Synthesis and biological evaluation of analogues of the peptaibol ampullosporin A

Rare Glutamic Acid Methyl Ester Peptaibols from Sepedonium ampullosporum Damon KSH 534 Exhibit Promising Antifungal and Anticancer Activity

Fungal species of genus Sepedonium are rich sources of diverse secondary metabolites (e.g., alkaloids, peptaibols), which exhibit variable biological activities. Herein, two new peptaibols, named ampullosporin F (1) and ampullosporin G (2), together with five known compounds, ampullosporin A (3), peptaibolin (4), chrysosporide (5), c(Trp-Ser) (6) and c(Trp-Ala) (7), have been isolated from the culture of Sepedonium ampullosporum Damon strain KSH534. The structures of 1 and 2 were elucidated based on ESI-HRMSⁿ experiments and intense 1D and 2D NMR analyses. The sequence of ampullosporin F (1) was determined to be Ac-Trp¹-Ala²-Aib³-Aib⁴-Leu⁵-Aib⁶-Gln⁷-Aib⁸-Aib⁹-Aib¹⁰-GluOMe¹¹-Leu¹²-Aib¹³-Gln¹⁴-Leuol¹⁵, while ampullosporin G (2) differs from 1 by exchanging the position of Gln⁷ with GluOMe¹¹. Furthermore, the total synthesis of 1 and 2 was carried out on solid-phase to confirm the absolute configuration of all chiral amino acids as L. In addition, ampullosporin F (1) and G (2) showed significant antifungal activity against B. cinerea and P. infestans, but were inactive against S. tritici. Cell viability assays using human prostate (PC-3) and colorectal (HT-29) cancer cells confirmed potent anticancer activities of 1 and 2. Furthermore, a molecular docking study was performed in silico as an attempt to explain the structure-activity correlation of the characteristic ampullosporins (1–3).

Conformational properties, membrane interaction, and antibacterial activity of the peptaibiotic chalciporin A: Multitechnique spectroscopic and biophysical investigations on the natural compound and labeled analogs

In this work, an extensive set of spectroscopic and biophysical techniques (including FT-IR absorption, CD, 2D-NMR, fluorescence, and CW/PELDOR EPR) was used to study the conformational preferences, membrane interaction, and bioactivity properties of the naturally occurring synthetic 14-mer peptaibiotic chalciporin A, characterized by a relatively low (≈20%), uncommon proportion of the strongly helicogenic Aib residue. In addition to the unlabeled peptide, we gained in-depth information from the study of two labeled analogs, characterized by one or two residues of the helicogenic, nitroxyl radical-containing TOAC. All three compounds were prepared using the SPPS methodology, which was carefully modified in the course of the syntheses of TOAC-labeled analogs in view of the poorly reactive α-amino function of this very bulky residue and the specific requirements of its free-radical side chain. Despite its potentially high flexibility, our results point to a predominant, partly amphiphilic, α-helical conformation for this peptaibiotic. Therefore, not surprisingly, we found an effective membrane affinity and a remarkable penetration propensity. However, chalciporin A exhibits a selectivity in its antibacterial activity not in agreement with that typical of the other members of this peptide class.

An EPR study of ampullosporin A, a medium-length peptaibiotic, in bicelles and vesicles

EPR/CD spectroscopies reveal that the peptaibol ampullosporin A changes the orientation and conformation depending on its concentration and bilayer thickness.

