Photosensitized oxygenation of Nb-methoxycarbonyltryptamines. A new pathway to kynurenine derivatives

Bioinspired Total Synthesis and Structural Reidentification of Alstrostines

Alstrostine A and isoalstrostine A are monoterpenoid indole alkaloid glycosides with unique structures found in the plant families Apocynaceae and Rubiaceae. With molecular weights exceeding 900, nine chiral centers (excluding sugar rings), and complex fused-ring structures, the structural elucidation of these molecules using spectral analysis is highly challenging. Therefore, their structural identification through total synthesis is important in both natural product chemistry and synthetic organic chemistry. In this study, we successfully accomplished the first asymmetric total syntheses of these alkaloids in 18 or 19 steps. A key synthetic feature was a two- or three-component coupling reaction between secologanin and a pyrrolidinoindoline moiety based on our proposed biosynthetic pathway. This approach enabled the synthesis of all isomers of the pyrrolidinoindoline ring, which constitutes the upper fragment of the alstrostines, and allowed us to revise the stereochemistry of alstrostine A. Additionally, a compound previously isolated as alstrostine A from Palicourea luxurians (Rubiaceae) was successfully reidentified and renamed as epialstrostine A.

Strategies for total synthesis of bispyrrolidinoindoline alkaloids

Covering up to 2021Complex cyclotryptamine alkaloids with a bispyrrolidino[2,3-b]indoline (BPI) skeleton are an intriguing family of natural products, exhibiting wide systematic occurrences, large structural diversity, and multiple biological activities. Based on their structural characteristics, BPI alkaloids can be classified into chimonanthine-type BPI alkaloids, BPI diketopiperazines, and BPI epipolythiodiketopiperazines. These intricate molecules have captivated great attention soon after their isolation and identification in the 1960s. Due to the structural complexity, the total synthesis of these cyclotryptamine alkaloids is challenging. Nevertheless, remarkable progress has been achieved in the last six decades; in particular, several methods have been successfully established for the construction of vicinal all-carbon quaternary stereocenters. In this review, the structural diversity and chemical synthesis of these BPI alkaloids were summarized. BPI alkaloids are mainly synthesized by the methods of oxidative dimerization, reductive dimerization, and alkylation of bisoxindole. The purpose of this review is to present overall strategies for assembling the BPI skeleton and efforts towards controlling the stereocenters.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Superoxide radicals react with peptide-derived tryptophan radicals with very high rate constants to give hydroperoxides as major products

Oxidative damage is a common process in many biological systems and proteins are major targets for damage due to their high abundance and very high rate constants for reaction with many oxidants (both radicals and two-electron species). Tryptophan (Trp) residues on peptides and proteins are a major sink for a large range of biological oxidants as these side-chains have low radical reduction potentials. The resulting Trp-derived indolyl radicals (Trp^•) have long lifetimes in some circumstances due to their delocalized structures, and undergo only slow reaction with molecular oxygen, unlike most other biological radicals. In contrast, we have shown previously that Trp^• undergo rapid dimerization. In the current study, we show that Trp^• also undergo very fast reaction with superoxide radicals, O₂^•-, with k 1-2 × 10⁹ M^-1 s^-1. These values do not alter dramatically with peptide structure, but the values of k correlate with overall peptide positive charge, consistent with positive electrostatic interactions. These reactions compete favourably with Trp^• dimerization and O₂ addition, indicating that this may be a major fate in some circumstances. The Trp^• + O₂^•- reactions occur primarily by addition, rather than electron transfer, with this resulting in high yields of Trp-derived hydroperoxides. Subsequent degradation of these species, both stimulated and native decay, gives rise to N-formylkynurenine, kynurenine, alcohols and diols. These data indicate that reaction of O₂^•- with Trp^• should be considered as a major pathway to Trp degradation on peptides and proteins subjected to oxidative damage.

