Lasso Peptides: Exploring the Folding Landscape of Nature's Smallest Interlocked Motifs.

Lasso peptides make up a class of natural products characterized by a threaded structure. Given their small size and stability, chemical synthesis would offer tremendous potential for the development of novel therapeutics. However, the accessibility of the pre-folded lasso architecture has limited this advance. To better understand the folding process de novo, simulations are used herein to characterize the folding propensity of microcin J25 (MccJ25), a lasso peptide known for its antimicrobial properties. New algorithms are developed to unambiguously distinguish threaded from nonthreaded precursors and determine handedness, a key feature in natural lasso peptides. We find that MccJ25 indeed forms right-handed pre-lassos, in contrast to past predictions but consistent with all natural lasso peptides. Additionally, the native pre-lasso structure is shown to be metastable prior to ring formation but to readily transition to entropically favored unfolded and nonthreaded structures, suggesting that de novo lasso folding is rare. However, by altering the ring forming residues and appending thiol and thioester functionalities, we are able to increase the stability of pre-lasso conformations. Furthermore, conditions leading to protonation of a histidine imidazole side chain further stabilize the modified pre-lasso ensemble. This work highlights the use of computational methods to characterize lasso folding and demonstrates that de novo access to lasso structures can be facilitated by optimizing sequence, unnatural modifications, and reaction conditions like pH.

Chemoselective, Oxidation-Induced Macrocyclization of Tyrosine-Containing Peptides.

Inspired by nature's wide range of oxidation-induced modifications to install cross-links and cycles at tyrosine (Tyr) and other phenol-containing residue side chains, we report a Tyr-selective strategy for the preparation of Tyr-linked cyclic peptides. This approach leverages N4-substituted 1,2,4-triazoline-3,5-diones (TADs) as azo electrophiles that react chemoselectively with the phenolic side chain of Tyr residues to form stable C-N1-linked cyclic peptides. In the developed method, a precursor 1,2,4-triazolidine-3,5-dione moiety, also known as urazole, is readily constructed at any free amine revealed on a solid-supported peptide. Once prepared, the N4-substituted urazole peptide is selectively oxidized using mild, peptide-compatible conditions to generate an electrophilic N4-substituted TAD peptide intermediate that reacts selectively under aqueous conditions with internal and terminal Tyr residues to furnish Tyr-linked cyclic peptides. The approach demonstrates good tolerance of native residue side chains and enables access to cyclic peptides ranging from 3- to 11-residues in size (16- to 38-atom-containing cycles). The identity of the installed Tyr-linkage, a stable covalent C-N1 bond, was characterized using NMR spectroscopy. Finally, we applied the developed method to prepare biologically active Tyr-linked cyclic peptides bearing the integrin-binding RGDf epitope.

New Role for Radical SAM Enzymes in the Biosynthesis of Thio(seleno)oxazole RiPP Natural Products.

Ribosomally synthesized post-translationally modified peptides (RiPPs) are ubiquitous and represent a structurally diverse class of natural products. The ribosomally encoded precursor polypeptides are often extensively modified post-translationally by enzymes that are encoded by coclustered genes. Radical S-adenosyl-l-methionine (SAM) enzymes catalyze numerous chemically challenging transformations. In RiPP biosynthetic pathways, these transformations include the formation of C-H, C-C, C-S, and C-O linkages. In this paper, we show that the Geobacter lovleyi sbtM gene encodes a radical SAM protein, SbtM, which catalyzes the cyclization of a Cys/SeCys residue in a minimal peptide substrate. Biochemical studies of this transformation support a mechanism involving H-atom abstraction at the C-3 of the substrate Cys to initiate the chemistry. Several possible cyclization products were considered. The collective biochemical, spectroscopic, mass spectral, and computational observations point to a thiooxazole as the product of the SbtM-catalyzed modification. To our knowledge, this is the first example of a radical SAM enzyme that catalyzes a transformation involving a SeCys-containing peptide and represents a new paradigm for formation of oxazole-containing RiPP natural products.

Photosensitized Oxidative Dimerization at Tyrosine by a Water-Soluble 4-Amino-1,8-naphthalimide.

