3-hydroxyl Group Research Articles

Recent Advances on 3-Hydroxyflavone Derivatives: Structures and Properties.

Flavonoids are naturally occurring phenolic compounds named as 2-phenyl-1-benzopyran- 4-one and classified into several subclasses according to their basic chemical structures such as flavones, flavonols, flavanones, flavanonols, and flavanols. Among them, flavonols (3-hydroxyflavone), containing a 3-hydroxyl group in association with its 4-keto group and double bond between 2- and 3- carbon atoms, have the best electron conjugated skeleton in flavonoid family, providing several resonance structures and chelating ability, through which various pharmacological activities such as antiviral, antitumour, anti-inflammatory, anticholinesterase, cytotoxicity and particularly high antioxidant activity are demonstrated. They prevent generation of free radicals and are used as food preservatives. Their strong electron conjugation provides them with fluorescent property, which make them used as imaging agents for therapeutic purpose.

Discrimination of Stereoisomers by Their Enantioselective Interactions with Chiral Cholesterol-Containing Membranes.

Discrimination between enantiomers is an important subject in medicinal and biological chemistry because they exhibit markedly different bioactivity and toxicity. Although stereoisomers should vary in the mechanistic interactions with chiral targets, their discrimination associated with the mode of action on membrane lipids is scarce. The aim of this study is to reveal whether enantiomers selectively act on chiral lipid membranes. Different classes of stereoisomers were subjected at 5–200 μM to reactions with biomimetic phospholipid membranes containing ~40 mol % cholesterol to endow the lipid bilayers with chirality and their membrane interactions were comparatively evaluated by measuring fluorescence polarization. All of the tested compounds interacted with cholesterol-containing membranes to modify their physicochemical property with different potencies between enantiomers, correlating to those of their experimental and clinical effects. The rank order of membrane interactivity was reversed by changing cholesterol to C3-epimeric α-cholesterol. The same selectivity was also obtained from membranes prepared with 5α-cholestan-3β-ol and 5β-cholestan-3α-ol diastereomers. The opposite configuration allows molecules to interact with chiral sterol-containing membranes enantioselectively, and the specific β configuration of cholesterol’s 3-hydroxyl group is responsible for such selectivity. The enantioselective membrane interaction has medicinal implications for the characterization of the stereostructures with higher bioactivity and lower toxicity.

A Novel Steroid-Coenzyme A Ligase from Novosphingobium sp. Strain Chol11 Is Essential for an Alternative Degradation Pathway for Bile Salts.

Bile salts such as cholate are steroid compounds with a C5 carboxylic side chain and occur ubiquitously in vertebrates. Upon their excretion into soils and waters, bile salts can serve as growth substrates for diverse bacteria. Novosphingobium sp. strain Chol11 degrades 7-hydroxy bile salts via 3-keto-7-deoxy-Δ4,6 metabolites by the dehydration of the 7-hydroxyl group catalyzed by the 7α-hydroxysteroid dehydratase Hsh2. This reaction has not been observed in the well-studied 9-10-seco degradation pathway used by other steroid-degrading bacteria indicating that strain Chol11 uses an alternative pathway. A reciprocal BLASTp analysis showed that known side chain degradation genes from other cholate-degrading bacteria (Pseudomonas stutzeri Chol1, Comamonas testosteroni CNB-2, and Rhodococcus jostii RHA1) were not found in the genome of strain Chol11. The characterization of a transposon mutant of strain Chol11 showing altered growth with cholate identified a novel steroid-24-oyl-coenzyme A ligase named SclA. The unmarked deletion of sclA resulted in a strong growth rate decrease with cholate, while growth with steroids with C3 side chains or without side chains was not affected. Intermediates with a 7-deoxy-3-keto-Δ4,6 structure, such as 3,12-dioxo-4,6-choldienoic acid (DOCDA), were shown to be likely physiological substrates of SclA. Furthermore, a novel coenzyme A (CoA)-dependent DOCDA degradation metabolite with an additional double bond in the side chain was identified. These results support the hypothesis that Novosphingobium sp. strain Chol11 harbors an alternative pathway for cholate degradation, in which side chain degradation is initiated by the CoA ligase SclA and proceeds via reaction steps catalyzed by so-far-unknown enzymes different from those of other steroid-degrading bacteria.IMPORTANCE This study provides further evidence of the diversity of metabolic pathways for the degradation of steroid compounds in environmental bacteria. The knowledge about these pathways contributes to the understanding of the CO2-releasing part of the global C cycle. Furthermore, it is useful for investigating the fate of pharmaceutical steroids in the environment, some of which may act as endocrine disruptors.

