Asymmetric synthesis of spiroketal, spiroether, and oxabicycle building blocks via stereoselective spiro- and bicycloannulation of 2-hydroxy dihydropyrans

A modular asymmetric synthesis of spiroketal, spiroether, and oxabicycle building blocks is described based on the spiro- and bicycloannulation of alpha-hydroxy dihydropyrans, which were obtained from sulfoximine-substituted homoallylic alcohols. Key steps of the syntheses are stereoselective Ferrier-type O- and C-glycosidation, ring-closing metathesis, and stereoselective Prins cyclization.     

Water-soluble tripeptide Abeta (9-11) forms amyloid-like fibrils and exhibits neurotoxicity

A water-soluble, hydrophilic tripeptide GYE, having sequence identity with the N-terminal segment of amyloid peptides Abeta(9-11), upon self-association exhibits amyloid-like fibrils and significant neurotoxicity towards the Neuro2A cell line. However, the tripeptides GFE and GWE, in which the centrally located tyrosine residue has been replaced by phenylalanine or tryptophan, fail to show amyloidogenic behavior and exhibit little or no neurotoxicity.     

Development and validation of HPLC method for the resolution of drug intermediates: DL-3-Phenyllactic acid, DL-O-acetyl-3-phenyllactic acid and (+/-)-mexiletine acetamide enantiomers

Sensitive and specific, high-performance liquid chromatography (HPLC) methods have been developed and validated for linearity, accuracy and precision for the quantification of dl-3-phenyllactic acid, dl-O-acetyl-3-phenyllactic acid and (+/-)-mexiletine acetamide enantiomers. Chromatographic separations were performed on a Chiralcel OJ-H column (0.46 mm x 250 mm, 5 microm, Daicel Chemical Industries, Japan) based on cellulose tris-(4-methyl benzoate) chiral stationary phase. The mobile phase consists of hexane and isopropanol (IPA) in the ratio of 90:10 containing 0.1% trifluoroacetic acid (in case of dl-3-phenyllactic acid and dl-O-acetyl-3-phenyllactic acid) and hexane and IPA (95:5) containing 0.1% triethylamine (in case of (+/-)-mexiletine acetamide) and the flow rate was set at 0.5 ml/min at 25 degrees C. The detection was carried out at 261 nm for dl-3-phenyllactic acid and dl-O-acetyl-3-phenyllactic acid and at 254 nm for (+/-)-mexiletine acetamide. The developed methods were utilized for monitoring the progress of lipase catalyzed enantioselective synthesis of O-acetyl-3-phenyllactic acid and mexiletine acetamide from dl-3-phenyllactic acid and (+/-)-mexiletine, respectively.     

Ionic liquid promoted interrupted Feist-Benary reaction with high diastereoselectivity

A basic ionic liquid, 1-butyl-3-methylimidazolium hydroxide promotes the interrupted Feist-Benary reaction at room temperature under organic solvent-free conditions to produce a variety of substituted hydroxydihydrofurans. The hydroxydihydrofurans are converted to furans (Feist-Benary products) using the ionic liquid, 1-methyl-3-pentylimidazolium bromide at 70-75 °C. The reactions are very clean, high yielding and highly stereoselective.     

Analysis of hairy root culture of Rauvolfia serpentina using direct analysis in real time mass spectrometric technique

The applicability of a new mass spectrometric technique, DART (direct analysis in real time) has been studied in the analysis of the hairy root culture of Rauvolfia serpentina. The intact hairy roots were analyzed by holding them in the gap between the DART source and the mass spectrometer for measurements. Two nitrogen-containing compounds, vomilenine and reserpine, were characterized from the analysis of the hairy roots almost instantaneously. The confirmation of the structures of the identified compounds was made through their accurate molecular formula determinations. This is the first report of the application of DART technique for the characterization of compounds that are expressed in the hairy root cultures of Rauvolfia serpentina. Moreover, this also constitutes the first report of expression of reserpine in the hairy root culture of Rauvolfia serpentina.     

Effect of mycolic acid on surface activity of binary surfactant lipid monolayers

In pulmonary tuberculosis, Mycobacterium tuberculosis lies in close physical proximity to alveolar surfactant. Cell walls of the mycobacteria contain loosely bound, detachable surface-active lipids. In this study, the effect of mycolic acid (MA), the most abundant mycobacterial cell wall lipid, on the surface activity of phospholipid mixtures from lung surfactant was investigated using Langmuir monolayers and atomic force microscopy (AFM). In the presence of mycolic acid, all the surfactant lipid mixtures attained high minimum surface tensions (between 20 and 40 mN/m) and decreased surface compressibility moduli <50 mN/m. AFM images showed that the smooth surface topography of surfactant lipid monolayers was altered with addition of MA. Aggregates with diverse heights of at least two layer thicknesses were found in the presence of mycolic acid. Mycolic acids could aggregate within surfactant lipid monolayers and result in disturbed monolayer surface activity. The extent of the effect of mycolic acid depended on the initial state of the monolayer, with fluid films of DPPC-POPC and DPPC-CHOL being least affected. The results imply inhibitory effects of mycolic acid toward lung surfactant lipids and could be a mechanism of lung surfactant dysfunction in pulmonary tuberculosis.     

