Fluorine-18 labeling PEGylated 6-boronotryptophan for PET scanning of mice for assessing the pharmacokinetics for boron neutron capture therapy of brain tumors.

Two tryptophan compound classes 5- and 6-borono PEGylated boronotryptophan derivatives have been prepared for assessing their aqueous solubility as formulation of injections for boron neutron capture therapy (BNCT). The PEGylation has improved their aqueous solubility thereby increasing their test concentration in 1 mM without suffering from toxicity. In-vitro uptake assay of PEGylated 5- and 6-boronotryptophan showed that the B-10 concentration can reach 15-50 ppm in U87 cell whereas the uptake in LN229 cell varies. Shorter PEG compound 6-boronotryptophanPEG200[18F] was obtained in 1.7 % radiochemical yield and the PET-derived radioradioactivity percentage in 18 % was taken up by U87 tumor at the limb of xenograft mouse. As high as tumor to normal uptake ratio in 170 (T/N) was obtained while an inferior radioactivity uptake of 3 % and T/N of 8 was observed in LN229 xenografted mouse.

Abiotic synthesis of amino acids in the recesses of the oceanic lithosphere.

Abiotic hydrocarbons and carboxylic acids are known to be formed on Earth, notably during the hydrothermal alteration of mantle rocks. Although the abiotic formation of amino acids has been predicted both from experimental studies and thermodynamic calculations, its occurrence has not been demonstrated in terrestrial settings. Here, using a multimodal approach that combines high-resolution imaging techniques, we obtain evidence for the occurrence of aromatic amino acids formed abiotically and subsequently preserved at depth beneath the Atlantis Massif (Mid-Atlantic Ridge). These aromatic amino acids may have been formed through Friedel-Crafts reactions catalysed by an iron-rich saponite clay during a late alteration stage of the massif serpentinites. Demonstrating the potential of fluid-rock interactions in the oceanic lithosphere to generate amino acids abiotically gives credence to the hydrothermal theory for the origin of life, and may shed light on ancient metabolisms and the functioning of the present-day deep biosphere.

Design, synthesis, and activity evaluation of novel multitargeted l-tryptophan derivatives with powerful antioxidant activity against Alzheimer's disease.

Alzheimer's disease (AD) is a multifactorial neurological disease, and the multitarget directed ligand (MTDL) strategy may be an effective approach to delay its progression. Based on this strategy, 27 derivatives of l-tryptophan, 3a-1-3d-1, were designed, synthesized, and evaluated for their biological activity. Among them, IC50 (inhibitor concentration resulting in 50% inhibitory activity) values of compounds 3a-18 and 3b-1 were 0.58 and 0.44 µM for human serum butyrylcholinesterase (hBuChE), respectively, and both of them exhibited more than 30-fold selectivity for human serum acetylcholinesterase. Enzyme kinetics studies showed that these two compounds were mixed inhibitors of hBuChE. In addition, these two derivatives possessed extraordinary antioxidant activity in OH radical scavenging and oxygen radical absorption capacity fluorescein assays. Meanwhile, these compounds could also prevent ß-amyloid (Aß) self-aggregation and possessed low toxicity on PC12 and AML12 cells. Molecular modeling studies revealed that these two compounds could interact with the choline binding site, acetyl binding site, and peripheral anionic site to exert submicromolar BuChE inhibitory activity. In the vitro blood-brain barrier permeation assay, compounds 3a-18 and 3b-1 showed enough blood-brain barrier permeability. In drug-likeness prediction, compounds 3a-18 and 3b-1 showed good gastrointestinal absorption and a low risk of human ether-a-go-go-related gene toxicity. Therefore, compounds 3a-18 and 3b-1 are potential multitarget anti-AD lead compounds, which could work as powerful antioxidants with submicromolar selective inhibitory activity for hBuChE as well as prevent Aß self-aggregation.

Visualization of Platelet Integrins via Two-Photon Microscopy Using Anti-transmembrane Domain Peptides Containing a Blue Fluorescent Amino Acid.

