Crucial Moieties Research Articles

Synthesis, biochemical screening and in-silico investigations of arylsulfonamides bearing linear and cyclic tails

A small series of arylsulfonamide derivatives was designed and synthesized to study linear and cyclic inhibitors targeting human Carbonic Anhydrases (hCAs EC 4.2.1.1) as essential enzymes regulating (patho)-physiological processes. Particularly, the synthesis of these ten compounds was inspired to the well-known arylsulfonamides having flexible or constrained linkers able to maintain the two crucial moieties, anchoring zinc group and hydrophobic tail, in the optimized orientation within CA cavities of tumor-expressed isoforms hCA IX and hCA XII. The synthesized imine derivatives and related cyclic 1,3-thiazin-4-ones were screened in a stopped-flow carbon dioxide hydrase assay and proved to be effective inhibitors against hCA IX and hCA XII isoforms with Ki values ranging of 3.7–215.7 nM and 5.7–415.0 nM, respectively. Molecular docking studies of both series of arylsulfonamides were conducted to propose their binding mode within hCA IX and hCA XII active sites thus highlighting their distinct ability to occupy the two catalytic cavities. Moreover, the 4-[(3-cyanophenyl)methylidene]aminobenzene-1-sulfonamide 7 proved to reduce the cell viability of breast carcinoma (MCF-7) and colon rectal carcinoma (HCT-116) human cell lines under the fixed doses of 10 μM. These results encouraged us to continue our efforts in developing potent and efficient arylsulfonamides targeting hCA IX and hCA XII isoforms.

Designing weakly and strongly solvating polymer electrolytes: Systematically boosting high‐voltage lithium metal batteries

AbstractPractical high‐voltage lithium metal batteries hold promise for high energy density applications, but face stability challenges in electrolytes for both 4 V‐class cathodes and lithium anode. To address this, we delve into the positive impacts of two crucial moieties in electrolyte chemistry: fluorine atom (‐F) and cyano group (‐CN) on the electrochemical performance of polyether electrolytes and lithium metal batteries. Cyano‐bearing polyether electrolytes possess strong solvation, accelerating Li+ desolvation with minimal SEI impact. Fluorinated polyether electrolytes possess weak solvation, and stabilize the lithium anode via preferential decomposition of F‐segment, exhibiting nearly 6000‐h stable cycling of lithium symmetric cell. Furthermore, the electron‐withdrawing properties of ‐F and ‐CN groups significantly bolster the high‐voltage tolerance of copolymer electrolyte, extending its operational range up to 5 V. This advancement enables the development of 4 V‐class lithium metal batteries compatible with various cathodes, including 4.45 V LiCoO2, 4.5 V LiNi0.8Co0.1Mn0.1O2, and 4.2 V LiNi0.5Co0.2Mn0.3O2. These findings provide insights into design principles centered around polymer components for high‐performance polymer electrolytes.

Metal‐Free Electrocatalytic Diacetoxylation of Alkenes

Abstract1,2‐Dioxygenation of alkenes leads to a structural motif ubiquitous in organic synthons, natural products and active pharmaceutical ingredients. Straightforward and green synthesis protocols starting from abundant raw materials are required for facile and sustainable access to these crucial moieties. Especially industrially abundant aliphatic alkenes have proven to be arduous substrates in sustainable 1,2‐dioxygenation methods. Here, we report a highly efficient electrocatalytic diacetoxylation of alkenes under ambient conditions using a simple iodobenzene mediator and acetic acid as both the solvent and an atom‐efficient reactant. This transition metal‐free method is applicable to a wide range of alkenes, even challenging feedstock alkenes such as ethylene and propylene, with a broad functional group tolerance and excellent faradaic efficiencies up to 87 %. In addition, this protocol can be extrapolated to alkenoic acids, resulting in cyclization of the starting materials to valuable lactone derivatives. With aromatic alkenes, a competing mechanism of direct anodic oxidation exists which enables reaction under catalyst‐free conditions. The synthetic method is extensively investigated with cyclic voltammetry.

Metal-Free Electrocatalytic Diacetoxylation of Alkenes.