Trichogin GA IV: a versatile template for the synthesis of novel peptaibiotics

Trichogin GA IV, isolated from the fungus Trichoderma longibrachiatum, is the prototype of lipopeptaibols, the sub-class of short-length peptaibiotics exhibiting membrane-modifying properties. This peptaibol is predominantly folded in a mixed 3(10)-/α- helical conformation with a clear, albeit modest, amphiphilic character, which is likely to be responsible for its capability to perturb bacterial membranes and to induce cell death. In previous papers, we reported on the interesting biological properties of trichogin GA IV, namely its good activity against Gram positive bacteria, in particular methicillin-resistant S. aureus strains, its stability towards proteolytic degradation, and its low hemolytic activity. Aiming at broadening the antimicrobial activity spectrum by increasing the peptide helical amphiphilicity, in this work we synthesized, by solution and solid-phase methodologies, purified and fully characterized a set of trichogin GA IV analogs in which the four Gly residues at positions 2, 5, 6, 9, lying in the poorly hydrophilic face of the helical structure, are substituted by one (position 2, 5, 6 or 9), two (positions 5 and 6), three (positions 2, 5, and 9), and four (positions 2, 5, 6, and 9) Lys residues. The conformational preferences of the Lys-containing analogs were assessed by FT-IR absorption, CD and 2D-NMR techniques in aqueous, organic, and membrane-mimetic environments. Interestingly, it turns out that the presence of charged residues induces a transition of the helical conformation adopted by the peptaibols (from 3(10)- to α-helix) as a function of pH in a reversible process. The role played in the analogs by the markedly increased amphiphilicity was further tested by fluorescence leakage experiments in model membranes, protease resistance, antibacterial and antifungal activities, cytotoxicity, and hemolysis. Taken together, our biological results provide evidence that some of the least substituted among these analogs are good candidates for the development of new membrane-active antimicrobial agents.

Synthesis of mutual azo prodrugs of anti-inflammatory agents and peptides facilitated by α-aminoisobutyric acid

Reported is the synthesis of azo mutual prodrugs of the nonsteroidal anti-inflammatory agents (NSAIDs) 4-aminophenylacetic acid (4-APAA) or 5-aminosalicylic acid (5-ASA) with peptides, including an antibiotic peptide temporin analogue modified at the amino terminal by an α-aminoisobutyric acid (Aib) residue. These prodrugs are designed for colonic delivery of two agents to treat infection and inflammation by the bacterial pathogen Clostridium difficile .

Synthesis, preferred conformation, protease stability, and membrane activity of heptaibin, a medium-length peptaibiotic

The medium-length peptaibiotics are characterized by a primary structure of 14-16 amino acid residues. Despite the interesting antibiotic and antifungal properties exhibited by these membrane-active peptides, their exact mechanism of action is still unknown. Here, we present our results on heptaibin, a 14-amino acid peptaibiotic found to exhibit antimicrobial activity against Staphylococcus aureus. We carried out the very challenging synthesis of heptaibin on solid phase and a detailed conformational analysis in solution. The peptaibiotic is folded in a mixed 3₁₀-/α-helix conformation which exhibits a remarkable amphiphilic character. We also find that it is highly stable toward degradation by proteolytic enzymes and nonhemolytic. Finally, fluorescence leakage experiments using small unilamellar vesicles of three different compositions revealed that heptaibin, although uncharged, is a selective compound for permeabilization of model membranes mimicking the overall negatively charged surface of Gram-positive bacteria. This latter finding is in agreement with the originally published antimicrobial activity data.

Hypersensitive-like response to the pore-former peptaibol alamethicin in Arabidopsis thaliana

In Arabidopsis thaliana cell cultures, the peptaibol alamethicin induced a form of active cell death that was associated with cell shrinkage and DNA fragmentation. The transfer of mature A. thaliana plants from a peptide-free medium to a medium containing a moderate concentration of alamethicin caused the development of lesions in leaves after a few days. These lesions were characterized by cell death, deposition of callose, production of autofluorescent phenolic compounds, and transcription of defense genes, just like in the hypersensitive response to a pathogen attack. The induction of defense-like responses in Arabidopsis by other membrane-disrupting peptides was also evaluated. The peptides selected for comparison included the natural antimicrobial melittin and the peptaibol ampullosporin A, as well as synthetic analogues of the peptaibols cervinin and trichogin. The response amplitude in A. thaliana increased with the peptaibol's ability to permeabilize biological membranes through a pore-forming mechanism and was strongly associated with their content in the helicogenic α-aminoisobutyric acid residue.