Characterization of naturally occurring pentacyclic triterpenes as novel inhibitors of deubiquitinating protease USP7 with anticancer activity in vitro.

Deubiquitinating protease USP7 is a promising therapeutic target for cancer treatment, and interest in developing USP7 inhibitors has greatly increased. In the present study, we reported a series of natural pentacyclic triterpenes with USP7 inhibitory activity in vitro. Among them, both the ursane triterpenes and oleanane triterpenes were more active than the lupine triterpenes, whereas ursolic acid was the most potent with IC50 of 7.0±1.5 μmol/L. Molecular docking studies showed that ursolic acid might occupy the ubiquitin binding pocket of USP7, with the 17-carboxyl group and 3-hydroxyl group playing a vital role in the USP7-ursolic acid interaction. Using the cellular thermal shift assay, we demonstrated that ursolic acid interacted with USP7 in RPMI8226 human myeloma cells. Ursolic acid dose-dependently inhibited the proliferation of the myeloma cells with IC50 of 6.56 μmol/L, accompanied by reductions in USP7 substrates such as MDM2, UHRF1 and DNMT1. Overexpression of USP7 partially, but significantly attenuated ursolic acid-induced cell death as well as downregulation of MDM2, UHRF1 and DNMT1. In conclusion, we demonstrate for the first time that pentacyclic triterpenes represent a novel scaffold for developing USP7 inhibitors and that USP7 inhibition contributes to the anti-cancer effect of ursolic acid.

2,4,5-Trihydroxy-3-methylacetophenone: A Cellulosic Chromophore as a Case Study of Aromaticity.

The title compound (2,4,5-trihydroxy-3-methylacetophenone, 1) was isolated as chromophore from aged cellulosic pulps. The peculiar feature of the compound is its weak aromatic system that can be converted into nonaromatic (quinoid or cyclic aliphatic) tautomers, depending on the conditions and reaction partners. In alkaline media, the participation of quinoid canonic forms weakens aromaticity, whereas in neutral and acidic media, the strong hydrogen bond between the 2-hydroxyl group and the acetyl moiety plays an important role in favoring quinoid tautomers. As a result, compound 1, with quinoid contributions being already “preset”, is relatively stable toward oxidation and hardly undergoes alkylation or nitration at CH-6, whereas the 2,4,5-trimethoxyderivative, being “properly” aromatic and even more sterically hindered, is readily alkylated or nitrated. The lability of the aromatic system is best demonstrated by the unusual reaction of 1 with hydroxylamine, producing a tetroxime that is derived from its 2,4,5-triketo tautomer. The high oxidative stability and low reactivity of the compound hinder oxidative bleaching of this chromophore in cellulosic pulps and detection reactions for analytical purposes.

Synthesis and cytotoxicity against KB and NCI-H187 cell lines of sporogen AO-1 analogues

20 Analogues of sporogen AO-1 were synthesized by chemical modification at α,β-unsaturated carbonyl, 3-hydroxyl and vinylic methyl groups of sporogen AO-1 precursor, and were evaluated for their cytotoxic activities against human oral epidermoid carcinoma (KB) and human small cell lung (NCI-H187) cancer cell lines. Structure-activity relationship study indicated the importance of α,β-unsaturated carbonyl moiety for both cancer cell lines. Vinylic methyl and R-configuration of 3-hydroxyl group were crucial for cytotoxicity toward KB cells. In contrast, conversion of vinylic methyl and 3-hydroxyl groups to ketone moieties afforded triketone 19 which displayed comparable cytotoxicity against NCI-H187 cells lines to sporogen AO-1, and was more potent than ellipticine, a standard drug. Interestingly, compound 19 was weakly cytotoxic toward Vero cells, whereas sporogen AO-1 showed strong cytotoxicity.