Formation and detection of oxidant-generated tryptophan dimers in peptides and proteins

Free radicals are produced during physiological processes including metabolism and the immune response, as well as on exposure to multiple external stimuli. Many radicals react rapidly with proteins resulting in side-chain modification, backbone fragmentation, aggregation, and changes in structure and function. Due to its low oxidation potential, the indole ring of tryptophan (Trp) is a major target, with this resulting in the formation of indolyl radicals (Trp^•). These undergo multiple reactions including ring opening and dimerization which can result in protein aggregation. The factors that govern Trp^• dimerization, the rate constants for these reactions and the exact nature of the products are not fully elucidated. In this study, second-order rate constants were determined for Trp^• dimerization in Trp-containing peptides to be 2-6 × 10⁸M^-1s^-1 by pulse radiolysis. Peptide charge and molecular mass correlated negatively with these rate constants. Exposure of Trp-containing peptides to steady-state radiolysis in the presence of NaN₃ resulted in consumption of the parent peptide, and detection by LC-MS of up to 4 different isomeric Trp-Trp cross-links. Similar species were detected with other oxidants, including CO₃^•- (from the HCO₃^- -dependent peroxidase activity of bovine superoxide dismutase) and peroxynitrous acid (ONOOH) in the presence or absence of HCO₃^-. Trp-Trp species were also isolated and detected after alkaline hydrolysis of the oxidized peptides and proteins. These studies demonstrate that Trp^• formed on peptides and proteins undergo rapid recombination reactions to form Trp-Trp cross-linked species. These products may serve as markers of radical-mediated protein damage, and represent an additional pathway to protein aggregation in cellular dysfunction and disease.

Ab-initio HF and density functional theory investigations on the synthesis mechanism, conformational stability, molecular structure and UV spectrum of N’-Formylkynurenine

Tryptophan methyl ester (Trp-ME) degrades with singlet oxygen and produce compounds which are photosensitizers and may react to form other derivatives such as N’-Formylkynurénine (NFK) and kynurenine, which are the final products of this oxidation. In order to study and optimize the molecular structure of NFK and determine its different thermodynamic properties, we performed a conformational analysis by DFT/B3LYP method with 3-21G basis set. Six most stable conformations were observed through the analysis of the potential energy surfaces, obtained by a relaxed scan of the dihedral angles.

The most stable form of NFK has been registered for D[Formula: see text], D[Formula: see text], D[Formula: see text], D[Formula: see text], D[Formula: see text], and D[Formula: see text]. The study was conducted by HF and DFT/B3LYP with 6-31G(d,p), 6-3[Formula: see text](d,p) and 6-31[Formula: see text](d,p) basis sets, on the optimized geometry of the most stable conformation and its thermodynamic and orbital properties. Two absorption bands were recorded at [Formula: see text][Formula: see text]nm and at [Formula: see text][Formula: see text]nm and were also determined by TD-DFT method. They showed good agreement with the UV experimental spectrum which confirms that it is a powerful tool to determine the dynamic and static properties of molecules. The surface of the electrostatic potential (ESP) of the NFK was also analyzed.

Ketoprofen-induced formation of amino acid photoadducts: possible explanation for photocontact allergy to ketoprofen

Photocontact allergy is a well-known side effect of topical preparations of the nonsteroidal anti-inflammatory drug ketoprofen. Photocontact allergy to ketoprofen appears to induce a large number of photocross allergies to both structurally similar and structurally unrelated compounds. Contact and photocontact allergies are explained by structural modification of skin proteins by the allergen. This complex is recognized by the immune system, which initiates an immune response. We have studied ketoprofen's interaction with amino acids to better understand ketoprofen's photoallergenic ability. Irradiation of ketoprofen and amino acid analogues resulted in four different ketoprofen photodecarboxylation products (6-9) together with a fifth photoproduct (5). Dihydroquinazoline 5 was shown to be a reaction product between the indole moiety of 3-methylindole (Trp analogue) and the primary amine benzylamine (Lys analogue). In presence of air, dihydroquinazoline 5 quickly degrades into stable quinazolinone 12. The corresponding quinazolinone (17) was formed upon irradiation of ketoprofen and the amino acids N-acetyl-l-Trp ethyl ester and l-Lys ethyl ester. The formation of these models of an immunogenic complex starts with the ketoprofen-sensitized formation of singlet oxygen, which reacts with the indole moiety of Trp. The formed intermediate subsequently reacts with the primary amino functionality of Lys, or its analogue, to form a Trp-Lys adduct or a mimic thereof. The formation of a specific immunogenic complex that does not contain the allergen but that can still induce photocontact allergy would explain the large number of photocross allergies with ketoprofen. These allergens do not have to be structurally similar as long as they can generate singlet oxygen. To the best of our knowledge, there is no other suggested explanation for ketoprofen's photoallergenic properties that can account for the observed photocross allergies. The formation of a specific immunogenic complex that does not contain the allergen is a novel hypothesis in the field of contact and photocontact allergy.