Structure and alignment of the membrane-associated peptaibols ampullosporin A and alamethicin by oriented 15N and 31P solid-state NMR spectroscopy

Ampullosporin A and alamethicin are two members of the peptaibol family of antimicrobial peptides. These compounds are produced by fungi and are characterized by a high content of hydrophobic amino acids, and in particular the alpha-tetrasubstituted amino acid residue ?-aminoisobutyric acid. Here ampullosporin A and alamethicin were uniformly labeled with (15)N, purified and reconstituted into oriented phophatidylcholine lipid bilayers and investigated by proton-decoupled (15)N and (31)P solid-state NMR spectroscopy. Whereas alamethicin (20 amino acid residues) adopts transmembrane alignments in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes the much shorter ampullosporin A (15 residues) exhibits comparable configurations only in thin membranes. In contrast the latter compound is oriented parallel to the membrane surface in 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine and POPC bilayers indicating that hydrophobic mismatch has a decisive effect on the membrane topology of these peptides. Two-dimensional (15)N chemical shift -(1)H-(15)N dipolar coupling solid-state NMR correlation spectroscopy suggests that in their transmembrane configuration both peptides adopt mixed alpha-/3(10)-helical structures which can be explained by the restraints imposed by the membranes and the bulky alpha-aminoisobutyric acid residues. The (15)N solid-state NMR spectra also provide detailed information on the helical tilt angles. The results are discussed with regard to the antimicrobial activities of the peptides.

Nonaqueousversus aqueous capillary electrophoresis of α-helical polypeptides: Effect of secondary structure on separation selectivity

The CE separation of alpha-helical polypeptides composed of 14-31 amino acid residues has been investigated using aqueous and nonaqueous BGEs. The running buffers were optimized with respect to pH. Generally, higher separation selectivities were observed in nonaqueous electrolytes. This may be explained by a change in the secondary structure when changing from water to organic solvents. Circular dichroism spectra revealed a significant increase in helical structures in methanol-based buffers compared to aqueous buffers. This change in secondary structure of the polypeptides contributed primarily to the different separation selectivity observed in aqueous CE and NACE. For small oligopeptides of two to five amino acid residues no significant effect of the solvent was observed in some cases while in other cases a reversal of the migration order occurred when changing from aqueous to nonaqueous buffers. As these peptides cannot adopt secondary structures the effect may be attributed to a shift of the pKa values in organic solvents compared to water.

Detection of new amino acid sequences of alamethicins F30 by nonaqueous capillary electrophoresis–mass spectrometry

The microheterogeneous alamethicin F30 (ALM F30) isolated from the fermentation of Trichoderma viride strain NRRL 3199 was analyzed by nonaqueous capillary electrophoresis coupled to electrospray ion-trap mass spectrometry (ESI-IT-MS) and electrospray time-of-flight mass spectrometry (ESI-TOF-MS). Tandem ESI-IT-MS was used for elucidation of the amino acid sequence based on the fragmentation pattern of selected parent ions. The MS/MS spectra using the [M + 3H](3+) or [M + 2H](2+) ions as precursor ions displayed the respective b- and the y-type fragments resulting from cleavage of the particularly labile Aib-Pro bond. The MS(3) of these fragments generated the b acylium ion series, as well as internal fragment ion series. Eleven amino acid sequences were identified, characterized by the exchange of Ala to Aib in position 6, Gln to Glu in positions 7 or 19 as well as the loss of the C-terminal amino alcohol. In addition, two truncated pyroglutamyl peptaibols were found. Overall, seven new sequences are reported compared to earlier LC-MS studies. The composition of the components was confirmed by on-line ESI-TOF-MS detection. Mass accuracy well below 5 ppm was observed. Quantification of the individual components was achieved by a combination of UV and TOF-MS detection.