Pyrolytic behavior of lignin-related α-O-4 contained model compound with addition of methanol

Pyrolysis of lignin-related α-O-4 contained model compound, benzyl phenyl ether (BPE, C6H5CH2OC6H5) was conducted in a fixed-bed reactor. The compounds in liquid product from the pyrolysis process were analyzed by GC/FID and GC/MS. Compared to the other three alcohols (CH3CH2OH, CH3CH2CH2OH and CH3CHOHCH3), methanol was estimated as the optimal hydrogen-donor reagent for the co-pyrolysis with BPE. The yield of the monomeric aromatic compounds was dramatically increased with the addition of methanol to BPE during the co-pyrolysis process (an increment of 29.70wt.% for BPE with mass concentration of 0.06g/mL in methanol). An isotopic tracing method (CH3OD as the tracer) was employed for understanding the role of hydrogen-donor reagent on the cracking of the α-O-4 linkage. Hydroxylic hydrogen in methanol participated in the formation of a certain percentage of monomeric aromatics (except benzene) for the co-pyrolysis process of BPE and methanol. Benzyl (C6H5CH2) and phenoxy (C6H5O) radicals can be produced from the initial pyrolytic cracking of BPE, while the hydrogen from hydroxylic group in methanol attracted by phenoxy radical is predominant over that of benzyl radical.

Energetic and electronic computation of the two-hydrogen atom donation process in catecholic and non-catecholic anthocyanidins

Antioxidant activity of anthocyanidins is greatly affected by the 3-hydroxyl group and/or a catecholic moiety. The two-hydrogen atom donation process is frequently used to explain the high antioxidant activity of polyphenolic compounds leading to the formation of stable diketones e.g. 1,2-quinones. Thermodynamic parameters, HOMO and spin density were computed to identify the favoured path, either through the 3-hydroxyl group or through the catecholic moiety in a series of catecholic and non-catecholic 3-oxy- (and deoxy)-anthocyanidins. DFT calculations showed that the donation process in non-catecholic anthocyanidins depended on the substituents on ring B. Anthocyanidins with 3′,5′-diOMe groups showed donation through 3,4′-OH or, otherwise, through 3,5-OH groups. Catecholic 3-oxyanthocyanidins, on the other hand, showed donation through the 3,4′-OH path rather than the catecholic path (4′,3′-path). The 3,4′-path was favoured by the formation of planar 3-radicals in the first step and the stabilization of 4′-radicals in the second step by H-bonding with the 3′-OH group.

Attempt to simultaneously generate three chiral centers in 4-hydroxyisoleucine with microbial carbonyl reductases

A panel of microorganisms was screened for selective reduction ability towards a racemic mixture of prochiral 2-amino-3-methyl-4-ketopentanoate (rac-AMKP). Several of the microorganisms tested produced greater than 0.5mM 4-hydroxyisoleucine (HIL) from rac-AMKP, and the stereoselectivity of HIL formation was found to depend on the taxonomic category to which the microorganism belonged. The enzymes responsible for the AMKP-reducing activity, ApAR and FsAR, were identified from two of these microorganisms, Aureobasidium pullulans NBRC 4466 and Fusarium solani TG-2, respectively. Three AMKP reducing enzymes, ApAR, FsAR, and the previously reported BtHILDH, were reacted with rac-AMKP, and each enzyme selectively produced a specific composition of HIL stereoisomers. The enzymes appeared to have different characteristics in recognition of the stereostructure of the substrate AMKP and in control of the 4-hydroxyl group configuration in the HIL product.

Effect of Ca2+ to Sphingomyelin Investigated by Sum Frequency Generation Vibrational Spectroscopy