The oxidation of proteins generates reactive amino acid (AA) residue intermediates, leading to protein modification and cross-linking. Aerobic studies with peptides and photosensitizers allow for the controlled generation of reactive oxygen species (ROS) and reactive AA residue intermediates, providing mechanistic insights as to how natural protein modifications form. Such studies have inspired the development of abiotic methods for protein modification and crosslinking, including applications of biomedical importance. Dityrosine linkages derived from oxidation at tyrosine (Tyr) residues represent one of the more well-understood oxidation-induced modifications. Here we demonstrate an aerobic, visible light-dependent oxidation reaction of Tyr-containing substrates promoted by a water-soluble 4-amino-1,8-naphthalimide-based photosensitizer. The developed procedure converts Tyr-containing substrates into o,o'-Tyr-Tyr linked dimers. The regioselectively formed o,o'-Tyr-Tyr linkage is consistent with dimeric standards prepared using a known enzymatic method. A crossover study with two peptides provides a statistical mixture of three distinct o,o'-Tyr-Tyr linked dimers, supporting a mechanism that involves Tyr residue oxidation followed by intermolecular combination.

Cascading Auto-oxidative Biproline Guanylations Form Optically Active Dispacamide Dimers and Permit an Eight-Step Synthesis of (-)-Ageliferin.

A nickel catalyzed synthesis of isomeric 3,3'-biproline esters is described. When those materials are doubly acylated with the acid chloride of pyrrole-2-carboxylic acid, they become susceptible to auto-oxidation in the presence of guanidine. Through proper staging of reaction conditions, it is possible to initiate two consecutive oxidative guanylations prior to in situ cycloisomerization to afford spirocyclic bis-glycocyamidines. This unique outcome reflects a cascade of no fewer than 10 reactions occurring sequentially in one flask. The chemistry provides rapid access to advanced intermediates useful for the preparation of complex, optically active pyrrole/imidazole alkaloids.

Total Chemical Synthesis and Folding of All-l and All-d Variants of Oncogenic KRas(G12V).

The Ras proteins are essential GTPases involved in the regulation of cell proliferation and survival. Mutated oncogenic forms of Ras alter effector binding and innate GTPase activity, leading to deregulation of downstream signal transduction. Mutated forms of Ras are involved in approximately 30% of human cancers. Despite decades of effort to develop direct Ras inhibitors, Ras has long been considered "undruggable" due to its high affinity for GTP and its lack of hydrophobic binding pockets. Herein, we report a total chemical synthesis of all-l- and all-d-amino acid biotinylated variants of oncogenic mutant KRas(G12V). The protein is synthesized using Fmoc-based solid-phase peptide synthesis and assembled using combined native chemical ligation and isonitrile-mediated activation strategies. We demonstrate that both KRas(G12V) enantiomers can successfully fold and bind nucleotide substrates and binding partners with observable enantiodiscrimination. By demonstrating the functional competency of a mirror-image form of KRas bound to its corresponding enantiomeric nucleotide triphosphate, this study sets the stage for further biochemical studies with this material. In particular, this protein will enable mirror-image yeast surface display experiments to identify all-d peptide ligands for oncogenic KRas, providing a useful tool in the search for new therapeutics against this challenging disease target.

Fully Synthetic Granulocyte Colony-Stimulating Factor Enabled by Isonitrile-Mediated Coupling of Large, Side-Chain-Unprotected Peptides.

Human granulocyte colony-stimulating factor (G-CSF) is an endogenous glycoprotein involved in hematopoiesis. Natively glycosylated and nonglycosylated recombinant forms, lenograstim and filgrastim, respectively, are used clinically to manage neutropenia in patients undergoing chemotherapeutic treatment. Despite their comparable therapeutic potential, the purpose of O-linked glycosylation at Thr133 remains a subject of controversy. In light of this, we have developed a synthetic platform to prepare G-CSF aglycone with the goal of enabling access to native and designed glycoforms with site-selectivity and glycan homogeneity. To address the synthesis of a relatively large, aggregation-prone sequence, we advanced an isonitrile-mediated ligation method. The chemoselective activation and coupling of C-terminal peptidyl Gly thioacids with the N-terminus of an unprotected peptide provide ligated peptides directly in a manner complementary to that with conventional native chemical ligation-desulfurization strategies. Herein, we describe the details and application of this method as it enabled the convergent total synthesis of G-CSF aglycone.