1,2-Dioxygenation of alkenes leads to a structural motif ubiquitous in organic synthons, natural products and active pharmaceutical ingredients. Straightforward and green synthesis protocols starting from abundant raw materials are required for facile and sustainable access to these crucial moieties. Especially industrially abundant aliphatic alkenes have proven to be arduous substrates in sustainable 1,2-dioxygenation methods. Here, we report a highly efficient electrocatalytic diacetoxylation of alkenes under ambient conditions using a simple iodobenzene mediator and acetic acid as both the solvent and an atom-efficient reactant. This transition metal-free method is applicable to a wide range of alkenes, even challenging feedstock alkenes such as ethylene and propylene, with a broad functional group tolerance and excellent faradaic efficiencies up to 87 %. In addition, this protocol can be extrapolated to alkenoic acids, resulting in cyclization of the starting materials to valuable lactone derivatives. With aromatic alkenes, a competing mechanism of direct anodic oxidation exists which enables reaction under catalyst-free conditions. The synthetic method is extensively investigated with cyclic voltammetry.

Discovery of seven-membered ring berberine analogues as highly potent and specific hCES2A inhibitors

Human carboxylesterase 2A (hCES2A) is a key serine hydrolase responsible for the metabolic clearance of large number of compounds bearing the ester- or amide-bond(s). Inhibition of hCES2A can relieve the chemotherapy-induced toxicity and alter the pharmacokinetic bahaviors of some orally administrate esters-containing agents. However, most of the hCES2A inhibitors show poor cell-membrane permeability and poor specificity. Herein, guided by the structure activity relationships (SAR) of fifteen natural alkaloids against hCES2A, fifteen new seven-membered ring berberine analogues were designed and synthesized, and their anti-hCES2A activities were evaluated. Among all tested compounds, compound 28 showed potent anti-hCES2A effect (IC50 = 1.66 μM) and excellent selectivity over hCES1A (IC50 > 100 μM). The SAR analysis revealed that the seven-membered ring of these berberine analogues was a crucial moiety for hCES2A inhibition, while the secondary amine group of the ring-C is important for improving their specificity over other serine hydrolases. Inhibition kinetic analyses and molecular dynamic simulation demonstrated that 28 strongly inhibited hCES2A in a mixed-inhibition manner, with an estimated Ki value of 1.035 μM. Moreover, 28 could inhibit intracellular hCES2A in living HepG2 cells and exhibited suitable metabolic stability. Collectively, the SAR of seven-membered ring berberine analogues as hCES2A inhibitors were studied, while compound 28 acted as a promising candidate for developing highly selective hCES2A inhibitors.

A new series of thiosemicarbazone-based anti-inflammatory agents exerting their action through cyclooxygenase inhibition.

In an endeavor to identify potent anti-inflammatory agents, new thiosemicarbazones (TSCs) incorporated into a diaryl ether framework (2a-2l) were prepared and screened for their in vitro inhibitory effects on cyclooxygenases (COXs). 4-[4-(Piperidin-1-ylsulfonyl)phenyl]-1-[4-(4-cyanophenoxy)benzylidene]thiosemicarbazide (2c) was the most potent and selective COX-1 inhibitor in this series, with an IC50 value of 1.89 ± 0.04 µM. On the other hand, 4-[4-(piperidin-1-ylsulfonyl)phenyl]-1-[4-(4-nitrophenoxy)benzylidene]thiosemicarbazide (2b) was identified as a nonselective COX inhibitor (COX-1 IC50 = 13.44 ± 0.65 µM, COX-2 IC50 = 12.60 ± 0.78 µM). Based on molecular docking studies, the diaryl ether and the TSC groups serve as crucial moieties for interactions with pivotal amino acid residues in the active sites of COXs. According to MTT test, compounds 2b and 2c showed low cytotoxic activity toward NIH/3T3 cells. Their in vivo anti-inflammatory and antioxidant potencies were also assessed using the lipopolysaccharide-induced sepsis model. Compounds 2b and 2c diminished high-sensitivity C-reactive protein, myeloperoxidase, nitric oxide, and malondialdehyde levels. Both compounds also caused a significant decrease in aspartate aminotransferase levels as well as alanine aminotransferase levels. In silico pharmacokinetic studies suggest that compounds 2b and 2c possess favorable drug-likeness and oral bioavailability. It can be concluded that these compounds may act as orally bioavailable anti-inflammatory and antioxidant agents.

Visible-Light-Induced Dual Acylation of Alkenes for the Construction of 3-Substituted Chroman-4-ones.

