The Critical Roles of Conserved Glu 21 and Asp 23 of a Staphylococcal Anti-Anti-Sigma Factor.

Staphylococcus aureus and similar bacteria cope with stressful environments using a set of conserved proteins including σ, an alternative sigma factor. The initiation of transcription by σ is obstructed by RsbW, an anti-sigma factor. RsbW also associates and phosphorylates RsbV, an anti-anti-sigma factor.

Synthetic Studies toward the Total Synthesis of Scabrolide A.

Herein, we report a concise route to the [5-5-6]-fused tricyclic core of scabrolide A and our efforts toward the construction of the fourth cycloheptane ring of the molecule via a 7- radical cyclization. The tricyclic cyclohexenone core was assembled by a ring-closing metathesis (RCM) reaction followed by oxidation and concomitant isomerization of the double bond. These promising results have potential implications in the synthesis of similar tricyclic cores of many other congeners within this family of furanobutenolide-derived polycyclic cembranoids and norcembranoids.

A staphylococcal capsule-producing enzyme that unfolds via multiple intermediates predominantly exists as the trimers at low concentrations.

CapG, an enzyme expressed by , catalyzes an epimerization reaction to synthesize -acetyl-L-fucosamine, a constituent of capsule involved in pathogenesis. This protein has two domains, exists as the homohexamers in the solution, and usually produces products at hundred-nanomolar concentrations. To determine the folding-unfolding mechanism and the oligomeric form of CapG, particularly at low concentrations, we have investigated a recombinant CapG (rCapG) using different probes.

Staphylococcus aureus major cell division protein FtsZ assembly is inhibited by silibinin, a natural flavonolignan that also blocked bacterial growth and biofilm formation.

The bacterial cell division protein FtsZ has been considered a potential therapeutic target due to its rapid treadmilling that induces cellular wall construction in bacteria. The current study discovered a novel antimicrobial compound, silibinin, a natural flavonolignan and its impact on the recombinant S. aureus FtsZ (SaFtsZ).

Modeling and monitoring the effects of three partly conserved Ile residues in the dimerization domain of a Mip-like virulence factor from .

FKBP22, an -made peptidyl-prolyl - isomerase, has shown considerable homology with Mip-like virulence factors. While the C-terminal domain of this enzyme is used for executing catalytic function and binding inhibitor, the N-terminal domain is employed for its dimerization. To precisely determine the underlying factors of FKBP22 dimerization, its structural model, developed using a suitable template, was carefully inspected.

CpTiCl-Promoted Radical Cyclization to Six- and Seven-Membered Carbocycles: Synthesis of (±)-Isoclavukerin A.

A systematic study is undertaken to investigate the less explored radical cyclization in activated olefin-appended epoxides using CpTiCl. The radical generated by the Ti(III)-promoted reductive opening of the epoxy ring promptly underwent cyclization, giving access to differently 1,3-disubstituted six- and seven-membered carbocycles in good yields and diastereoselectivity. This protocol was successfully employed in the construction of 5,7- and 6,7-fused bicyclic frameworks entailing a de novo synthesis of (±)-isoclavukerin A belonging to tri--guaiane class of sesquiterpene natural products in eight simple steps from commercially available starting materials.

An attempt to construct an indole-fused azabicyclo[3.3.1]nonane framework radical cyclization.

The structural motif of an indole-fused azabicyclo[3.3.1]nonane is common in many biologically significant indole-based natural products.

One-Pot Tandem Aldol-Cycloetherification Protocol in the Enantioselective Synthesis of Davanoids.

Total synthesis of and diastereomers of prenylated davanoids like davanone, nordavanone, and davana acid ethyl ester was achieved in an enantioselective strategy. Various other davanoids could also be synthesized using standard procedures from the Weinreb amides derived from davana acids. Enantioselectivity in our synthesis was achieved employing a Crimmins' non-Evans syn aldol reaction that fixed the stereochemistry of the C3-hydroxyl group, while the C2-methyl group was epimerized in a late stage of the synthesis.

The cofactors and domains of a staphylococcal capsule-producing enzyme preserve its structure, stability, shape and dimerization ability.

CapF, a staphylococcal capsule-producing enzyme, binds Zn2+ ion and NADPH using its C-terminal domain (CTD) and N-terminal domain (NTD), respectively. To elucidate the roles of cofactors and domains, we have systematically investigated the related recombinant proteins, rCapF, rCTD, recombinant NTD (rNTD) and the Zn2+-free rCapF/rCTD, Apo-rCapF/Apo-rCTD. The results show that the secondary structure, tertiary structure, shape and surface hydrophobicity of Apo-rCapF and Apo-rCTD are different from those of rCapF and rCTD.

Serine 106 preserves the tertiary structure, function, and stability of a cyclophilin from .