A widespread methylotroph acyl-homoserine lactone synthase produces a new quorum sensing signal that regulates swarming in Methylobacterium fujisawaense.

IMPORTANCE: Bacteria known as pink-pigmented facultative methylotrophs colonize many diverse environments on earth, play an important role in the carbon cycle, and in some cases promote plant growth. However, little is known about how these organisms interact with each other and their environment. In this work, we identify one of the chemical signals commonly used by these bacteria and discover that this signal controls swarming motility in the pink-pigmented facultative methylotroph Methylobacterium fujisawaense DSM5686. This work provides new molecular details about interactions between these important bacteria and will help scientists predict these interactions and the group behaviors they regulate from genomic sequencing information.

Development of a 213Bi-Labeled Pyridyl Benzofuran for Targeted α-Therapy of Amyloid-ß Aggregates.

Alzheimer disease is a neurodegenerative disorder with limited treatment options. It is characterized by the presence of several biomarkers, including amyloid-ß aggregates, which lead to oxidative stress and neuronal decay. Targeted α-therapy (TAT) has been shown to be efficacious against metastatic cancer. TAT takes advantage of tumor-localized α-particle emission to break disease-associated covalent bonds while minimizing radiation dose to healthy tissues due to the short, micrometer-level, distances traveled. We hypothesized that TAT could be used to break covalent bonds within amyloid-ß aggregates and facilitate natural plaque clearance mechanisms. Methods: We synthesized a 213Bi-chelate-linked benzofuran pyridyl derivative (BiBPy) and generated [213Bi]BiBPy, with a specific activity of 120.6 GBq/µg, dissociation constant of 11 ± 1.5 nM, and logP of 0.14 ± 0.03. Results: As the first step toward the validation of [213Bi]BiBPy as a TAT agent for the reduction of Alzheimer disease-associated amyloid-ß, we showed that brain homogenates from APP/PS1 double-transgenic male mice (6-9 mo old) incubated with [213Bi]BiBPy exhibited a marked reduction in amyloid-ß plaque concentration as measured using both enzyme-linked immunosorbent and Western blotting assays, with a half-maximal effective concentration of 3.72 kBq/pg. Conclusion: This [213Bi]BiBPy-concentration-dependent activity shows that TAT can reduce amyloid plaque concentration in vitro and supports the development of targeting systems for in vivo validations.

Chemical cyclization of tyrosine-containing peptides via in situ generated triazolinedione peptides.

Tyr-derived cyclic peptide natural products are formed by enzymatic manifolds that oxidatively cross-link embedded phenolic side chains of tyrosine (Tyr) and 4-hydroxyphenylglycine residues during their controlled production. Bioactive Tyr-derived cyclic peptides, such as the arylomycins and vancomycins, continue to motivate the development of enzymatic and chemical strategies for their de novo assembly and modification. However, chemical access to these structurally diverse natural cycles can be challenging and step intensive. Therefore, we developed an oxidative procedure to selectively convert Tyr-containing N4-substituted 1,2,4-triazolidine-3,5-dione peptides (urazole peptides) into stable Tyr-linked cyclic peptides. We show that Tyr-containing urazole peptides are simple to prepare and convert into reactive N4-substituted 1,2,4-triazoline-3,5-dione peptides by oxidation, which then undergo spontaneous cyclization under mildly basic aqueous conditions to form a cross-linkage with the phenol side chain of embedded Tyr residues. Using this approach, we have demonstrated access to over 25 Tyr-linked cyclic peptides (3- to 11-residue cycles) with good tolerance of native residue side chain functionalities. Importantly, this method is simple to perform, and product formation can be quickly confirmed by mass spectrometric and 1H NMR spectroscopic analyses.

Nonthreaded Isomers of Sungsanpin and Ulleungdin Lasso Peptides Inhibit H1299 Cancer Cell Migration.