Heterocyclic compounds, especially oxygen-containing heterocyclic compounds, are crucial moieties in bioactive compounds and drug leads. Substituted chroman-4-ones are a kind of the most significant structural skeletons. Herein, we report a visible-light-induced dual acylation of alkenes for constructing 3-substituted chroman-4-ones, which undergoes a radical tandem cyclization reaction through carbon-carbon bond cleavage of oxime esters by a nitrogen-centered radical strategy. A series of 3-substituted chroman-4-ones were prepared with up to 86% yield.

Pharmacophore mapping of the crucial mediators of acetylcholinesterase and butyrylcholinesterase dual inhibition in Alzheimer's disease.

Optimization and re-optimization of bioactive molecules usingin silicomethods have found application in the design of more active ones. Herein, we applied a pharmacophore modeling approach to screen potent dual inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) aimed at Alzheimer's disease (AD) treatment. The investigation entails molecular dynamics simulation, docking, pharmacophore modeling, drug-like screening, and binding energy analysis. We prepared a pharmacophore model from approved inhibitors of AChE and BuChE to predict the crucial moieties required for optimum molecular interaction with these proteins. The obtained pharmacophore model, used for database screening via some critical criteria, showed 229 hit molecules. Further analyses showed 42 likely dual inhibitors of AChE/BuChE with drug-like and pharmacokinetics properties the same as the approved cholinesterase inhibitors. Finally, we identified 14 dual molecules with improved potentials over the existing inhibitors and simulated ZINC92385797 bound to human AChE and BuChE structure after noticing that these 14 molecules are similar. The selected compound maintained relative stability at the active sites of both proteins over 120ns simulation. Our integrated protocols showed the pertinent recipes of anti-AD drug design through the in silico pipeline.

Cover Feature: Oxygen‐Doped PAH Electrochromes: Difurano, Dipyrano, and Furano‐Pyrano Containing Naphthalene‐Cored Molecules (Eur. J. Org. Chem. 2/2022)

The Cover Feature shows the color changes achieved, upon oxidation, by newly synthesized electrochromic oxygen-doped small molecules. The crucial moieties are furano and pyrano. Incorporation of these groups into other organic molecules can be used to greatly expand the library of electrochromic small molecules, giving very good control over color transition observed. Such controlled color changes could take electrochromism from the niche applications it currently occupies, to the wider world. Cover design by Adriana Lopez-Biagi. More information can be found in the Full Paper by D. Bonifazi et al.

A probable means to an end: exploring P131 pharmacophoric scaffold to identify potential inhibitors of Cryptosporidium parvum inosine monophosphate dehydrogenase

Compound P131 has been established to inhibit Cryptosporidium parvum's inosine monophosphate dehydrogenase (CpIMPDH). Its inhibitory activity supersedes that of paromomycin, which is extensively used in treating cryptosporidiosis. Through the per-residue energy decomposition approach, crucial moieties of P131 were identified and subsequently adopted to create a pharmacophore model for virtual screening in the ZINC database. This search generated eight ADMET-compliant hits that were examined thoroughly to fit into the active site of CpIMPDH via molecular docking. Three compounds ZINC46542062, ZINC58646829, and ZINC89780094, with favorable docking scores of - 8.3 kcal/mol, - 8.2 kcal/mol, and - 7.5 kcal/mol, were selected. The potential inhibitory mechanism of these compounds was probed using molecular dynamics simulation and Molecular Mechanics Generalized Poisson Boltzmann Surface Area (MM/PBSA) analyses. Results revealed that one of the hits (ZINC46542062) exhibited a lower binding free energy of - 39.52 kcal/mol than P131, which had - 34.6 kcal/mol. Conformational perturbation induced by the binding of the identified hits to CpIMPDH was similar to P131, suggesting a similarity in inhibitory mechanisms. Also, in silico investigation of the properties of the hit compounds implied superior physicochemical properties with regards to their synthetic accessibility, lipophilicity, and number of hydrogen bond donors and acceptors in comparison with P131. ZINC46542062 was identified as a promising hit compound with the highest binding affinity to the target protein and favorable physicochemical and pharmacokinetic properties relative to P131. The identified compounds can serve as a basis for conducting further experimental investigations toward the development of anticryptosporidials, which can overcome the challenges of existing therapeutic options. Graphical abstract P131 and the identified compounds docked in the NAD+ binding site of Cryptosporidium parvum IMPDH.