Synthesis, DNA binding and antitumor evaluation of styelsamine and cystodytin analogues

A series of N-14 sidechain substituted analogues of styelsamine (pyrido[4,3,2-mn]acridine) and cystodytin (pyrido[4,3,2-mn]acridin-4-one) alkaloids have been prepared and evaluated for their DNA binding affinity and antiproliferative activity towards a panel of human tumor cell lines. Overall it was found that styelsamine analogues were stronger DNA binders, with the natural products styelsamines B and D having particularly high affinity (K_app 5.33 × 10⁶ and 3.64 × 10⁶ M⁻¹, respectively). In comparison, the cystodytin iminoquinone alkaloids showed lower affinity for DNA, but were typically just as active as styelsamine analogues at inhibiting proliferation of tumor cells in vitro. Sub-panel selectivity towards non-small cell lung, melanoma and renal cancer cell lines were observed for a number of the analogues. Correlation was observed between whole cell activity and clogP, with the most potent antiproliferative activity being observed for 3-phenylpropanamide analogues 37 and 41 (NCI panel average GI₅₀ 0.4 μM and 0.32 μM, respectively) with clogP ~4.0–4.5.

A valid model for the mechanism of oxidation of tryptophan to formylkynurenine-25 years later

The dye-sensitized photooxygenation of DL-tryptophan in aqueous solution leads to the tricyclic compound 2-carboxy-3a-hydroperoxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole which, on reduction with dimethyl sulfide, furnishes two diastereoisomeric alcohols separable by fractional crystallization into a higher melting (mp 254 degrees -256 degrees ) and a lower melting (mp 228 degrees ) diastereoisomer. Each of these alcohols was correlated with one of the analogous pair of isomeric 1,2-dicarbomethoxy analogs by alkaline hydrolysis and by x-ray analysis. In this way, the 3a-hydroxy-1,2-dimethoxycarbonyl- 1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole, mp 163 degrees -164 degrees , was shown to have the trans configuration with regard to the relative positions of the hydroxyl and carbomethoxy groups and that, on alkaline hydrolysis, it produced the isomer with mp 228 degrees , which therefore also has the trans configuration. The mechanism of the smooth thermal rearrangement of the (presumably ring-chain tautomeric) tryptophan hydroperoxy intermediates to formylkynurenine is discussed with its implications for the biological oxidation by tryptophan 2,3-dioxygenase.

Scalable total syntheses of N-linked tryptamine dimers by direct indole-aniline coupling: psychotrimine and kapakahines B and F

This report details the invention of a method to enable syntheses of psychotrimine (1) and the kapakahines F and B (2, 3) on a gram scale and in a minimum number of steps. Mechanistic inquiries are presented for the key enabling quaternization of indole at the C3 position by electrophilic attack of an activated aniline species. Excellent chemo-, regio-, and diastereoselectivities are observed for reactions with o-iodoaniline, an indole cation equivalent. Additionally, the scope of this reaction is broad with respect to the tryptamine and aniline components. The anti-cancer profiles of 1-3 have also been evaluated.

