Some aspects of the relationship between the structure of a bitter diketopiperazine and its receptor

A conformation of a diketopiperazine, cyclo(L -leucyl-L -tryptophyl), was determined by nmr and x-ray analysis. The receptor for this bitter diketopiperazine neither recognizes chirality nor requires a strict conformation of the substrate. A good correlation was found between the bitterness of diketopiperazine and its hydrophobicity. Experiments using liposome as a model of the receptor membrane suggested that a lecithin-like lipid is a candidate for a binding site of the bitter diketopiperazine. Furthermore, affinity chromatography using cyclo(L -leucyl-L -phenylalanyl) as a bitter ligand indicated that phosphoglycerides of the lipids tested showed a particular affinity to the bitter diketopiperazine.     

The optical properties of alanine and proline diketopiperazines

The optical properties of the diketopiperazine chromophore of the cyclic dipeptides have been investigated as a function of molecular conformation. The rotatory strengths of L -alanyl–L -alanine diketopiperazine and L -prolyl–L -proline diketopiperazine have been calculated as a function of the angle of fold of the diketopiperazine ring. The results of these theoretical calculations have been compared with experimental circular dichroism and optical rotatory dispersion data. It is shown that the observed optical properties of these molecules can be explained only if their diketopiperazine rings are folded in opposite directions. The direction of fold is established for each molecule. In solution, the diketopiperazine ring of L -alanyl-L -alanine diketopiperazine is folded in the direction opposite to that found by X-ray diffraction analysis of crystals. It has been observed that the degree of conservatism of the π → π* couplet of L -propyl–L -proline diketopiperazine depends markedly upon the nature of the solvent that is used. In addition, a shoulder has been discovered in the CD spectrum of L -alanyl–L -alanine diketopiperazine, which may not be directly attributable to the n → π* and π → π* transitions of the peptide chromophores.     

Antagonism of ethanol-induced decrease in rat brain cGMP concentration by histidyl-proline diketopiperazine, a thyrotropin releasing hormone metabolite.

Intraperitoneal administration of thyrotropin releasing hormone (50 μmol/kg) produced an approximately 2-fold increase in rat brain cGMP concentration within 15 min. Histidyl-proline diketopiperazine, a metabolite of thyrotropin releasing hormone, produced a similar effect, but the response was faster and shorter-lasting. Intraperitoneal administration of ethanol (1.5 g/kg) decreased brain cGMP concentration approximately 50% within 10–15 min; thyrotropin releasing hormone or histidyl-proline diketopiperazine, injected 5 min after ethanol, antagonized the ethanol-induced decrease in cGMP. Antagonism of the ethanol-induced decrease in the cGMP level required 10 μmol/kg of thyrotropin releasing hormone but was observed with 5 μmol/kg of histidyl-proline diketopiperazine. These data suggest that the metabolic conversion of thyrotropin releasing hormone to histidylproline diketopiperazine might explain the previous observation that thyrotropin releasing hormone elevated the level of brain cGMP and antagonized the ethanolinduced decrease in brain cGMP concentration.     

Thermoregulation in rats: opposing effects of thyrotropin releasing hormone and its metabolite histidyl-proline diketopiperazine.

Intraventricular administration of histidyl-proline diketopiperazine to rats produces a dose-dependent hypothermia at 4° or 24°, but not at 31°. At 4°, administration of thyrotropin releasing hormone elicits a dose-dependent hypothermia up to 0.1 μmole/kg which is not evoked at higher doses. At 24°, thyrotropin releasing hormone administration results in no change in core temperature, whereas it induces hyperthermia at 31°. At 4°, thyrotropin releasing hormone antagonizes and thyrotropin releasing hormone antiserum potentiates the hypothermic effect of histidyl-proline diketopiperazine, suggesting opposing actions of thyrotropin releasing hormone and histidyl-proline diketopiperazine on thermoregulation.     

Small molecule biaryl FSH receptor agonists. Part 2: Lead optimization via parallel synthesis

Potent small molecule biaryl diketopiperazine FSH receptor agonists such as 10c (EC(50)=13 nM) and 11f (EC(50)=1.2 nM) were discovered through the design, synthesis and evaluation of three biaryl diketopiperazine optimization libraries with over 300 compounds. These libraries were prepared via solid-phase parallel synthesis using a cyclization-release method.     

Prebiotic thermal polymerization of crystals of amino acids via the diketopiperazine reaction

In this work, we continue our studies on the thermal prebiotic oligomerization of amino acids. The next step is to consider all four types of electromagnetic interactions that our model may admit. In addition, only the polymerization of amino acids via the formation of diketopiperazine, which arises from the cyclodehydration of two amino acids, will be considered. By assuming that only one residue group of two will predominate in the diketopiperazine molecule, it is possible to reduce the three-body problem to a simpler situation with the two objects that we have already solved.     