Lasso peptides are a structurally distinct class of biologically active natural products defined by their short sequences with impressively interlocked tertiary structures. Their characteristic peptide [1]rotaxane motif confers marked proteolytic and thermal resiliency, and reports on their diverse biological functions have been credited to their exceptional sequence variability. Because of these unique properties, taken together with improved technologies for their biosynthetic production, lasso peptides are emerging as a designable scaffold for peptide-based therapeutic discovery and development. Although the defined structure of lasso peptides is recognized for its remarkable properties, the role of the motif in imparting bioactivity is less understood. For example, sungsanpin and ulleungdin are natural lasso peptides that similarly exhibit encouraging cell migration inhibitory activities in A549 lung carcinoma epithelial cells, despite sharing only one-third of the sequence homology. We hypothesized that the shape of the lasso motif is beneficial for the preorganization of the conserved residues, which might be partially retained in variants lacking the threaded structure. Herein, we describe solid-phase peptide synthesis strategies to prepare acyclic, head-to-side chain (branched), and head-to-tail (macrocyclic) cyclic variants based on the sungsanpin (Sun) and ulleungdin (Uln) sequences. Proliferation assays and time-lapse cell motility imaging studies were used to evaluate the cell inhibitory properties of natural Sun compared with the synthetic Sun and Uln isomers. These studies demonstrate that the lasso motif is not a required feature to slow cancer cell migration and more generally show that these nonthreaded isomers can retain similar activity to the natural lasso peptide despite the differences in their overall structures.

Design and Evaluation of Ambiphilic Aryl Thiol-Iminium-Based Molecules for Organocatalyzed Thioacyl Aminolysis.

Progress toward the design and synthesis of ambiphilic aryl thiol-iminium-based small molecules for organocatalyzed thioacyl aminolysis is reported. Here we describe the synthesis of a novel tetrahydroisoquinoline-derived scaffold, bearing both thiol and iminium functionalities, capable of promoting the transthioesterification and subsequent amine capture reactions necessary to achieve organocatalyzed thioacyl aminolysis. Model studies demonstrate the ability of this designed organocatalyst to deliver critical intermediates capable of undergoing these individual reactions necessary for the proposed process. Future design improvements and directions toward cysteine-independent organocatalyzed native chemical ligation are discussed.

Deaminative ring contraction for the synthesis of polycyclic heteroaromatics: a concise total synthesis of toddaquinoline.

A concise strategy to prepare polycyclic heteroaromatics involving a deaminative contraction cascade is detailed. The efficient deaminative ring contraction involves the in situ methylation of a biaryl-linked dihydroazepine to form a cyclic ammonium cation that undergoes a base-induced [1,2]-Stevens rearrangement/dehydroamination sequence. The presence of pseudosymmetry guides the retrosynthetic analysis of pyridyl-containing polycyclic heteroaromatics, enabling their construction by the reductive cyclization and deaminative contraction of tertiary amine precursors.

A Promiscuous rSAM Enzyme Enables Diverse Peptide Cross-linking.

Ribosomally produced and post-translationally modified polypeptides (RiPPs) are a diverse group of natural products that are processed by a variety of enzymes to their biologically relevant forms. PapB is a member of the radical S-adenosyl-l-methionine (rSAM) superfamily that introduces thioether cross-links between Cys and Asp residues in the PapA RiPP. We report that PapB has high tolerance for variations in the peptide substrate. Our results demonstrate that branched side chains in the thiol- and carboxylate-containing residues are processed and that lengthening of these groups to homocysteine and homoglutamate does not impair the ability of PapB to form thioether cross-links. Remarkably, the enzyme can even cross-link a peptide substrate where the native Asp carboxylate moiety is replaced with a tetrazole. We show that variations to residues embedded between the thiol- and carboxylate-containing residues are tolerated by PapB, as peptides containing both bulky (e.g., Phe) and charged (e.g., Lys) side chains in both natural L- and unnatural D-forms are efficiently cross-linked. Diastereomeric peptides bearing (2S,3R)- and (2S,3S)-methylaspartate are processed by PapB to form cyclic thioethers with markedly different rates, suggesting the enzymatic hydrogen atom abstraction event for the native Asp-containing substrate is diastereospecific. Finally, we synthesized two diastereomeric peptide substrates bearing E- and Z-configured γ,δ-dehydrohomoglutamate and show that PapB promotes addition of the deoxyadenosyl radical (dAdo•) instead of hydrogen atom abstraction. In the Z-configured γ,δ-dehydrohomoglutamate substrate, a fraction of the dAdo-adduct peptide is thioether cross-linked. In both cases, there is evidence for product inhibition of PapB, as the dAdo-adducts likely mimic the native transition state where dAdo• is poised to abstract a substrate hydrogen atom. Collectively, these findings provide critical insights into the arrangement of reacting species in the active site of the PapB, reveal unusual promiscuity, and highlight the potential of PapB as a tool in the development peptide therapeutics.