The fluorescent reporters commonly used to visualize proteins can perturb both protein structure and function. Recently, we found that 4-cyanotryptophan (4CN-Trp), a blue fluorescent amino acid, is suitable for one-photon imaging applications. Here, we demonstrate its utility in two-photon fluorescence microscopy by using it to image integrins on cell surfaces. Specifically, we used solid-phase peptide synthesis to generate CHAMP peptides labeled with 4-cyanoindole (4CNI) at their N-termini to image integrins on cell surfaces. CHAMP (computed helical anti-membrane protein) peptides spontaneously insert into membrane bilayers to target integrin transmembrane domains and cause integrin activation. We found that 4CNI labeling did not perturb the ability of CHAMP peptides to insert into membranes, bind to integrins, or cause integrin activation. We then used two-photon fluorescence microscopy to image 4CNI-containing integrins on the surface of platelets. Compared to a 4CNI-labeled scrambled peptide that uniformly decorated cell surfaces, 4CNI-labeled CHAMP peptides were present in discrete blue foci. To confirm that these foci represented CN peptide-containing integrins, we co-stained platelets with integrin-specific fluorescent monoclonal antibodies and found that CN peptide and antibody fluorescence coincided. Because 4CNI can readily be biosynthetically incorporated into proteins with little if any effect on protein structure and function, it provides a facile way to directly monitor protein behavior and protein-protein interactions in cellular environments. In addition, these results clearly demonstrate that the two-photon excitation cross section of 4CN-Trp is sufficiently large to make it a useful two-photon fluorescence reporter for biological applications.

Tryptophan Analogues with Fixed Side-Chain Orientation: Expanding the Scope.

A generalized synthetic strategy is proposed here for the synthesis of asymmetric ß-indoylated amino acids by 8-aminoquinoline (8AQ)-directed C(sp3)-H functionalization of suitably protected precursors. Peptides containing one of the four stereoisomers of (indol-3-yl)-3-phenylalanine at position 2 of the parent peptide KwFwLL-NH2 (w=d-Trp) cover a wide range of activities as ghrelin receptor inverse agonists, among them the most active described until now. This application exemplarily shows how ß-indoylated amino acids can be used for the systematic variation of the position of an indole group in a bioactive peptide.

Structural and biophysical characterization of the Burkholderia pseudomallei IspF inhibitor L-tryptophan hydroxamate.

The enzyme 2-methylerythritol 2,4-cyclodiphosphate synthase, IspF, is essential for the biosynthesis of isoprenoids in most bacteria, some eukaryotic parasites, and the plastids of plant cells. The development of inhibitors that target IspF may lead to novel classes of anti-infective agents or herbicides. Enantiomers of tryptophan hydroxamate were synthesized and evaluated for binding to Burkholderia pseudomallei (Bp) IspF. The L-isomer possessed the highest potency, binding BpIspF with a KD of 36 µM and inhibited BpIspF activity 55% at 120 µM. The high-resolution crystal structure of the L-tryptophan hydroxamate (3)/BpIspF complex revealed a non-traditional mode of hydroxamate binding where the ligand interacts with the active site zinc ion through the primary amine. In addition, two hydrogen bonds are formed with active site groups, and the indole group is buried within the hydrophobic pocket composed of side chains from the 60 s/70 s loop. Along with the co-crystal structure, STD NMR studies suggest the methylene group and indole ring are potential positions for optimization to enhance binding potency.

Tryptophan derivatives regulate the seed germination and radicle growth of a root parasitic plant, Orobanche minor.

Root parasitic plant germination is induced by the host-derived chemical, strigolactone (SL). We found that a major microbial culture broth component, tryptone, inhibits the SL-inducible germination of a root parasitic plant, Orobanche minor. l-tryptophan (l-Trp) was isolated as the active compound from tryptone. We further found that l-Trp related compounds (1b-11), such as a major plant hormone auxin (8, indole-3-acetic acid; IAA), also inhibit the germination and post-radicle growth of O. minor. We designed a hybrid chemical (13), in which IAA is attached to a part of SL, and found that this synthetic analog induced the germination of O. minor, and also inhibited post-radicle growth. Moreover, contrary to our expectations, we found that N-acetyl Trp (9) showed germination stimulating activity, and introduction of a substitution at C-5 position increased its activity (12a-12f). Our data, in particular, the discovery of a structurally hybrid compound that has two activities that induce spontaneous germination and inhibit subsequent radical growth, would provide new types of germination regulators for root parasitic plants.