Singlet-oxygen-mediated amino acid and protein oxidation: formation of tryptophan peroxides and decomposition products

Proteins are major biological targets for oxidative damage within cells owing to their high abundance and rapid rates of reaction with radicals and excited-state species, including singlet oxygen. Reaction of Tyr, Trp, and His residues, both free and on proteins, with singlet oxygen generates peroxides in high yield. Peroxides have also been detected on proteins within intact cells on exposure to visible light in the presence of a photosensitizer. The structures of some of these materials have been elucidated for free amino acids, but less is known about peptide- and protein-bound species. In this study we have characterized Trp-derived peroxides, radicals, and breakdown products generated on free Trp and Trp residues in peptides and proteins, using LC/MS/MS. With free Trp, seven major photoproducts were characterized, including two isomeric hydroperoxides, two alcohols, two diols, and N-formylkynurenine, consistent with singlet oxygen-mediated reactions. The hydroperoxides decompose rapidly at elevated temperatures and in the presence of reductants to the corresponding alcohols. Some of these materials were detected on proteins after complete enzymatic (Pronase) hydrolysis and LC/MS/MS quantification, providing direct evidence for peroxide formation on proteins. This approach may allow the quantification of protein modification in intact cells arising from singlet oxygen formation.

Studies of the electron-promoted cope cyclization of 2,5-phenyl-substituted 1,5-hexadiene radical anions

This work describes studies of the electron-promoted Cope cyclization of 2,5-phenyl-1,5-hexadiene radical anions in a flowing afterglow triple quadrupole mass spectrometer. The electronic properties of the hexadienes have been systematically modified by using aromatic substituents at the 2- and 5-positions of the hexedienes, including those with nitro, trifluoromethyl, fluoro, chloro, and acetyl groups. Ions were formed by the thermal attachment of electrons in the gas phase. Structures of the molecular radical anions were probed to determine whether they undergo cyclization to a cyclohexane-1,4-diyl anion structure by examining chemical reactivity with neutral reagents including carbon dioxide, carbon disulfide, and nitric oxide. First-order rate constants for the reactions of ions were measured, and the reaction efficiencies were determined. Based on the reactivity results, a thermochemical model has been developed, which predicts the reaction thermochemistry by using thermochemical properties of model systems. The observed reactivity from ion-molecule reactions and the study of reaction rates show that the ion of 2,5-dicyanohexadiene and 2,5-di(4,4'-trifluoromethyl phenyl)-1,5-hexadiene undergo Cope cyclization, whereas the radical anions having substituents such as the fluoro, nitro, chloro, and acetyl groups do not.

Singlet oxygen-mediated damage to proteins and its consequences

Proteins comprise approximately 68% of the dry weight of cells and tissues and are therefore potentially major targets for oxidative damage. Two major types of processes can occur during the exposure of proteins to UV or visible light. The first of these involves direct photo-oxidation arising from the absorption of UV radiation by the protein, or bound chromophore groups, thereby generating excited states (singlet or triplets) or radicals via photo-ionisation. The second major process involves indirect oxidation of the protein via the formation and subsequent reactions of singlet oxygen generated by the transfer of energy to ground state (triplet) molecular oxygen by either protein-bound, or other, chromophores. Singlet oxygen can also be generated by a range of other enzymatic and non-enzymatic reactions including processes mediated by heme proteins, lipoxygenases, and activated leukocytes, as well as radical termination reactions. This paper reviews the data available on singlet oxygen-mediated protein oxidation and concentrates primarily on the mechanisms by which this excited state species brings about changes to both the side-chains and backbone of amino acids, peptides, and proteins. Recent work on the identification of reactive peroxide intermediates formed on Tyr, His, and Trp residues is discussed. These peroxides may be important propagating species in protein oxidation as they can initiate further oxidation via both radical and non-radical reactions. Such processes can result in the transmittal of damage to other biological targets, and may play a significant role in bystander damage, or dark reactions, in systems where proteins are subjected to oxidation.

Photo-oxidation of proteins and its role in cataractogenesis

Proteins comprise approximately 68% of the dry weight of cells and tissues and are therefore potentially major targets for photo-oxidation. Two major types of processes can occur with proteins. The first of these involves direct photo-oxidation arising from the absorption of UV radiation by the protein, or bound chromophore groups, thereby generating excited states (singlet or triplets) or radicals via photo-ionisation. The second major process involves indirect oxidation of the protein via the formation and subsequent reactions of singlet oxygen generated by the transfer of energy to ground state (triplet) molecular oxygen by either protein-bound, or other, chromophores. The basic principles behind these mechanisms of photo-oxidation of amino acids, peptides and proteins and the potential selectivity of damage are discussed. Emphasis is placed primarily on the intermediates that are generated on amino acids and proteins, and the subsequent reactions of these species, and not the identity or chemistry of the sensitizer itself, unless the sensitizing group is itself intrinsic to the protein. A particular system is then discussed--the cataractous lens--where UV photo-oxidation may play a role in the aetiology of the disease, and tryptophan-derived metabolites act as UV filters.