Analysis of the lipophilic peptaibol alamethicin by nonaqueous capillary electrophoresis-electrospray ionization-mass spectrometry

The microheterogeneous peptaibol alamethicin F30 isolated from the culture broth of Trichoderma viride was analyzed by nonaqueous CE-electrospray-MS using an IT and a TOF mass analyzer. Compared to aqueous buffers, higher separation selectivity was observed for methanolic BGE allowing the detection of more minor components. The low electrophoretic mobility observed for neutral analytes under nonaqueous conditions may be explained by ion-dipole interactions between the peptide analytes and electrolyte ions. The amino acid sequences of the individual components were derived from MS(n) using the doubly or triply charged pseudomolecular ions as well as characteristic fragments as precursor ions. The exchange of Ala by alpha-aminoisobutyric acid (Aib) which is frequently observed for peptaibols was detected for several components. Additional variations included the exchange of Gln to Glu, and the loss of the C-terminal amino alcohol or of the first six amino acids from the N-terminus with concomitant formation of pyroglutamyl residues. In most cases comigration of the Aib peptaibols with the respective Ala component was observed as the mass difference of 14 Da as the result of the amino acid exchange was not sufficient to translate into an electrophoretic separation under the conditions applied. However, proper selection of the precursor ions allowed the unequivocal analysis of the components. Additional TOF-MS measurements were performed in order to resolve the ammonium adducts from comigrating compounds (i.e., Aib-Ala exchange) and to confirm the amino acid composition of the individual components. Except for neutral compounds migrating close to the EOF the mass accuracy was better than 4 ppm for the doubly charged pseudomolecular ions and better than 2 ppm for triply charged ions.

Membrane association and activity of 15/16-membered peptide antibiotics: zervamicin IIB, ampullosporin A and antiamoebin I

Permeabilization of the phospholipid membrane, induced by the antibiotic peptides zervamicin IIB (ZER), ampullosporin A (AMP) and antiamoebin I (ANT) was investigated in a vesicular model system. Membrane-perturbing properties of these 15/16 residue peptides were examined by measuring the K(+) transport across phosphatidyl choline (PC) membrane and by dissipation of the transmembrane potential. The membrane activities are found to decrease in the order ZER>AMP>>ANT, which correlates with the sequence of their binding affinities. To follow the insertion of the N-terminal Trp residue of ZER and AMP, the environmental sensitivity of its fluorescence was explored as well as the fluorescence quenching by water-soluble (iodide) and membrane-bound (5- and 16-doxyl stearic acids) quenchers. In contrast to AMP, the binding affinity of ZER as well as the depth of its Trp penetration is strongly influenced by the thickness of the membrane (diC(16:1)PC, diC(18:1)PC, C(16:0)/C(18:1)PC, diC(20:1)PC). In thin membranes, ZER shows a higher tendency to transmembrane alignment. In thick membranes, the in-plane surface association of these peptaibols results in a deeper insertion of the Trp residue of AMP which is in agreement with model calculations on the localization of both peptide molecules at the hydrophilic-hydrophobic interface. The observed differences between the membrane affinities/activities of the studied peptaibols are discussed in relation to their hydrophobic and amphipathic properties.

Crystal structure and conformational analysis of ampullosporin A

Ampullosporin A is a 15-mer peptaibol type polypeptide that induces pigment formation by the fungus Phoma destructiva, forms voltage-dependent ion channels in membranes and exhibits hypothermic effects in mice. The structure of ampullosporin A has been determined by x-ray crystallography. This is the first three-dimensional (3D) structure of the peptaibol subfamily SF6. From the N-terminus to residue 13 the molecule adopts an approximate right-handed alpha-helical geometry, whereas a less regular structure pattern with beta-turn characteristics is found in the C-terminus. Even though ampullosporin A does not contain a single proline or hydroxyproline it is significantly bent. It belongs to both the shortest and the most strongly bent peptaibol 3D structures. The straight structure part encompasses residues Ac-Trp(1)-Aib(10) and is thus less extended than the alpha-helical subunit. The 3D structure of ampullosporin A is discussed in relation to other experimentally determined peptaibol structures and in the context of its channel-forming properties. As a part of this comparison a novel bending analysis based on a 3D curvilinear axis describing the global structural characteristics has been proposed and applied to all 3D peptaibol structures. A sampling of 2500 conformations using different molecular dynamics protocols yields, for the complete ampullosporin A structure, an alpha-helix as the preferred conformation in vacuo with almost no bend. This indicates that solvent or crystal effects may be important for the experimentally observed peptide backbone bending characteristics of ampullosporin A.