The interactions between Ca2+ ions and sphingomyelin play crucial roles in a wide range of cellular activities. However, little is known about the molecular details of the interactions at interfaces. In this work, we investigated the interactions between Ca2+ ions and egg sphingomyelin (ESM) Langmuir monolayers at the air/water interface by subwavenumber high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS). We show that Ca2+ ions can induce ordering of the acyl chains in the ESM monolayer. An analysis of the one alkyl-chain-deuterated ESM revealed that the Ca2+ ions do not affect the N-linked saturated fatty acid chain, although they make the sphingosine backbone become ordered. Further analysis of the SFG-VS spectra shows that the interactions between ESM and Ca2+ ions make the orientation of the methyl group at the end of sphingosine backbone change from pointing downward to pointing upward. Moreover, a large blue shift of the phosphate group at the CaCl2 solution interface indicates, to our knowledge, new cation binding modes. Such binding causes the phosphate moiety to dehydrate, resulting in the conformation change of the phosphate moiety. Based on these results, we propose the molecular mechanism that Ca2+ ions can bind to the phosphate group and subsequently destroy the intramolecular hydrogen bond between the 3-hydroxyl group and the phosphate oxygen, which results in an ordering change of the sphingosine backbone. These findings illustrate the potential application of HR-BB-SFG-VS to investigate lipid-cation interactions and the calcium channel modulated by lipid domain formation through slight structural changes in the membrane lipid. It will also shed light on the interactions of complex molecules at surfaces and interfaces.

Synthesis of paricalcitol: A novel strategy combining chemical and microbial transformations

A novel strategy was developed for the preparation of paricalcitol (1α,25-dihydroxy-19-norvitamin D2) via seven-step chemical transformation and one-step microbial transformation. Using vitamin D2 as a starting material, 1α- and 25-hydroxyl groups of paricalcitol were introduced by Pseudonocardia autotrophica CGMCC5098 at the late stage of the synthesis, allowing us to avoid harsh reaction conditions and tedious steps. The overall yield of paricalcitol was 0.75%.

Acid-Base Catalysis in Glycosidations: A Nature Derived Alternative to the Generally Employed Methodology.

Inverting glycosyltransferases enforce in the active site an intramolecular, acid-base catalyzed glycosidation that, due to proximity of the donor anomeric carbon and the acceptor hydroxyl group, follows an SN2-type reaction. Spacers, tethering donor and acceptor via nonreacting functional groups, led in intramolecular glycosidations to excellent yields and, independent of the donor anomeric configuration, to either the α- or the β-anomer. The requirement of a demanding protecting group pattern confines the application of this efficient method. Only the method where the 2-hydroxyl group of a mannopyranosyl donor is tethered via an acetal spacer to the reacting acceptor functional group is used for β-mannopyranoside synthesis. The most elegant method, tethering donor and acceptor covalently to the spacer via the leaving group and the reacting functional group, was so far not as efficient as hoped. This method is very efficient when donor and acceptor are temporarily assembled through a hydrogen-bond facilitating a stretched hexagon-like transition state. This follows from the stereoselective O-glucopyranosyl trichloroacetimidate transformation into O-glucopyranosyl phosphate with dibenzyl phosphoric acid as acceptor that can be regarded as A═B-C-H acceptor type. Generalizing this concept to the use of alcohols as acceptors requires reversible generation of an A-B-C-H adduct where A-H represents the acceptor (RO-H) and B═C a catalyst that has to fulfill several criteria. Among these criteria are low affinity to nitrogen, avoiding glycosyl donor activation in the absence of acceptor, and high affinity to oxygen in order to generate the A-B-C-H adduct with increased proton acidity. Thus, hydrogen-bond mediated self-assembly of donor and acceptor and concomitant donor activation via a transition state is available, which enforces an acid-base catalyzed SN2-type reaction. It could be shown that PhBF2, Ph2BF, and PhSiF3 are such catalysts that fulfill the desired four functions: reversible adduct formation with the acceptor, hydrogen-bond mediated tethering of this adduct with the donor, and acid- and base-catalysis of the glycosidation. Also Lewis acidic metal salts, particularly the dimeric gold(III) chloride, turned out to exhibit excellent B═C type catalyst properties. Worth mentioning in this context is the ability of gold(III) chloride to regioselectively activate diols. As thioureas have high affinity to anions and also to neutral compounds through strong hydrogen bonds, their binding to alcohols and concomitant activation of O-glycosyl trichloroacetimidates was of interest. Yet, even the acidic N,N'-bis[3,5-bis(trifluoromethyl)phenyl]-thiourea was unable to catalyze glycosidations. However, as a cocatalyst to acids, thiourea exerts a strong effect that, based on NMR studies, leads first to a hydrogen-bond mediated catalyst-cocatalyst-acceptor complex. This complex activates the donor in an intramolecular, acid-base catalyzed reaction that is again closely related to the action of inverting glycosyltransferases. Thus, from O-(α-glycosyl) trichloroacetimidates, good yields of the inversion products, that is, the β-glycosides, are obtained. This novel conceptual approach to glycosidation revealed that for retention of configuration in addition a catalytic nucleophile is required that enables formation of the α-glucoside from the α-trichloroacetimidate. Preliminary studies with a catalyst possessing this 5-fold function, that is, adduct formation with the acceptor, hydrogen-bonding between the reactants, acid and base catalysis, and a catalytic nucleophile as part of a chiral framework supporting facial selection, exhibited good chances for final success in this endeavor.