Modification of the lead molecule: Tryptophan and piperidine appended triazines reversing inflammation and hyeperalgesia in rats.

The structural optimization of the molecules making them to fit into the active site pocket of COX-2 occupying the same space as covered by the natural substrate arachidonic acid helped in the emergence of compound 10 as an efficacious anti-inflammatory agent. Selective for COX-2 over COX-1, compound 10 exhibited IC50 0.02 µM for COX-2 and reversed acetic acid induced inflammation in rats by 73% when used at 10 mg kg-1 dose and the same dose of the compound also rescued the animals from inflammatory phase of formalin induced hyperalgesia. As evidenced by the results of molecular modeling studies supported by the nuclear Overhauser enhancement data, the appropriate geometry of the molecule in the active site pocket of COX-2 contributing to its H-bond/hydrophobic interactions with Ser530, Trp387 and Tyr385 seems responsible for the enzyme inhibitory activity of the compound.

Rational design of specific ligands for human serum albumin separation and applications.

Human serum albumin is widely used in clinical practice, and the development of new ligands with high affinity is beneficial to improve its separation efficiency. The Site II of human serum albumin is an active binding site of various molecules such as l-tryptophan, which was studied with molecular simulation to obtain insights for the design of new ligands. The results showed that the carboxyl and indolyl groups of l-tryptophan were critical for the binding on Site II. Seven ligands containing carboxyl groups and indolyl groups were designed, and molecular simulation showed that indole-3-pentanoic acid was the best ligand. A new ligand combined indole-3-acetic acid and cysteine was designed for easier resin preparation, and molecular simulation also indicated that the new ligand bound strongly to Site II. Resins with the new ligand designed was prepared and static adsorption experiments indicated that the new resin had high adsorption capacity of human serum albumin and strong salt tolerance. Finally, recombinant human serum albumin was separated from yeast broth with high purity of 90.4% and recovery of 94.2%, which indicated that the new resin had good adsorption selectivity and strong potential for applications.

Design and synthesis of enantiopure 18 F-labelled [18 F]trifluoromethyltryptophan from 2-halotryptophan derivatives via copper(I)-mediated [18 F]trifluoromethylation and evaluation of its in vitro characterization for the serotonergic system imaging.

We synthesized [18 F]trifluoromethyl-l-tryptophan ([18 F]CF3 -l-Trp) using Cu(I)-mediated [18 F]trifluoromethylation to image serotonergic system. Radiochemical yield was 6 ± 1.5% (n = 9), and radiochemical purity was over 99%. The molar activity was 0.44 to 0.76 GBq/µmol. [18 F]CF3 -l-Trp was stable for up to 6 hours in mouse and human sera at 37°C. Protein-binding was 0.26 ± 0.03% and 0.34 ± 0.02% in human and mouse serum at 60 minutes, respectively. In conclusion, enantiopure [18 F]CF3 -l-Trp was synthesized as a feasible imaging agent for the serotonergic system.

Total Synthesis of Sarpagine-Related Bioactive Indole Alkaloids.

Extension of the asymmetric Pictet-Spengler reaction to bulkier Nb -alkylated tryptophan derivatives resulted in an improved stereospecific access to the key bicyclo[3.3.1]nonane core of bioactive C-19 methyl substituted sarpagine/macroline/ajmaline indole alkaloids with excellent diastereoselectivity by internal asymmetric induction. Complete stereocontrol of the C-19 methyl function in either the α- or ß-configuration was achieved, which enables the total synthesis of any member from this group of thirty alkaloids. We report herein, the total synthesis of macrocarpines (A-C) 1-3, talcarpine 4, N(4)-methyl-N(4),21-secotalpinine 5, dihydroperaksine 8 and deoxyperaksine 9.

Improved Synthesis of 4-Cyanotryptophan and Other Tryptophan Analogues in Aqueous Solvent Using Variants of TrpB from Thermotoga maritima.

