环肽的合成方法逐一进行了描述.     

Two New Cyclopeptides from Arenaria oreophila (Caryophyllaceae)

Two new cyclopeptides, named arenariphilin A ( 1 ) and arenariphilin B ( 2 ), were isolated from the whole plants of Arenaria oreophila. Their structures were determined as cyclo‐(Thr‐Gly) ( 1 ) and cyclo‐(Ser₁‐Gly ‐Ser₂‐Ile ‐Phe₁‐Phe₂) ( 2 ) on the basis of spectral data, especially by 2D‐NMR.     

An efficient route to VEGF-like peptide porphyrin conjugates via microwave-assisted ‘click-chemistry’

Synthetic cyclopeptides, and particularly those derived from VEGF sequence, present considerable interest for the development of nanodevices devoted to tumour imaging or targeting. In order to provide selective peptide-targeted tetrapyrrolic structures, we designed two meso-porphyrin derivatives anchored to a 17-residue-long cyclopeptide, potent antagonist of VEGF receptors, via a flexible tetraethylene glycol chain. Anchoring was achieved by two different strategies: a classical secondary amide bond formation and microwave-assisted Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (‘click-chemistry’). These compounds appear to be promising candidates for applications in PDT.     

Analysis of Rhizonin Biosynthesis Reveals Origin of Pharmacophoric Furylalanine Moieties in Diverse Cyclopeptides

Rhizonin A and B are hepatotoxic cyclopeptides produced by bacterial endosymbionts (Mycetohabitans endofungorum) of the fungus Rhizopus microsporus. Their toxicity critically depends on the presence of 3-furylalanine (Fua) residues, which also occur in pharmaceutically relevant cyclopeptides of the endolide and bingchamide families. The biosynthesis and incorporation of Fua by non-ribosomal peptide synthetases (NRPS), however, has remained elusive. By genome sequencing and gene inactivation we elucidated the gene cluster responsible for rhizonin biosynthesis. A suite of isotope labeling experiments identified tyrosine and l -DOPA as Fua precursors and provided the first mechanistic insight. Bioinformatics, mutational analysis and heterologous reconstitution identified dioxygenase RhzB as necessary and sufficient for Fua formation. RhzB is a novel type of heme-dependent aromatic oxygenases (HDAO) that enabled the discovery of the bingchamide biosynthesis gene cluster through genome mining.     

Analysis of the 5p6p configuration of Xe V

The spectrum of four times ionized Xenon (XeV), has been observed in the 500–6800 Å range and 84 new lines have been identified as transitions between levels of 5s5p ³, 5s ²5p5d, 5s ²5p6s, 5s ²5p ², and 5s ²5p6p configurations. Nine new levels belonging to the configuration 5p6p have been determined. The results of this analysis are supported by Hartree-Fock calculations. The configurations are interpreted by fitting the theoretical energy parameters to the observed levels using least-squares techniques.     

New levels and calculations in the Nd I spectrum

The energies of all the levels of Nd I 4f ⁴ 6s ^{2 5} F and of⁵ S ₂ as well as of 4f ⁴ 5d 6s ⁷ H ₂ have been determined by means of laser-induced fluorescence in a hollow cathode. Their interpretation was carried out with the use of hyperfine structure data and a new parametric calculation of (4f ⁴ 6s ² + 4f ⁴ 5d 6s). Furthermore, the energies andJ values of 5 new odd levels were found.     

Diméthyl 3-benzyl-2-(4-méthyl-2,5-dioxoimidazolidin-1-yl)butanedioate

There are two symmetry-independent formula units of the title compound, dimethyl 3-benzyl-2-(4-methyl-2,5-di­oxo­imid­azol­idin-1-yl)­butane­dioate, C₁₇H₂₀N₂O₆, per cell. The two symmetry-independent molecules differ in their configuration and are diastereomers. This structural study confirms a new side reaction during the synthesis of seven-membered cyclopeptides. The stereochemistry of both diastereomers has been established.     

Determination of the k 0-values for 75Se, 110mAg, 115Cd–115mIn, 131Ba, and 153Sm by irradiation in highly thermalized neutron flux

The k ₀-values were determined for five high Q ₀(n,γ) reactions, including ⁷⁴Se(n,γ) ⁷⁵Se, ¹⁰⁹Ag(n,γ) ^110mAg, ¹¹⁴Cd(n,γ) ¹¹⁵Cd–^115mIn, ¹³⁰Ba(n,γ) ¹³¹Ba, and ¹⁵²Sm(n,γ) ¹⁵³Sm. These determinations were carried out under favorable experiment conditions: the irradiations were performed in a highly thermalized neutron flux, the irradiated target samples were counted at a far distance from HPGe detector with an efficiency carefully calibrated, and the k ₀-values were calculated against an internal comparator. When compared to the new values from this work, the 2003 recommended ^110mAg k ₀-values are confirmed. The other confirmed recommended k ₀-value is that of ⁷⁵Se 400.7 keV line. However, for the other ⁷⁵Se γ-lines, the new k ₀-values are 4–10 % higher. It is assumed that an inaccurate efficiency calibration was used when the recommended k ₀-values were measured. For the other three nuclides, the new k ₀-values are higher by 4 % for the ¹¹⁵Cd–^115mIn γ-lines, lower by 6–8 % for the ¹³¹Ba γ-lines, and lower by 8.8 % for the ¹⁵³Sm 103.2 keV γ-line.     

New Dunham coefficients of the A 1 ∑ +-State of NaH and NaD

We determined new Dunham coefficients of the A ¹ ∑ ⁺-State of NaH and NaD from degenerate Four- Wave-Mixing (DFWM) spectra in the near UV and in the blue spectral region. In the case of NaH we combined these data with results of Rafi et al. and Orth et al.. The new set of coefficients describes the vibrational dependence of the rotational constants B and D and of the band origins from ? = 0 up to 25. The spectral positions of the lines in our DFWM-spectra can be reproduced by this coefficients with an accuracy better than 0.3 cm^-1 for J-values ≤ 15 and 2 cm^-1 for J-values ≤ 25. Especially for high J-values this is an improvement up to 30 times in comparison to Dunham coefficients recommended before. A RKR-potential of the A ¹ ∑ ⁺-state was calculated with the new coefficients. The Dunham coefficients of NaD were obtained by scaling the NaH coefficients with the reduced masses of the molecules. A comparison of our results to molecular constants determined from the measured NaD-spectra shows good agreement.     

Preparation of new monomers aza-β-aminoacids for solid-phase syntheses of aza-β-peptides

The preparation of new N^β-Fmoc-protected aza-β³-amino acids (aza-β³-aa) with proteinogenic side chains as well as their N^β-Fmoc, N^β-Cbz or N^β-Boc aza-β³-amino esters (from Pro, Asn, Asp, Glu, Gln) by successive nucleophilic substitutions will be described.