SaCyp, a staphylococcal cyclophilin involved in both protein folding and pathogenesis, has a Ser residue at position 106 and a Trp residue at position 136. While Ser 106 of SaCyp aligned with a cyclosporin A (CsA) binding Ala residue, its Trp 136 aligned with a Trp or a Phe residue of most other cyclophilins. To demonstrate the exact roles of Ser 106 and Trp 136 in SaCyp, we have elaborately studied rCyp[S106A] and rCyp[W136A], two-point mutants of a recombinant SaCyp (rCyp) harboring an Ala substitution at positions 106 and 136, respectively.

CpTiCl-Mediated Reductive Cyclization: Total Synthesis of Pestalotiolactone A, Myrotheciumone A, and Scabrol A.

The first stereoselective total syntheses of fungal secondary metabolites monoterpenoid (+)-pestalotiolactone A, meroterpenoid (-)-myrotheciumone A, and iridoid lactone (+)-scabrol A have been accomplished in an expedient unified approach starting from d-(+)-malic acid employing an epoxide opening-radical cyclization protocol initiated by CpTi(III)Cl as a key step to assemble the core bicyclic lactone moieties of these molecules with complete diastereoselective control. Finally, the deoxygenation and methylation delivered the target natural products.

Environment-Sensitive Fluorescence of 7-Nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-Labeled Ligands for Serotonin Receptors.

Serotonin is a neurotransmitter that plays a crucial role in the regulation of several behavioral and cognitive functions by binding to a number of different serotonin receptors present on the cell surface. We report here the synthesis and characterization of several novel fluorescent analogs of serotonin in which the fluorescent NBD (7-nitrobenz-2-oxa-1,3-diazol-4-yl) group is covalently attached to serotonin. The fluorescent ligands compete with the serotonin receptor specific radiolabeled agonist for binding to the receptor.

Structurally distinct unfolding intermediates formed from a staphylococcal capsule-producing enzyme retained NADPH binding activity.

CapF, a capsule-producing enzyme expressed by , binds NADPH and exists as a dimer in the aqueous solution. Many other capsule-producing virulent bacteria also express CapF orthologs. To understand the folding-unfolding mechanism of CapF, herein a recombinant CapF (rCapF) was individually investigated using urea and guanidine hydrochloride (GdnCl).

Total Synthesis of Panaginsene.

The total synthesis of panaginsene has been accomplished in 11 linear steps starting from methyl 3,3-dimethyl-5-oxocyclopent-1-ene-1-carboxylate. The key steps are a Sharpless asymmetric epoxidation and Ti(III)-mediated reductive epoxide opening-radical cyclization to construct the chiral quaternary carbon stereocenter followed by a very challenging HWE olefination reaction on an 1,3-keto aldehyde and a late stage McMurry olefination using low valent titanium to construct the highly constrained angular tetrasubstituted olefin in a five-membered ring.

Alternative Sigma Factor of Interacts with the Cognate Antisigma Factor Primarily Using Its Domain 3.

σ, an alternative sigma factor, is usually employed to tackle the general stress response in and other Gram-positive bacteria. This protein, involved in -mediated pathogenesis, is typically blocked by RsbW, an antisigma factor having serine kinase activity. σ, a σ-like sigma factor, harbors three conserved domains designated σ, σ, and σ.

A conserved arginine residue in a staphylococcal anti-sigma factor is required to preserve its kinase activity, structure, and stability.

RsbW, σ, and RsbV, encoded by and related bacteria, act as an anti-sigma factor, an sigma factor, and an anti-anti-sigma factor, respectively. The interaction between RsbW and σ blocks the transcription initiation activity of the latter protein. RsbW also functions as a serine kinase and phosphorylates RsbV in the presence of ATP.

Understanding the structure, stability, and anti-sigma factor-binding thermodynamics of an anti-anti-sigma factor from .

and many related bacteria encode both anti-sigma factor RsbW and anti-anti-sigma factor RsbV to control stress response by σ, an alternative sigma factor. Our structural and thermodynamic studies of a recombinant RsbV (rRsbV) show that the monomeric protein contains five α-helices and a mostly parallel but mixed β-sheet composed of five β-strands, and interacts with a chimeric RsbW (rRsbW) . In addition, rRsbV binds rRsbW with a of 0.

Application of CpTiCl-Promoted Radical-Induced Cyclization: An Expeditious Access to []-Annelated Indoles.

An efficient and novel route for assembling pyrrolo/piperido[1,2-]indoles is portrayed involving a radical-mediated reductive epoxide opening reaction of -tethered epoxy-indoles that trigger facile intramolecular cyclization followed by an oxidative quenching step. Capitalizing on the operational simplicity of the method involving just two steps and use of an efficient C-C bond-forming reaction, this radical-based protocol enables the modular assembly of an important class of -fused indole derivatives with versatile functional and structural diversity.