The use of enzymes has become increasingly widespread in synthesis as chemists strive to reduce their reliance on organic solvents in favor of more environmentally benign aqueous media. With this in mind, we previously endeavored to engineer the tryptophan synthase ß-subunit (TrpB) for production of noncanonical amino acids that had previously been synthesized through multistep routes involving water-sensitive reagents. This enzymatic platform proved effective for the synthesis of analogues of the amino acid tryptophan (Trp), which are frequently used in pharmaceutical synthesis as well as chemical biology. However, certain valuable compounds, such as the blue fluorescent amino acid 4-cyanotryptophan (4-CN-Trp), could only be made in low yield, even at elevated temperature (75 °C). Here, we describe the engineering of TrpB from Thermotoga maritima that improved synthesis of 4-CN-Trp from 24% to 78% yield. Remarkably, although the final enzyme maintains high thermostability ( T50 = 93 °C), its temperature profile is shifted such that high reactivity is observed at ∼37 °C (76% yield), creating the possibility for in vivo 4-CN-Trp production. The improvements are not specific to 4-CN-Trp; a boost in activity at lower temperature is also demonstrated for other Trp analogues.

Conformational stabilization of a ß-hairpin through a triazole-tryptophan interaction.

Molecular tools to stabilize the ß-hairpin conformation are needed as ß-hairpin peptides are useful molecules for pharmaceutical, biological and materials applications. We explored the use of a "triazole bridge", a covalent link between two ß-hairpin strands obtained through Cu-catalyzed alkyne-azide cycloaddition, combined with an aromatic-aromatic interaction. Highly conformationally stable peptides were identified by NMR screening of a small collection of cyclic peptides based on the Trpzip2 scaffold. The characteristic Trp-Trp interaction of Trpzip2 was replaced by a diagonal triazole bridge of variable length. NMR and CD analyses showed that triazole and indole rings could favorably interact to stabilize a ß-hairpin conformation. The conformational stabilization depends on the length of the triazole bridge and the reciprocal position between the aromatic rings. Combining aromatic interactions and the covalent inter-strand triazole bridge is a useful strategy to obtain peptides with a high ß-hairpin content.

Painting argyrins blue: Negishi cross-coupling for synthesis of deep-blue tryptophan analogue ß-(1-azulenyl)-l alanine and its incorporation into argyrin C.

The argyrins are a family of non-ribosomal peptides that exhibits different biological activities through only small structural changes. Ideally, a biologically active molecule can be tracked and observed in a variety of biological and clinical settings in a non-invasive manner. As a step towards this goal, we report here a chemical synthesis of unnatural deep blue amino acid ß-(1-azulenyl)-l alanine with different fluorescence and photophysical properties, which allows a spectral separation from the native tryptophan signal. This might be especially useful for cell localization studies and visualizing the targeted proteins. In particular, the synthesis of ß-(1-azulenyl)-l alanine was achieved through a Negishi coupling which proved to be a powerful tool for the synthesis of unnatural tryptophan analogs. Upon ß-(1-azulenyl)-l alanine incorporation into argyrin C, deep blue octapeptide variant was spectrally and structurally characterized.

Automated GMP-production of α-[11 C]methyl-L-tryptophan using a tracer production system (TPS).

The radiosynthesis and GMP validation of [11 C]AMT for human use are described. Three consecutive batches were produced giving 940-3790 MBq (4%-17% RCY, decay corrected, based on [11 C]CO2 ) of the tracer. The molar activity at the end of synthesis was 19 to 35 GBq/µmol, the radiochemical purity was ≥98%, and the enantiomeric purity was >99%. While the synthesis method was automated using a new generation of synthesis equipment, tracer production system developed in house, the method should be readily applicable to other synthesis platforms with minor modifications.

Mild, Aqueous α-Arylation of Ketones: Towards New Diversification Tools for Halogenated Metabolites and Drug Molecules.