Development of New Neurotrophic Compounds Based on Talaumidin

(-)-2S,3S,4S,5S-Talaumidin (1) exhibits potent neurotrophic activities such as neurite-outgrowth promotion and neuroprotection in primary cultured rat cortical neurons and in nerve growth factor (NGF)-differentiated PC12 cells. To examine the stereochemistry-activity relationship of 1, systematic syntheses of seven stereoisomers of 1 were accomplished via Evans aldol reaction, diastereoselective hydroboration, and Mitsunobu reaction. All stereoisomers showed moderate-to-potent neurite-outgrowth promotion in NGF-differentiated PC12 cells. In particular, (-)-2S,3R,4S,5R-1e having all-cis-substituted configuration exhibited the most significant neurite-outgrowth promotion. Subsequently, we developed a short-step synthetic route for talaumidin derivatives so as to explore new neurotrophic compounds that could be obtained on a large scale. First, we synthesized derivative 2a, which is a racemic compound of (-)-1e, and compared its neurotrophic activity with (-)-1e. It was found that the neurotrophic activity of racemic 2a was similar to that of (-)-1e. Using the same synthetic route, several talaumidin derivatives were synthesized to optimize the oxy-functionality on aromatic rings. Thereby, bis(methylenedioxybenzene) derivative 2b was found to exhibit the highest neurotrophic activity. In addition, examination of the structure-activity relationship of 2b indicated that the 2,5-diphenyl structure was crucial moiety, and the two methyl groups on the tetrahydrofuran (THF) ring could enhance the neurotrophic activity. Furthermore, 2a and 2b were found to induce mouse optic nerve regeneration in vivo.

Discovery of novel 1,2,3-triazole derivatives as anticancer agents using QSAR and in silico structural modification.

Considerable attention has been given on the search for novel anticancer drugs with respect to the disease sequelae on human health and well-being. Triazole is considered to be an attractive scaffold possessing diverse biological activities. Structural modification on the privileged structures is noted as an effective strategy towards successful design and development of novel drugs. The quantitative structure–activity relationships (QSAR) is well-known as a powerful computational tool to facilitate the discovery of potential compounds. In this study, a series of thirty-two 1,2,3-triazole derivatives (1–32) together with their experimentally measured cytotoxic activities against four cancer cell lines i.e., HuCCA-1, HepG2, A549 and MOLT-3 were used for QSAR analysis. Four QSAR models were successfully constructed with acceptable predictive performance affording RCV ranging from 0.5958 to 0.8957 and RMSECV ranging from 0.2070 to 0.4526. An additional set of 64 structurally modified triazole compounds (1A–1R, 2A–2R, 7A–7R and 8A–8R) were constructed in silico and their predicted cytotoxic activities were obtained using the constructed QSAR models. The study suggested crucial moieties and certain properties essential for potent anticancer activity and highlighted a series of promising compounds (21, 28, 32, 1P, 8G, 8N and 8Q) for further development as novel triazole-based anticancer agents.Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-015-1352-5) contains supplementary material, which is available to authorized users.

Key Structures and Interactions for Binding of Mycobacterium tuberculosis Protein Kinase B Inhibitors from Molecular Dynamics Simulation.

Substituted aminopyrimidine inhibitors have recently been introduced as antituberculosis agents. These inhibitors show impressive activity against protein kinase B, a Ser/Thr protein kinase that is essential for cell growth of M.tuberculosis. However, up to now, X-ray structures of the protein kinase B enzyme complexes with the substituted aminopyrimidine inhibitors are currently unavailable. Consequently, structural details of their binding modes are questionable, prohibiting the structural-based design of more potent protein kinase B inhibitors in the future. Here, molecular dynamics simulations, in conjunction with molecular mechanics/Poisson-Boltzmann surface area binding free-energy analysis, were employed to gain insight into the complex structures of the protein kinase B inhibitors and their binding energetics. The complex structures obtained by the molecular dynamics simulations show binding free energies in good agreement with experiment. The detailed analysis of molecular dynamics results shows that Glu93, Val95, and Leu17 are key residues responsible to the binding of the protein kinase B inhibitors. The aminopyrazole group and the pyrimidine core are the crucial moieties of substituted aminopyrimidine inhibitors for interaction with the key residues. Our results provide a structural concept that can be used as a guide for the future design of protein kinase B inhibitors with highly increased antagonistic activity.