Development of Minimal Diguanosinyl Motif toward RNA G-Quadruplex-Like Structures in Solution.

Among the non-canonical structures of B-DNA, the G-quadruplex is of particular interest because of its well-defined conformation, high stability, and versatility. Herein we report our studies on the development of an amide-linked minimal diguanosinyl motif that forms a G-quadruplex-like structure in solution in the presence of potassium cations; various linear guanosine amino acid dimers were synthesized with linkers of different chain lengths to investigate the optimum flexibility required to form such structures.

Titanocene(III)-Mediated 5-exo-trig Radical Cyclization: En Route to Spirooxindole-Based Tetrahydrofuran and Bicyclic Lactone.

The isatin core system is of immense importance due to the highly reactive prochiral C-3 position, which paves an easy way to construct large arrays of spirooxindole heterocyclic motifs. Herein, we depict an isatin-derived and 3,3'-disubstituted oxindole-appended epoxy-acrylate undergoing CpTi(III)Cl-mediated reductive oxirane-ring opening with concomitant intramolecular 5-exo-trig radical cyclization leading to tetrahydrofuran-based oxa-spirooxindole systems. The fused spirooxindole structural feature is embedded in many natural products and tends to exhibit a wide spectrum of biological activities.

Removal of an atypical region from a staphylococcal cyclophilin affects its structure, function, stability, and shape.

SaCyp, a cyclophilin having 197 amino acid residues, acts both as a protein-folding catalyst and a virulence factor in Staphylococcus aureus. Interestingly, a region, homologous to the SaCyp region carrying 121-148 amino acid residues, is present in many putative cyclophilins but absent in well-studied cyclophilins. To determine the exact roles of this unusual region in SaCyp and related proteins, we have investigated a deletion mutant (rCypΔ) of a recombinant SaCyp (rCyp) using various probes.

Studies on sugar puckering and glycosidic stabilities of 3'-amino-5'-carboxymethyl-3',5'-dideoxy nucleoside mimics.

The synthesis of nucleoside amino acid monomers and dimers has been carried out to evaluate and characterize the impact of the neutral amide backbone on key attributes like puckering of the sugar rings and glycosidic bond strengths of these analogs. The conformational analysis suggests that amide-linked nucleotides have a high predilection towards N-type conformers. The glycosidic bond strength was found to be slightly weaker compared to ribonucleosides under acidic conditions at high temperatures.

Application of CpTiCl-Promoted Radical Cyclization: A Unified Strategy for the Syntheses of Iridoid Monoterpenes.

An expedient approach toward the unified total syntheses of (+)-iridomyrmecin, (-)-isoiridomyrmecin, (+)-7- epi-boschnialactone, (+)-teucriumlactone, and (-)-dolichodial in chirally pure forms starting from readily available (+)-β-citronellene is delineated combining step economy and simplicity. Highlights include a Ti(III)-mediated reductive epoxide opening-cyclization for the construction of the core cyclopenta[ c]pyran skeleton of the iridoid lactones with complete diastereoselectivity for the newly created bridgehead stereogenic centers. Subsequent transformations facilitate a short access to (+)-teucriumlactone and (-)-dolichodial and formal access to potentially other iridoids.

Conformation Analysis of GalNAc-Appended Sugar Amino Acid Foldamers as Glycopeptide Mimics.

Sugar amino acid (SAA)-based foldamers with well-defined secondary structures were appended with N-acetylgalactosamine (GalNAc) sugars to access sequence-defined, multidentate glycoconjugates with full control over number, spacing and position. Conformation analysis of these glycopeptides by extensive NMR spectroscopic studies revealed that the appended GalNAc units had a profound influence on the native conformational behaviour of the SAA foldamers. Whereas the 2,5-cis glycoconjugate showed a helical structure in water, comprising of two consecutive 16-membered hydrogen bonds, its 2,5-trans congener displayed an unprecedented 16/10-mixed turn structure not seen before in any glycopeptide foldamer.

Diversity-Oriented Approach to N-Heterocyclic Compounds from α-Phenyl-β-enamino Ester via a Mitsunobu-Michael Reaction Sequence.

Herein we delineate a novel route for the diastereoselective construction of diversely substituted N-heterocyclic ring systems as valuable scaffolds for natural products and pharmaceuticals, starting from an easily accessible prochiral α-phenyl-β-enamino ester. The reaction sequence relies on the unexplored reactivity of α-phenyl-β-enamino ester as a nucleophilic partner in the Mitsunobu reaction to forge the N-tethered alkene-alcohol/thiol/amine intermediate, which was subjected to an intramolecular hetero-Michael addition reaction under mild conditions to furnish the respective N-heterocyclic compounds embedded with an exocyclic chiral center in high yields and excellent diastereoselectivities. The methodology is amenable for a broad range of substrates based on a metal-free approach.