Design and synthesis of astatinated benzothiazole compounds for their potential use in Targeted Alpha Therapy (TAT) strategies to treat Alzheimer's disease-associated amyloid plaques.

PURPOSE: Alzheimer's disease (AD) is a terminal neurodegenerative disease characterized by the buildup of amyloid fibrils, amorphous aggregates and tauopathies. Several treatment modalities, which rely on various biological processes to reduce disease burden, have been largely ineffective at treating Alzheimer's disease. Targeted alpha therapy (TAT) has demonstrated positive results in the treatment of cancer. Benzothiazole derivatives have been successfully shown to target these plaques and are used in several imaging applications. One such derivative, Flutemetamol (VizamylTM) is an FDA approved diagnostic tool for PET imaging of AD-associated plaques. We report the radiolabeling of benzothiazole derivatives with 211At, a 7.2 h alpha emitting radionuclide, using a copper catalyzed reaction with a boronic acid precursor molecule. Our final compound [211At]3'-At-PIB-OMe had a radiochemical yield of 55% and was found to be stable for at least 3 h in phosphate buffered saline.

Establishing Self-Dopant Design Principles from Structure-Function Relationships in Self-n-Doped Perylene Diimide Organic Semiconductors.

Self-doping is a particular doping method that has been applied to a wide range of organic semiconductors. However, there is a lack of understanding regarding the relationship between dopant structure and function. A structurally diverse series of self-n-doped perylene diimides (PDIs) is investigated to study the impact of steric encumbrance, counterion selection, and dopant/PDI tether distance on functional parameters such as doping, stability, morphology, and charge-carrier mobility. The studies show that self-n-doping is best enabled by the use of sterically encumbered ammoniums with short tethers and Lewis basic counterions. Additionally, water is found to inhibit doping, which concludes that thermal degradation is merely a phenomenological feature of certain dopants, and that residual solvent evaporation is the primary driver of thermally activated doping. In situ grazing-incidence wide-angle X-ray scattering studies show that sample annealing increases the π-π stacking distance and shrinks grain boundaries for improved long-range ordering. These features are then correlated to contactless carrier-mobility measurements with time-resolved microwave conductivity before and after thermal annealing. The collective relationships between structural features and functionality are finally used to establish explicit self-n-dopant design principles for the future design of materials with improved functionality.

Photoactivation Properties of Self-n-Doped Perylene Diimides: Concentration-dependent Radical Anion and Dianion Formation.

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R •- ) and dianions (D ••2- ). We observe R •- formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D ••2- formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R •- to the strongly reducing doublet excited state 2[R •- ]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state 1[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R •- and D ••2- formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

Design, synthesis and characterization of structurally dynamic cyclic N,S-acetals.

We report the synthesis, characterization and comparison of a series of electronically perturbed, cyclic N,S-acetals. Inspired by electrophilic auxiliaries utilized for amine capture and concomitant peptide ligation, we studied these N,S-acetal systems and evaluated their propensity to generate zwitterionic intermediates in situ. Certain N,S-acetals in this study exhibit structurally dynamic properties through a solvent and pH-dependent ability to ring-open and ring-close via C1-S bond ionization at room temperature.

Exploring symmetry-based logic for a synthesis of palau'amine.

A synthetic approach to palau'amine is described that exploits veiled symmetry in the structure. Bis-alkylidenes i have been prepared and found susceptible to halogenative desymmetrization using t-BuOCl. This oxidation forms the imbedded spirocyclopentane motif observed in the natural product. A host of atypical reactions and processes developed during these studies are discussed, as are plans for completing total syntheses of this compound class.