Stable right- and left-handed peptide helices containing C(alpha)-tetrasubstituted alpha-amino acids

Cytotoxic Peptaibols from Trichoderma strigosum

Five new lipopeptaibols (1-5) and eight new 19-residue peptaibols (8-15) along with two known lipopeptaibols, lipovelutibols C (6) and D (7) were isolated from Trichoderma strigosum. The planar structures of the newly discovered peptaibols (1-5, 8-15) were elucidated using 1D and 2D NMR, and UPLC-MS/MS data. The absolute configurations for new peptaibols (1-5, 8-15) were elucidated using the advanced Marfey's method and GITC (2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate) derivatization. Through analysis of CD spectra, these peptabols were found to have right-handed helical conformations. While most of the new compounds were significantly more active than the positive control, 9, 10, 12, and 15 containing Ser and Leu at positions 10 and 11, respectively, were the most cytotoxic against MDA-MB-231, SNU449, SKOV3, DU145, and HCT116 cancer cell lines, and the 19-residue peptaibols were generally more potent than lipopeptaibols.

Cytotoxic Peptaibols from Trichoderma guizhouense, a Fungus Isolated from an Urban Soil Sample

Soil sustains human life by nourishing crops, storing food sources, and housing microbes, which may affect the nutrition and biosynthesis of secondary metabolites, some of which are used as drugs. To identify lead compounds for a new class of drugs, we collected soil-derived fungal strains from various environments, including urban areas. As various human pathogens are assumed to influence the biosynthetic pathways of metabolites in soil fungi, leading to the production of novel scaffolds, we focused our work on densely populated urban areas and tourist attractions. A soil-derived fungal extract library was screened against MDA-MB-231 cells to derive their cytotoxic activity. Notably, 10 μg/mL of the extract of Trichoderma guizhouense (DS9-1) was found to exhibit an inhibitory effect of 71%. Fractionation, isolation, and structure elucidation efforts led to the identification of nine new peptaibols, trichoguizaibols A-I (1-9), comprising 14 amino acid residues (14-AA peptaibols), and three new peptaibols, trichoguizaibols J-L (10-12), comprising 18 amino acid residues (18-AA peptaibols). The chemical structures of 1-12 were determined based on their 1D and 2D NMR spectra, HRESIMS, electronic circular dichroism data, and results of the advanced Marfey's method. The 18-AA peptaibols were found to exhibit cytotoxicity against MDA-MB-231, SK-Hep1, SKOV3, DU145, and HCT116 cells greater than that of the 14-AA peptaibols. Among these compounds, 10-12 exhibited potent sub-micromolar IC50 values. These results are expected to shed light on a new direction for developing novel scaffolds as anticancer agents.

New Antibacterial Peptaibiotics against Plant and Fish Pathogens from the Deep-Sea-Derived Fungus Simplicillium obclavatum EIODSF 020

Bacterial diseases could severely harm agricultural production. To develop new antibacterial agents, the secondary metabolites of a deep-sea-derived fungus Simplicillium obclavatum EIODSF 020 with antibacterial activities against plant and fish pathogens were investigated by a bioassay-guided approach, which led to the isolation of 11 new peptaibiotics, simplicpeptaibs A-K (1-11). They contain 16-19 residues, including β-alanine, tyrosine, or tyrosine O-sulfate, that were rarely present in peptaibiotics. Their structures were elucidated by spectroscopic analyses (NMR, HRMS, HRMS2, and ECD) and Marfey's method. The primary and secondary structures of novel sulfated peptaibiotic 9 were reconfirmed by single-crystal X-ray diffraction analysis. Genome sequencing of S. obclavatum EIODSF 020 allowed the detection of a gene cluster encoding two individual NRPSs (totally containing 19 modules) that was closely related to simplicpeptaib biosynthesis. Antibacterial investigations of 1-11 together with the previously isolated linear and cyclic peptides from this strain suggested the antibacterial property of this fungus was attributed to the peptaibiotics and cyclic lipopeptides. Among them, compounds 4, 6, 7, and 9 showed significant activity against the tobacco pathogen Ralstonia solanacearum or tilapia pathogens Streptococcus iniae and Streptococcus agalactiae. The antibacterial activity of 6 against R. solanacearum could be enhanced by the addition of 1% NaCl. The structure-bioactivity relationship of simplicpeptaibs was discussed.

β-Turn Induction by a Diastereopure Azepane-Derived Quaternary Amino Acid

β-Turns are one of the most common secondary structures found in proteins. In the interest of developing novel β-turn inducers, a diastereopure azepane-derived quaternary amino acid has been incorporated into a library of simplified tetrapeptide models in order to assess the effect of the azepane position and peptide sequence on the stabilization of β-turns. The conformational analysis of these peptides by molecular modeling, NMR spectroscopy, and X-ray crystallography showed that this azepane amino acid is an effective β-turn inducer when incorporated at the i + 1 position. Moreover, the analysis of the supramolecular self-assembly of one of the β-turn-containing peptide models in the solid state reveals that it forms a supramolecular helical arrangement while maintaining the β-turn structure. The results here presented provide the basis for the use of this azepane quaternary amino acid as a strong β-turn inducer in the search for novel peptide-based bioactive molecules, catalysts, and biomaterials.