The palladium-catalysed aqueous α-arylation of ketones was developed and tested for a large variety of reaction partners. These mild conditions enabled the coupling of aryl/alkyl-ketones with N-protected halotryptophans, heterocyclic haloarenes, and challenging base-sensitive compounds. The synthetic potential of this new methodology for the diversification of complex bioactive molecules was exemplified by derivatising prochlorperazine. The methodology is mild, aqueous and flexible, representing a means of functionalizing a wide range of halo-aromatics and therefore has the potential to be extended to complex molecule diversification.

Synthetic minimalistic tryptophan zippers as a chiroptical switch.

In this paper, we presented a new design strategy for a peptide-based chiral supramolecular assembly. A series of aryl linked peptides 1a-1f were designed and synthesized. The bis-urea peptides 1a-1c self-assembled into a helical supramolecular arrangement resembling Trp zipper (Trpzip) structures present in proteins. Interestingly, a dihydrogenphosphate anion, upon binding to the assembly, could invert the chirality of the supramolecular assembly which could be reverted to the original by the addition of water. This chiroptical behavior can be repeated several times. Microscopy analysis showed that the supramolecular helices were assembled to form spheres. In addition to that, we also found that the handedness of supramolecular chirality is dependent on the position of Trp residues on the aromatic scaffold. Both left and right handed helical supramolecular arrangements were obtained by placing l-Trp residues at different positions on the aromatic core. The unprecedented Trpzip in these designed small peptidomimetics will stimulate more work in the area of peptide-based assemblies.

Discovery of IDO1 and DNA dual targeting antitumor agents.

The development of small molecules for cancer immunotherapy is highly challenging and indoleamine 2,3-dioxygenase 1 (IDO1) represents a promising target. Inspired by the synergistic effects between IDO1 inhibitors and traditional antitumor chemotherapeutics, the first orally active dual IDO1 and DNA targeting agents were designed by the pharmacophore fusion strategy. The bifunctional hybrids exhibited enhanced IDO1 enzyme inhibitory activity and in vitro cytotoxicity as compared to IDO1 inhibitor 1-methyl-tryptophan and DNA alkylating agent melphalan. In a murine LLC tumor model, the dual targeting agents demonstrated excellent antitumor efficacy, highlighting the advantages of this novel design strategy to improve the efficacy of small molecule cancer immunotherapy.

Structure-activity relationship study towards non-peptidic positron emission tomography (PET) radiotracer for gastrin releasing peptide receptors: Development of [18F] (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide.

Gastrin-releasing peptide receptors (GRP-Rs, also known as bombesin 2 receptors) are overexpressed in a variety of human cancers, including prostate cancer, and therefore they represent a promising target for in vivo imaging of tumors using positron emission tomography (PET). Structural modifications of the non-peptidic GRP-R antagonist PD-176252 ((S)-1a) led to the identification of the fluorinated analog (S)-3-(1H-indol-3-yl)-N-[1-[5-(2-fluoroethoxy)pyridin-2-yl]cyclohexylmethyl]-2-methyl-2-[3-(4-nitrophenyl)ureido]propionamide ((S)-1m) that showed high affinity and antagonistic properties for GRP-R. This antagonist was stable in rat plasma and towards microsomal oxidative metabolism in vitro. (S)-1m was successfully radiolabeled with fluorine-18 through a conventional radiochemistry procedure. [18F](S)-1m showed high affinity and displaceable interaction for GRP-Rs in PC3 cells in vitro.

Stochastic thermodynamics of a chemical nanomachine: The channeling enzyme tryptophan synthase.

The enzyme tryptophan synthase is characterized by a complex pattern of allosteric interactions that regulate the catalytic activity of its two subunits and opening or closing of their ligand gates. As a single macromolecule, it implements 13 different reaction steps, with an intermediate product directly channeled from one subunit to another. Based on experimental data, a stochastic model for the operation of tryptophan synthase has been earlier constructed [D. Loutchko, D. Gonze, and A. S. Mikhailov, J. Phys. Chem. B 120, 2179 (2016)]. Here, this model is used to consider stochastic thermodynamics of such a chemical nanomachine. The Gibbs energy landscape of the internal molecular states is determined, the production of entropy and its flow within the enzyme are analyzed, and the information exchange between the subunits resulting from allosteric cross-regulations and channeling is discussed.