Metabolism of analogs of the phytoalexin brassinin by plant pathogenic fungi

he metabolism of analogs of the cruciferous phytoalexin brassinin by the phytopathogenic fungi Phoma lingam (Tode ex Fr.) "virulent" and "avirulent" groups (sexual stage Leptosphaeria maculans (Desm.) Ces. et de Not.) and Alternaria brassicae (Berk.) Sacc. is reported. It was established that each pathogen detoxified methyl tryptamine dithiocarbamate, although yielding different metabolic products. While the biotransformation by virulent P. lingam proceeded to yield methyl 3a-hydroxy-3,3a,8,8a-tetrahydropyrrolo[2,3-b]indol-1(2H)-yl carbodithioate and indole-3-acetic acid as final products, avirulent P. lingam yielded indole-3-carboxylic acid via Nb-acetyltryptamine, and A. brassicae yielded Nb-acetyltryptamine, as final products. Furthermore, to establish the importance of the dithiocarbamate group in antifungal activity against P. lingam and A. brassicae, carbamates were compared with dithiocarbamates and it was established that carbamates were devoid of antifungal activity. These products, contrary to methyl tryptamine dithiocarbamate, showed no inhibitory activity against either pathogen.Key words: Alternaria brassicae, brassinin, carbamates, detoxification, dithiocarbamates, Leptosphaeria maculans, Phoma lingam, phytoalexins.

Metabolism studies of indole derivatives using a staurosporine producer, Streptomyces staurosporeus

From a tryptophan metabolic study, 3-(hydroxyacetyl)indole, indole-3-carboxaldehyde, and o-aminobenzoic acid were obtained as tryptophan metabolites from a staurosporine (1) producer, Streptomyces staurosporeus. A new tryptamine metabolite, (3aR,8aS)-1-acetyl-1,2,3,3a,8,8a-hexahydropyrrolo[2,3- b]indol-3a-ol (2), was isolated along with Nb-acetyltryptamine using S. staurosporeus fed with tryptamine. The unusual compound 2 was proposed as a further metabolite of Nb-acetyltryptamine through an enzymatic oxidative cyclization. Examination of the metabolites from the feeding of 5- and 6-fluorotryptamines using the same microorganism afforded the two novel compounds 3 and 4 as the 5- and 6-fluoro derivatives of 2. However, 5-hydroxytryptamine failed to generate the 5-hydroxy derivative of 2. Indole-ring-substituted metabolites of staurosporine (1) were not observed.

Formation of diastereoisomeric 3a-hydroxypyrroloindoles from a tryptophan residue analog mediated by iron (II)-EDTA and L-ascorbate

Oxygenation of a tryptophan residue analog by ascorbate in the presence of catalytic amounts of iron(II) and ethylenediaminetetraacetic acid (EDTA) has been studied. Under physiological conditions, reaction of the tryptophan derivative (N-t-butoxycarbonyl-L-tryptophan) with Fe(II)-EDTA and ascorbate resulted mainly in the oxygenation of the indole moiety of the substrate. In this reaction, cis and trans diastereoisomeric alcohols 3a-hydroxy-1-t-butoxycarbonyl-1,2,3,3a,8,8a-hexahydropyrrolo[2,3- b]indoles have been successfully identified in the metal-catalyzed free radical oxidation of indole compounds. Hydroxylation at C-5 and C-6 and a ring opening reaction between C-2 and C-3 have also been confirmed. The reaction of Fe(II)-EDTA/ascorbate with the tryptophan derivative was apparently nonselective with regard to position and was significantly suppressed by the hydroxyl radical scavengers (mannitol and dimethylsulfoxide), suggesting the participation of the hydroxyl radical as the actual oxidizing species.