Circular dichroism studies of ampullosporin-A analogues

Ampullosporin A (AmpA), a 15mer peptalbol containing seven Aib residues is able to induce pigmentation on Phoma destructiva and hypothermia in mice, as well as to exhibit a neuroleptic effect. A circular dichroism study of ampullosporin A and its analogues was carried out in organic solvents with different polarities and detergent micelles to determine the relationship between their conformational flexibility and biological activities. The analogues were obtained by modifying the N- and C-termini of ampullosporin A. Furthermore, Gln and Leu were systematically substituted by Ala and Aib residues were replaced by Ala and/or Ac6c. To estimate the helicity of the analogues, the CD spectrum of AmpA recorded in acetonitrile was correlated to its crystal structure. All analogues displayed similar CD curve shapes in organic solvents with the ratio between two negative band intensities R = [theta]n-pi*/[theta]pi-pi* < 1. In acetonitrile, most of the analogues adopted a 70%-85% helical structure, which was higher than the average of 40%-60% obtained in TFE. In detergent micelles, the analogues were distinguishable by their CD profiles. For most of the biologically active analogues, the CD spectra in detergent micelles were characterized by a R ratio > 1 and increased helicity compared with those recorded in TFE, suggesting that the interaction of the peptides with the membrane and peptide association was necessary for their hypothermic effect.

The occurrence of peptaibols and structurally related peptaibiotics in fungi and their mass spectrometric identification via diagnostic fragment ions

Peptaibols and related peptide antibiotics (peptaibiotics) display diagnostically useful fragmentation patterns during mass spectrometry (FAB-MS, ESI-CID-MS/MS and CID-MSn]. The paper compiles fragmentation data of pseudo-molecular ions reported in the literature as a guide to the rational identification of recurrently isolated and new peptaibols and peptaibiotics. Taxonomic and ecological aspects of microorganisms producing peptaibols and peptaibiotics are discussed.

A nonribosomal peptide synthetase involved in the biosynthesis of ampullosporins inSepedonium ampullosporum

Recently, the saprophytic ascomycete Sepedonium ampullosporum strain HKI-0053 was isolated from a basidiomycete on account of its premature induction of pigment formation in Phoma destructiva, a process often related to the neuroleptic activity of the inducing compound. The active substance was identified as the 15-membered peptaibol type peptide Ampullosporin. Although to date more than 300 peptaibols have been discovered, their biosynthetic machinery has not been characterized yet. By improving the culture conditions it was possible to grow S. ampullosporum in a submerged culture and to increase Ampullosporin production by more than three times to 33 mg/l at reduced fermentation times. The appearance of two high molecular weight proteins, HMWP1 (1.5 MDa) and HMWP2 (350 kDa) was closely related to the production of Ampullosporin during the course of fermentation. Both proteins showed a cross-reaction with antibodies against a core fragment of nonribosomal peptide synthetases (NRPSs). Biochemical characterization of the partially purified enzymes exhibited selectivity for the substrate amino acid alpha-aminoisobutyric acid (Aib). substantiating their involvement in Ampullosporin biosynthesis. Our data suggest that Ampullosporin synthetase has been isolated, and provides the basis for the characterization of the entire biosynthetic gene cluster. Furthermore, this knowledge will enable the manipulation of its NRPS template, in order to engineer mutant strains of Sepedonium ampullosporum which could produce more potent analogues of Ampullosporin.