Face-selective Diels-Alder reactions between unsymmetrical cyclohexadienes and symmetric trans-dienophile: an experimental and computational investigation

A combined experimental and theoretical study of the Diels-Alder reactions between 2-trimethylsiloxy-1,3-cyclohexadienes (2-11) and (E)-1,4-diphenylbut-2-ene-1,4-dione (1) is reported. Two diastereomeric products, 5-endo-6-exo- (nx) and 5-exo-6-endo- (xn) dibenzoyl derivatives, are possible with symmetric trans-dienophile (1). While in many cases 5-endo-6-exo product is preferred over the corresponding 5-exo-6-endo product, the product ratio nx:xn is found to vary with the position of substituents on the diene. The density functional theory studies with the mPW1PW91/6-31G* as well as the B3LYP/6-31G* levels reveal that the electrostatic repulsion between the oxygen lone pairs on the diene and the dienophile is critical to the observed product selectivities. The optimized transition state geometries though appeared to involve secondary orbital interactions, careful examination of the frontier Kohn-Sham orbitals as well as calculations with the natural bond orbital (NBO) analyses confirm the absence of SOI in these transition states. In the case of methyl-substituted dienes, a cumulative effect of steric and electrostatic interactions between the diene and the dienophile is found to be the controlling element toward the observed selectivity.     

Structure, self-assembly, and dual role of a beta-defensin-like peptide from the Chinese soft-shelled turtle eggshell matrix

Biomineral matrix formation and molecular recognition are two important processes associated with eggshell biomineralization. To understand these two processes, a major intracrystalline peptide, pelovaterin, was isolated from turtle (Pelodiscus sinensis) eggshell and its tertiary and quaternary structures were established. The global fold of pelovaterin is similar to that of human beta-defensins but has a large hydrophobic core and a short hydrophilic N-terminal segment, which is not preserved in defensins. Pelovaterin exhibits strong antimicrobial activity against two pathogenic gram-negative bacteria, Pseudomonas aeruginosa and Proteus vulgaris, and stabilizes a thin film of metastable vaterite. We show that pelovaterin self-aggregates in the form of micellar nanospheres and the aggregation in solution is entropy-driven. It is suggested that the micellar aggregation of pelovaterin is responsible for the induction and stabilization of the metastable phase by altering the interfacial energy. The results demonstrate the adaptability of an extracellular matrix protein to perform multiple tasks: polymorph discrimination and protection of the contents of the egg against bacterial invasion.     

Interaction of 7-hydroxyflavone with human serum albumin: a spectroscopic study

Numerous recent investigations have revealed that various synthetic as well as therapeutically active natural flavonoids possess novel luminescence properties that can serve as highly sensitive monitors for exploring their interactions with relevant physiological targets. Here we report a detailed study on the interactions of the model flavone, 7-hydroxyflavone (7HF) with the plasma protein human serum albumin (HSA), employing electronic absorption, fluorescence (steady state and time resolved) and induced circular dichroism (ICD) spectroscopy. The spectral data indicate that in the protein matrix, the neutral 7HF molecules are predominantly transformed to a conjugate anion (7HFA) by a proton abstraction in the ground state. The protein (HSA) environment induces dramatic enhancements in the fluorescence emission intensity, anisotropy (r) and lifetime (tau) values, as well as pronounced changes in the fluorescence excitation and emission profiles of the fluorophore. Moreover, evidence for efficient F?rster type resonance energy transfer (FRET, from tryptophan to 7HFA) is presented, from which we infer that the binding site of 7HF in HSA is proximal (estimated distance, R=23.6A) to the unique tryptophan - 214 residue present in the inter-domain (between IIA and IIIA domains) loop region of the protein. The binding constant (K=9.44x10(4)M(-1)) and the Gibbs free energy change (DeltaG=-28.33kJ/mol) for 7HFA-HSA interaction have been estimated from the emission data. Finally, the near-UV circular dichroism (CD) studies show that the electronic transitions of 7HF are strongly perturbed on binding to the chiral host (HSA), leading to the appearance of ICD bands. Implications of these results are discussed.