Insights into cytochrome bc1 complex binding mode of antimalarial 2-hydroxy-1,4-naphthoquinones through molecular modelling.

BACKGROUND Malaria persists as a major public health problem. Atovaquone is a drug that inhibits the respiratory chain of Plasmodium falciparum, but with serious limitations like known resistance, low bioavailability and high plasma protein binding.OBJECTIVESThe aim of this work was to perform molecular modelling studies of 2-hydroxy-1,4-naphthoquinones analogues of atovaquone on the Qo site of P. falciparum cytochrome bc 1 complex (Pfbc1) to suggest structural modifications that could improve their antimalarial activity.METHODS We have built the homology model of the cytochrome b (CYB) and Rieske iron-sulfur protein (ISP) subunits from Pfbc1 and performed the molecular docking of 41 2-hydroxy-1,4-naphthoquinones with known in vitro antimalarial activity and predicted to act on this target.FINDINGS Results suggest that large hydrophobic R2 substituents may be important for filling the deep hydrophobic Qo site pocket. Moreover, our analysis indicates that the H-donor 2-hydroxyl group may not be crucial for efficient binding and inhibition of Pfbc1 by these atovaquone analogues. The C1 carbonyl group (H-acceptor) is more frequently involved in the important hydrogen bonding interaction with His152 of the Rieske ISP subunit.MAIN CONCLUSIONS Additional interactions involving residues such as Ile258 and residues required for efficient catalysis (e.g., Glu261) could be explored in drug design to avoid development of drug resistance by the parasite.

Comprehensive Characterization of Mouse UDP-Glucuronosyltransferase (Ugt) Belonging to the Ugt2b Subfamily: Identification of Ugt2b36 as the Predominant Isoform Involved in Morphine Glucuronidation.

UDP-Glucuronosyltransferases (UGTs) are classified into three subfamilies in mice: Ugt1a, 2b, and 2a. In the Ugt1a subfamily, Ugt1a1 and 1a6 appear to correspond to human UGT1A1 and 1A6 The mouse is an important animal for its use in investigations, but the substrate specificities of Ugt isoforms belonging to the 2b subfamily in mice remain largely unknown. To address this issue, we characterized the substrate specificity of all isoforms of the Ugt2b subfamily expressed in the mouse liver. The cDNAs of Ugt1a1, Ugt2a3, and all the Ugt2b isoforms expressed in the liver were reverse-transcribed from the total RNA of male FVB-mouse livers and then amplified. A baculovirus-Sf9 cell system for expressing each Ugt was established. Of all the Ugts examined, Ugt2b34, 2b36, and 2b37 exhibited the ability to glucuronidate morphine with Ugt2b36, the most active in this regard. Ugt1a1, but also Ugt2b34, 2b36, and 2b37 to a lesser extent, preferentially catalyzed the glucuronidation of 17β-estradiol on the 3-hydroxyl group (E3G). With these isoforms, E3G formation by Ugt1a1 was efficient; however, Ugt2b5 exhibited a preference for the 17β-hydroxyl group (E17G). Ugt2b1 and Ugt2a3 formed comparable levels of E3G and E17G. Ugt2b1 and 2b5 were the only isoforms involved in chloramphenicol glucuronidation. As Ugt2b36 is highly expressed in the liver, it is most likely that Ugt2b36 is a major morphine Ugt in mouse liver. Regarding E3G formation, Ugt1a1, like the human homolog, seems to play an important role in the liver.