Dissecting the structure of thiopeptides: assessment of thiazoline and tail moieties of baringolin and antibacterial activity optimization.

Several analogues of baringolin (1) were prepared to evaluate the role of its characteristic thiazoline ring and pentapeptidic tail with the aim of defining structure-activity relationships for these moieties. The thiazoline ring appeared as a crucial moiety to maintain a broad scope of activities against different Gram-positive bacteria. Further modifications were performed to simplify the structure of the natural product and assess the role of its tail, resulting in an enhanced in vitro performance. Analogue 25, with the thiazole-containing macrocycle and a 4-aminocyclohexane-1-carboxylic acid moiety in place of the pentapeptidic tail, was identified as a much more potent analogue, capable of overcoming the absence of the thiazoline ring and performing extraordinarily well against all strains tested. This is the first library of thiopeptide analogues produced by chemical synthesis alone, which demonstrates the robustness and convenience of the synthetic strategy used.

Novel Inhibitors of Human DOPA Decarboxylase Extracted fromEuonymus glabraRoxb.

Dopamine, a biogenic amine with important biological functions, is produced from l-DOPA by DOPA decarboxylase (DDC). DDC is a potential target to modulate the production of dopamine in several pathological states. Known inhibitors of DDC have been used for treatment of Parkinson's disease but suffered low specificity and diverse side effects. In the present study, we identified and characterized a novel class of natural-product-based selective inhibitors for DDC from the extract of Euonymus glabra Roxb. by a newly developed high-throughput enzyme assay. The structures of these inhibitors are dimeric diarylpropane, a unique chemical structure containing a divalent dopamine motif. The most effective inhibitors 5 and 6 have an IC50 of 11.5 ± 1.6 and 21.6 ± 2.7 μM in an in vitro purified enzyme assay, respectively, but did not inhibit other homologous enzymes. Compound 5 but not 6 dose-dependently suppressed the activity of hDDC and dopamine levels at low micromolar concentrations in cells. Furthermore, structure-activity relationship analyses revealed that p-benzoquinone might be a crucial moiety of these inhibitors for inhibiting hDDC. The natural-product-based selective inhibitors of hDDC could serve as a chemical lead for developing improved drugs for dopamine-related disease states.

Influence of ionic structure on tribological properties of poly(dimethylsiloxane–alkylene oxide) graft copolymers

Lubricants based on ionic liquid (IL) modified poly(dimethylsiloxane–alkylene oxide) graft co-polymers (GC) were synthesised by condensation of hydroxyalkyl groups linked to GC chains and to IL moieties. The tribological performances of neat and IL-modified co-polymers were investigated in reciprocating steel–steel and in rotating steel–PTFE contacts at 30 and 100°C. A clear dependence of friction and wear on temperature was observed for both neat and IL-modified co-polymers. Generally, friction decreased at higher temperature, especially pronounced for neat co-polymers. Neat as well as IL-modified co-polymers tend to increase wear at elevated temperature. At 30°C, the IL-modified co-polymers exhibited significantly improved anti-wear properties when compared to neat co-polymers with bis(trifluoromethylsulfonyl)amide anion being the crucial moiety involved in tribochemical reactions.

Recent Approaches to Novel Antibacterials Designed After LPS Structure and Biochemistry

Lipopolysaccharides (LPSs), which constitute the lipid portion of the outer leaflet of Gram-negative bacteria, are essential for growth, and are responsible for a variety of biological effects associated with Gram-negative sepsis. LPSs are amphiphilic molecules comprising three regions: lipid A, the core region, and a polysaccharide portion; the lipid A was proven to represent the toxic principle of endotoxic active lipopolysaccharides. In addition, it is known that the minimal conserved structure of LPS is the lipophylic oligoasaccharidic structure containing Kdo residues linked to the-LipA moiety. Thus, the design and development of novel antibacterial drugs can focus on different aspects, related to the biosynthesis and chemical features of LPS: 1) Inhibitors of lipid A biosynthesis 2) Inhibitors of Kdo biosynthesis. Both Kdo and Lipid A are needed for the construction of the minimum structural element Kdo2-LipidA, needed for bacterial survival. Any inhibitors acting on the biogenetic pathway of this molecule can act as antibacterial. 3) Antagonists of the interaction between endotoxins and the host receptors: LPS is recognised by the CD14 and the Toll-like receptor (TLR)-4/MD2 complex, where Lipid A is the crucial moiety in the interaction. Any drug acting as an antagonist of this process can have antisepsis potential. Considerable efforts have been made in this direction to identify natural or synthetic molecules able to interfere with the interaction between LPS and inflammatory cells. This review will highlight recent efforts in the design and biological activity of enzyme inhibitors and antagonist acting on the 3 key aspects outlined above.