Leveraging Peptaibol Biosynthetic Promiscuity for Next-Generation Antiplasmodial Therapeutics

Malaria remains a worldwide threat, afflicting over 200 million people each year. The emergence of drug resistance against existing therapeutics threatens to destabilize global efforts aimed at controlling Plasmodium spp. parasites, which is expected to leave vast portions of humanity unprotected against the disease. To address this need, systematic testing of a fungal natural product extract library assembled through the University of Oklahoma Citizen Science Soil Collection Program has generated an initial set of bioactive extracts that exhibit potent antiplasmodial activity (EC₅₀ < 0.30 μg/mL) and low levels of toxicity against human cells (less than 50% reduction in HepG2 growth at 25 μg/mL). Analysis of the two top-performing extracts from Trichoderma sp. and Hypocrea sp. isolates revealed both contained chemically diverse assemblages of putative peptaibol-like compounds that were responsible for their antiplasmodial actions. Purification and structure determination efforts yielded 30 new peptaibols and lipopeptaibols (1-14 and 28-43), along with 22 known metabolites (15-27 and 44-52). While several compounds displayed promising activity profiles, one of the new metabolites, harzianin NPDG I (14), stood out from the others due to its noteworthy potency (EC₅₀ = 0.10 μM against multi-drug-resistant P. falciparum line Dd2) and absence of gross toxicity toward HepG2 at the highest concentrations tested (HepG2 EC₅₀ > 25 μM, selectivity index > 250). The unique chemodiversity afforded by these fungal isolates serves to unlock new opportunities for translating peptaibols into a bioactive scaffold worthy of further development.

Helical screw-sense preferences of peptides based on chiral, Cα-tetrasubstituted α-amino acids

The preferred helical screw senses of chiral α-amino acids with a C(α)-tetrasubstituted α-carbon atom, as determined in the crystal state by X-ray diffraction analyses on derivatives and peptides, are reviewed. This survey covers C(α)-methylated and C(α)-ethylated α-amino acids, as well as α-amino acids cyclized on the α-carbon, including those characterized by the combination of lack of chirality at the α-carbon with either side-chain or axial chirality. Although, in general, chiral C(α)-tetrasubstituted α-amino acids show a less pronounced bias toward a single helical screw sense than their proteinogenic (C(α)-trisubstituted) counterparts, our analysis highlights significant differences in terms of magnitude and direction of such a bias among the various sub-families of residues, and between individual amino acids within each sub-family as well. The experimental findings can be rationalized, at least in part, on the basis of steric considerations.

Origin of Helical Screw Sense Selectivity Induced by Chiral Constrained Cα-Tetrasubstituted α-Amino Acids in Aib-based Peptides

The mechanisms behind the propensity of chiral constrained Cα-tetrasubstituted amino acids (cCTAAs) to induce one particular helical screw sense, when included in the Ac-Aib2-cCTAA-Aib2-NHMe peptide model, were studied through replica exchange molecular dynamics, potential of mean force, and quantum theory of atoms in molecules calculations. We observed that cCTAAs exert their effect on helical screw sense selectivity through the positioning of the side chain to generate steric hindrance in either the (-x, +y, +z) or (+x, +y, -z) sectors of a right-handed 3D Cartesian space, where the z axis corresponds to the axis of the helix and the Cα lies on the +y semiaxis (0, +y, 0). The different strengthening of the noncovalent interactions, also comprising C-H···O interactions, exerted by the cCTAA in the right-handed or left-handed helix was also found important to define the preference of a cCTAA for a particular helix screw sense.

Divergent, stereoselective access to heterocyclic α,α-quaternary- and β(2,3,3)-amino acid derivatives from a N-Pmp-protected Orn-derived β-lactam

A suitably protected Orn-derived (3S,4S)-β-lactam was used as common intermediate in the synthesis of conformationally constrained (3S,4S)-2-oxoazepane α,α- and (2S,3S)-2-oxopiperidine-β(2,3,3)-amino acid derivatives. Compared to alternative procedures using an N-p-methoxybenzyl group at the 2-azetidinone, the incorporation of a p-methoxyphenyl moiety is crucial for the excellent stereochemical outcomes in the preparation of these heterocyclic amino acids. Chemoselective 7- or 6-exo-trig cyclization was achieved through alternative sequences of Pmp-deprotection/Boc-activation, followed by inter- and intramolecular β-lactam ring opening, respectively.

Design and self-assembly of a leucine-enkephalin analogue in different nanostructures: application of nanovesicles

An opioid (leucine-enkephalin) conformational analogue forms diverse nanostructures such as vesicles, tubes, and organogels through self-assembly. The nanovesicles encapsulate the natural hydrophobic drug curcumin and allow the controlled release through cation-generated porogens in membrane mimetic solvent.

Design, conformational studies and analysis of structure-function relationships of PTH (1-11) analogues: the essential role of Val in position 2

The N-terminal 1-34 segment of parathyroid hormone (PTH) is fully active in vitro and in vivo and it elicits all the biological responses characteristic of the native intact PTH. Recent studies reported potent helical analogues of the PTH (1-11) with helicity-enhancing substitutions. This work describes the synthesis, biological activity, and conformational studies of analogues obtained from the most active non-natural PTH (1-11) peptide H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH2; specifically, the replacement of Val in position 2 with D-Val, L-(αMe)-Val and N-isopropyl-Gly was studied. The synthesized analogues were characterized functionally by in-cell assays and their structures were determined by CD and NMR spectroscopy. To clarify the relationship between the structure and activity, the structural data were used to generate a pharmacophoric model, obtained overlapping all the analogues. This model underlines the fundamental functional role of the side chain of Val2 and, at the same time, reveals that the introduction of conformationally constrained Cα-tetrasubstituted α-amino acids in the peptides increases their helical content, but does not necessarily ensure significant biological activity.