Temporal separation of steps in the biosynthesis of verruculogen

A mixture of 2[3H] mevalonate and U[14C] proline, given to Penicillium simplicissimum on days 2, 3, 4, 5 or 6 of stationary liquid culture, gave rise to radiolabelled verruculogen in which the ratio of 3H to 14C measured on day 8 increased from 2.0:1 to 9.5:1 over the period of administration of radiolabelled precursors. This suggests temporal separation of the formation of the diketopiperazine moiety and its subsequent prenylation. The putative precursor diketopiperazine, cyclo-L-tryptophanyl-L-proline, was isolated from 21 day culture broth.     

Macrophominol, a diketopiperazine from cultures of Macrophomina phaseolina

A diketopiperazine named macrophominol was isolated from the phytopathogenic fungus Macrophomina phaseolina.     

Conformation of histidine model peptides. I. Conformational energy calculations for L-alanyl-L-histidine diketopiperazine

Semiempirical conformational energy calculations were carried out for the cyclic dipeptide L -alanyl-L -histidine diketopiperazine. The results indicate that electrostatic effects are probably significant in determining the conformation assumed by this molecule. When the imidazole group is in its uncharged state the most stable conformations of the molecule are those with the imidazole ring folded over the diketopiperazine ring (χ¹ = 60°). Upon protonation of the imidazole group the folded conformation may be destabilized relative to conformations characterized by χ¹ positions near 180°.     

The effects of thyrotropin releasing hormone and histidyl-proline diketopiperazine on delta-9-tetrahydrocannabinol-induced hypothermia

The effects of central and peripheral administration of thyrotropin releasing hormone (TRH) and its postulated metabolite, histidyl-proline diketopiperazine (HPD) on △9-tetrahydrocannabinol (THC) induced hypothermia in mice were investigated. Intraperitoneal administration of THC produced hypothermia. The peak response was observed between 1 and 2 hours and the hypothermia lasted for 5 to 6 hours. Intracerebral or intraperitoneal administration of TRH prior to THC injection antagonized the hypothermic response of the latter. Similar effects were produced by histidyl-proline diketopiperazine given intracerebrally. However, HPD was completely ineffective when given intraperitoneally. The antagonism of THC-induced hypothermia by TRH may be mediated by its conversion to HPD in the central nervous system.     

Solid-phase synthesis of functionalized bis-peptides

We demonstrate the first solid-phase synthesis of highly functionalized bis-peptides. Bis-peptides are ladder oligomers composed of stereochemically pure, cyclic bis-amino acids joined by substituted diketopiperazine linkages. They have a shape-programmable backbone that is controlled by controlling the stereochemistry and sequence of the monomers within each oligomer. Functionalized bis-peptides are assembled using a new amide bond forming reaction (acyl-transfer coupling) that we have previously developed and a novel activation strategy that allows the sequential formation of penta- and hexa-substituted diketopiperazines from extremely hindered N-alkyl-alpha,alpha-disubstituted amino acids. We present mechanistic evidence that acyl-transfer coupling is competitive with direct acylation in the formation of hindered amide bonds. We also detail the synthesis of four functionalized bis-peptides, and that by combining bis-peptides with amino acids through diketopiperazine linkages, bis-peptides can mimic the display of residues i, i+4, i+7 of an alpha-helical peptide.     

Isolation and partial characterization of cyclosporin synthetase from a cyclosporin non-producing mutant of Beauveria nivea

Cyclosporin synthetase was isolated from a cyclosporin non-producing mutant of Beauveria nivea, strain YP 582. The enzyme has a molecular mass in the range of active cyclosporin synthetase and also contains 4'-phosphopantetheine as a prosthetic group. It is able to activate all constituent amino acids of cyclosporin A as thioesters and to carry out specific N-methylation reactions. Overall synthesis of the undecapeptide cyclosporin A in the presence of all necessary substrates was not observed, but the formation of the diketopiperazine cyclo-(D-alanyl-N-methyl-leucyl). This diketopiperazine represents a partial sequence of the cyclosporin molecule. It could be detected in the mycelium of the non-producing strain, whereas mycelium of the producing strain 7939/45 did not contain this compound. The results suggest that the inability of this mutant to produce cyclosporin A is caused by a mutation of the polypeptide chain of cyclosporin synthetase.     

Antifouling activity of synthesized peptide analogs of the sponge metabolite barettin

Barettin (cyclo [(6-bromo-8-en-tryptophan) arginine]), a diketopiperazine isolated from the marine sponge Geodia barretti, is a potent inhibitor of barnacle larvae settlement with an EC50-value of 0.9 microM. In the present study, 14 analogs of barettin and its structural congener dipodazine were synthezised and tested for their ability to inhibit larval settlement. Two of the analogs have an intact barettin skeleton. The remaining analogs have a dipodazine skeleton (a diketopiperazine where arginine is replaced with glycine). Six of the tested synthetic analogs displayed significant settlement inhibition with the most potent inhibitor being benzo[g]dipodazine, which displayed even stronger activity than barettin (EC50-value 0.034 microM). The effect of benzo[g]dipodazine was also shown to be readily reversible, when cyprids were transferred to filtered seawater (FSW).     