A Single Amino Acid Mutation Converts (R)-5-Diphosphomevalonate Decarboxylase into a Kinase

The biosynthesis of isopentenyl diphosphate, a fundamental precursor for isoprenoids, via the mevalonate pathway is completed by diphosphomevalonate decarboxylase. This enzyme catalyzes the formation of isopentenyl diphosphate through the ATP-dependent phosphorylation of the 3-hydroxyl group of (R)-5-diphosphomevalonate followed by decarboxylation coupled with the elimination of the 3-phosphate group. In this reaction, a conserved aspartate residue has been proposed to be involved in the phosphorylation step as the general base catalyst that abstracts a proton from the 3-hydroxyl group. In this study, the catalytic mechanism of this rare type of decarboxylase is re-investigated by structural and mutagenic studies on the enzyme from a thermoacidophilic archaeon Sulfolobus solfataricus The crystal structures of the archaeal enzyme in complex with (R)-5-diphosphomevalonate and adenosine 5'-O-(3-thio)triphosphate or with (R)-5-diphosphomevalonate and ADP are newly solved, and theoretical analysis based on the structure suggests the inability of proton abstraction by the conserved aspartate residue, Asp-281. Site-directed mutagenesis on Asp-281 creates mutants that only show diphosphomevalonate 3-kinase activity, demonstrating that the residue is required in the process of phosphate elimination/decarboxylation, rather than in the preceding phosphorylation step. These results enable discussion of the catalytic roles of the aspartate residue and provide clear proof of the involvement of a long predicted intermediate, (R)-3-phospho-5-diphosphomevalonate, in the reaction of the enzyme.

Structural and Functional Basis of C-Methylation of Coumarin Scaffolds by NovO.

C-methylation of aromatic small molecules by C-methyltransferases (C-MTs) is an important biological transformation that involves C-C bond formation using S-adenosyl-l-methionine (SAM) as the methyl donor. Here, two advances in the mechanistic understanding of C-methylation of the 8-position of coumarin substrates catalyzed by the C-MT NovO from Streptomyces spheroides are described. First, a crystal structure of NovO reveals the Arg116-Asn117 and His120-Arg121 motifs are essential for coumarin substrate binding. Second, the active-site His120 is responsible for deprotonation of the phenolic 7-hydroxyl group on the coumarin substrate, activating the rate-determining methyl transfer step from SAM. This work expands our mechanistic knowledge of C-MTs, which could be used in the downstream development of engineered biocatalysts for small molecule C-alkylations.

Molecular cloning and characterization of a flavonoid glucosyltransferase from Byungkyool (Citrus platymamma hort. ex Tanaka)

Uridine diphosphate glucosyltransferase (UGT) attaches glucoside to proteins, various flavonoids, and phenolic compounds. The modification of flavonoid affects its water solubility, stability, and bioavailability of flavonoids. In this study, citrus genomic sequence database was searched for UGTs of citrus, and a UDP-glucosyltransferase (bGT173) was isolated from Byungkyool (Citrus platymamma hort. ex Tanaka). The cloned cDNA gene was 1365 bp in length and encoded 456 amino acids. Phylogenetic analysis suggested that bGT173 was a member of the flavonoid 3-O-glucosyltransferase group. mRNA expression of bGT173 was higher in leaves compared to flowers, stems, and fruits. The recombinant protein of bGT173 was expressed in Escherichia coli, and tested for its activity on seven flavonoids (apigenin, eriodictyol, hesperetin, kaempferol, luteolin, naringenin, and quercetin). Both kaempferol and quercetin were good substrates for bGT173, demonstrating that bGT173 preferentially glucosylated the 3-hydroxyl group of flavonols. Furthermore, quercetin 3-O-glucoside produced in E. coli showed the same anti-migration activity on pancreatic cancer cells similar to the standard chemical, suggesting that bGT173 is a good candidate for bioconversion of quercetin to quercetin 3-O-glucoside.

The stereodivergent formation of 2,6-cis and 2,6-trans-tetrahydropyrans: experimental and computational investigation of the mechanism of a thioester oxy-Michael cyclization.