Psorospermin structural requirements for P-glycoprotein resistance reversal

Resistance to chemotherapy reduces its effectiveness, resulting in increased mortality. Psorospermin, a natural product, is a topoisomerase II-directed DNA alkylating agent active against multidrug-resistant (MDR) cell lines, including multiple myeloma. In this study, the mechanism of the P-glycoprotein (P-gp) modulation activity of psorospermin and that of its associated pharmacophore were examined. Flow cytometry shows that doxorubicin-resistant multiple myeloma cells (8226/D40) pretreated with psorospermin enhance intracellular retention of doxorubicin compared with control (75% versus 38%). Because the overexpression of P-gp is the primary cause of drug resistance in the 8226/D40 cells, psorospermin-induced sensitization was likely due to mdr1/P-gp expressional or functional inhibition. As shown by PCR and Western blot, neither transcription of mdr1 nor translation of P-gp was down-regulated by psorospermin treatment. Therefore, the mechanism of psorospermin-induced resistance reversal is most likely through a direct interaction between psorospermin and P-gp. Furthermore, because only the (2'R,3'R) isomer of psorospermin showed any resistance reversal activity, the side chain of psorospermin is apparently a crucial moiety for resistance reversal. By understanding the mechanism of psorospermin-induced MDR modulation, psorospermin and similar compounds can be combined with other chemotherapies to treat resistant cancers.

An Efficient Proton-Coupled Electron-Transfer Process during Oxidation of Ferulic Acid by Horseradish Peroxidase: Coming Full Cycle

Quantum mechanics/molecular mechanics calculations were utilized to study the process of oxidation of a native substrate (ferulic acid) by the active species of horseradish peroxidase (Dunford, H. B. Heme Peroxidases; Wiley-VCH: New York, 1999), Compound I and Compound II, and the manner by which the enzyme returns to its resting state. The results match experimental findings and reveal additional novel features. The calculations demonstrate that both oxidation processes are initiated by a proton-coupled electron-transfer (PCET) step, in which the active species of the enzyme participate only as electron-transfer partners, while the entire proton-transfer event is being relayed from the substrate to and from the His42 residue by a water molecule (W402). The reason for the observed (Henriksen, A; Smith, A. T.; Gajhede, M. J. Biol. Chem. 1999, 274, 35005-35011) similar reactivities of Compound I and Compound II toward ferulic acid is that the reactive isomer of Compound II is the, hitherto unobserved, Por(*)(+)Fe(III)OH isomer that resembles Compound I. The PCET mechanism reveals that His42 and W402 are crucial moieties and they determine the function of the HRP enzyme and account for its ability to perform substrate oxidation (Poulos, T. L. Peroxidases and Cytochrome P450. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 4, pp 189). In view of the results, the possibility of manipulating substrate oxidation by magnetic fields is an intriguing possibility.

Structure−Activity Study of Brassinin Derivatives as Indoleamine 2,3-Dioxygenase Inhibitors

A screen of indole-based structures revealed the natural product brassinin to be a moderate inhibitor of indoleamine 2,3-dioxygenase (IDO), a new cancer immunosuppression target. A structure-activity study was undertaken to determine which elements of the brassinin structure could be modified to enhance potency. Three important discoveries have been made, which will impact future IDO inhibitor development: (i) The dithiocarbamate portion of the brassinin lead is a crucial moiety, which may be binding to the heme iron of IDO; (ii) an indole ring is not necessary for IDO inhibition; and (iii) substitution of the S-methyl group of brassinin with large aromatic groups provides inhibitors that are three times more potent in vitro than the most commonly used IDO inhibitor, 1-methyl-tryptophan.