Conformation of cyclo-(L-threonine)2 and cyclo-(L-allothreonine)2. A proton and carbon n.m.r study

The diketopiperazines cyclo-(L-Thr)2 and cyclo-(L-allo Thr)2 in water and in dimethyl sulfoxide were studied by proton and carbon-13 nuclear magnetic resonance, and the dominant conformation were deduced from proton-proton and proton-carbon coupling constants. In cyclo-(L-Thr)2 the chi 1 = 60 degrees, hydroxyl over the ring, side chain conformation is favored; this conformation is also favored for cyclo-(L-Ser)2 and cyclo-(L-Ser-D-Ser). However, the important side chain conformation for cyclo-(L-allo Thr)2 is chi 1 = -60 degrees, methyl group over the diketopiperazine ring. The determining factors are apparently steric. The diketopiperazine ring of cyclo-(L-Thr)2 is puckered to hold the side chains more nearly axial than is that of cyclo-(L-allo Thr)2. although the degree of ring folding is probably not large.     

Isolation of gliovictin from the marine deuteromycete Asteromyces cruciatus

(−)Gliovictin, a diketopiperazine isolated from terrestrial fungi of the genera Helminthosporium and Penicillium, has been isolated from culture broths of the marine deuteromycete Astermyces cruciatus.     

Secondary metabolites in taxonomy of the <Emphasis Type="Italic">Penicillium</Emphasis> fungi

A correlation was established between species specificity and synthesis of specific secondary metabolites by the Penicillium fungi. Strains of the subgenus Aspergilloides usually synthesize metabolites of polyketide nature. Most strains of the subgenus Furcatum produce clavine ergot alkaloids and metabolites of diketopiperazine nature. The only clavine ergot alkaloids and diketopiperazine alkaloids produced by strains of the subgenus Biverticillium are rugulovasines and rugulosuvines, respectively. Species designations of the strains of the subgenus Penicillium isolated from permafrost soil, the Mir orbital complex, and sites undergoing anthropogenic load were refined based on the marker secondary metabolites. Changes in the taxonomic position of some strains in the genus Penicillium are suggested.     

Biosynthesis of alkaloids by mycelial fungi (review of the literature)

Evidence for the occurrence of alkaloids in mycelial fungi, pathways of their biosynthesis and types of regulation are presented. The effect of some factors on the alkaloid production is discussed. In literature, the biosynthesis and the metabolism of diketopiperazine and ergot alkaloids in Penicillium fungi are covered most completely.     

Apoptosis-inducing effect of diketopiperazine disulfides produced by Aspergillus sp. KMD 901 isolated from marine sediment on HCT116 colon cancer cell lines

Aims: Research is to identify the bioactive secondary metabolites produced by Aspergillus sp. KMD 901 isolated from marine sediment and to assess their apoptosis‐inducing effects. Methods and Results: Aspergillus sp. KMD 901 was isolated from marine sediment obtained from the East Sea of Korea. An ethyl acetate extract of KMD 901 exhibited potent cytotoxic activity towards five cancer cell lines (HCT116, AGS, A549, MCF‐7 and HepG2). Sequencing of the internal transcribed spacer (ITS) region in this strain allowed us to identify KMD 901 as a strain of Aspergillus versicolor. The cytotoxic compounds from Aspergillus sp. KMD 901 were purified by reversed‐phase high‐performance liquid chromatography and identified as diketopiperazine disulfides through spectroscopic analyses including extensive 2D NMR and mass spectrometry. The diketopiperazine disulfides were found to induce apoptosis in HCT116 cells based on cell morphology, DNA fragmentation observed by agarose gel electrophoresis, Annexin‐V/PI staining using a flow cytometer and cleavage of poly (ADP‐ribose) polymerase (PARP), caspase‐3, caspase‐8, caspase‐9 and Bcl‐2 family proteins (Bcl‐2, Bcl‐xL and Bax) using Western blotting analysis. Further study using an in vivo xenograft model showed inhibitory effects of acetylapoaranotin ( 2 ) on tumour proliferation. Conclusion: A new diketopiperazine disulfide, deoxyapoaranotin ( 3 ), along with previously described acetylaranotin ( 1 ) and acetylapoaranotin ( 2 ) was separated from Aspergillus sp. KMD 901 and found to have direct cytotoxic and apoptosis‐inducing effects towards HCT116 colon cancer cell lines. Significance and Impact of the Study: These results suggest that the diketopiperazine disulfides produced from Aspergillus sp., KMD 901, could be candidates for the development of apoptosis‐inducing antitumour agents. Also, this study indicates that marine natural products as potential source of pharmaceuticals.     

Nigrifortine,a diketopiperazine metabolite of Penicillium nigricans

The structure of a novel diketopiperazine metabolite, named nigrifortine, isolated from cultures of Penicillium nigricans is deduced from its ¹H NMR, mass and UV spectra together with biosynthetic reasoning.     

Synthesis of a reported calpain inhibitor isolated from Streptomyces griseus

The reported diketopiperazine calpain inhibitor, cis-L-L-3,6-bis-(4-hydroxybenzyl)-1,4-dimethylpiperazine-2,5-dione 1, and its analogues 3 and 4 were synthesized from the corresponding amino acids. The previously assigned structure of 1 is confirmed but neither synthetic 1 nor its N-methylphenylalanine analogues 3 and 4 inhibit porcine erythrocyte calpain I.