The origins of the stereodivergence in the thioester oxy-Michael cyclization for the formation of 4-hydroxy-2,6-cis- or 2,6-trans-substituted tetrahydropyran rings under different conditions was investigated both computationally and experimentally. Synthetic studies showed that the 4-hydroxyl group was essential for stereodivergence. When the 4-hydroxyl group was present, TBAF-mediated conditions gave the 2,6-trans-tetrahydropyran and trifluoroacetic acid-mediated conditions gave the 2,6-cis-tetrahydropyran. This stereodivergence vanished when the hydroxyl group was removed or protected. Computational studies revealed that: (i) the trifluoroacetic acid catalysed formation of 2,6-cis-tetrahydropyrans was mediated by a trifluoroacetate-hydroxonium bridge and proceeded via a chair-like transition state; (ii) the TBAF-mediated formation of 2,6-trans-tetrahydropyrans proceeded via a boat-like transition state, where the 4-hydroxyl group formed a crucial hydrogen bond to the cyclizing alkoxide; (iii) both reactions are under kinetic control. The utility of this stereodivergent approach for the formation of 4-hydroxy-2,6-substituted tetrahydropyran rings has been demonstrated by the total syntheses of the anti-osteoporotic natural products diospongin A and B.

Structure and conserved function of iso-branched sphingoid bases from the nematode Caenorhabditis elegans.

Sphingolipids are bio-active metabolites that show structural diversity among eukaryotes. They are essential for growth of all eukaryotic cells but when produced in an uncontrolled manner can lead to cell death and pathologies including auto-immune reactions, cancer, diabetes and neurodegeneration. Caenorhabditis elegans is an important genetic model organism both to find new drug-targets against parasitic nematodes and to study the conserved roles of sphingolipids in animals like their essential functions in very basic cellular processes ranging from maintenance of cell polarity and mitochondrial repair to growth and survival. C. elegans produces sphingoid bases which are structurally distinct from those of other animals as both iso- and anteiso-branched species have been reported. Using metabolic labeling we show that most worm sphingoid bases are iso-branched. We have synthesized the nematode-specific C17 iso-branched sphinganine and its 1-deoxy analogue and could show that both the iso-branch and the 1-hydroxyl group are essential to form functional nematode sphingolipids which are needed to maintain intestinal function. The organism specificity was examined by complementation experiments in Saccharomyces cerevisiae yeast cells lacking sphingoid base synthesis. We found that iso-branched sphingoid base did not support growth of mutant cells and was toxic to wild type yeast. 1-Deoxy sphingolipids have been linked to the hereditary disease HSAN1A and other metabolic disorders including diabetes. We found that in C. elegans the 1-deoxy analogue cannot rescue the intestinal phenotype caused by sphingoid base depletion. In fact, in wild-type animals with normal sphingoid base biosynthesis, exogenous 1-deoxy analogue had a disruptive effect on apical cytoskeletal organization of intestinal cells indicating that atypical bases can interfere with normal sphingolipid function.

Docking and molecular dynamics studies on triclosan derivatives binding to FabI.

FabI, enoyl-ACP reductase (ENR), is the rate-limiting enzyme in the last step for fatty acids biosynthesis in many bacteria. Triclosan (TCL) is a commercial bactericide, and as a FabI inhibitor, it can depress the substrate (trans-2-enoyl-ACP) binding with FabI to hinder the fatty acid synthesis. The structure-activity relationship between TCL derivatives and FabI protein has already been acknowledged, however, their combination at the molecular level has never been investigated. This paper uses the computer-aided approaches, such as molecular docking, molecular dynamics simulation, and binding free energy calculation based on the molecular mechanics/Poisson-Bolzmann surface area (MM/PBSA) method to illustrate the interaction rules of TCL derivatives with FabI and guide the development of new derivatives. The consistent data of the experiment and corresponding activity demonstrates that electron-withdrawing groups on side chain are better than electron-donating groups. 2-Hydroxyl group on A ring, promoting the formation of hydrogen bond, is vital for bactericidal effect; and the substituents at 4-position of A ring, 2'-position and 4'-position of B ring benefit antibacterial activity due to forming a hydrogen bond or stabilizing the conformation of active pocket residues of receptor. While the substituents at 3'-position and 5'-position of B ring destroy the π-π stacking interaction of A ring and NAD+ which depresses the antibacterial activity. This study provides a new sight for designing novel TCL derivatives with superior antibacterial activity.