Identification of critical residues of the serotype modifying O-acetyltransferase of Shigella flexneri

ABSTRACT: BACKGROUND: Thirteen serotypes of Shigella flexneri (S. flexneri) have been recognised, all of which are capable of causing bacillary dysentery or shigellosis. With the emergence of the newer S. flexneri serotypes, the development of an effective vaccine has only become more challenging. One of the factors responsible for the generation of serotype diversity is an LPS O-antigen modifying, integral membrane protein known as O-acetyltransferase or Oac. Oac functions by adding an acetyl group to a specific O-antigen sugar, thus changing the antigenic signature of the parent S. flexneri strain. Oac is a membrane protein, consisting of hydrophobic and hydrophilic components. Oac bears homology to several known and predicted acetyltransferases with most homology existing in the N-terminal transmembrane (TM) regions. RESULTS: In this study, the conserved motifs in the TM regions and in hydrophilic loops of S. flexneri Oac were targeted for mutagenesis with the aim of identifying the amino acid residues essential for the function of Oac. We previously identified three critical arginines-R73, R75 and R76 in the cytoplasmic loop 3 of Oac. Re-establishing that these arginines are critical, in this study we suggest a catalytic role for R73 and a structural role for R75 and R76 in O-acetylation. Serine-glycine motifs (SG 52-53, GS 138-139 and SYG 274-276), phenylalanine-proline motifs (FP 78-79 and FPV 282-84) and a tryptophan-threonine motif (WT141-142) found in TM segments and residues RK 110-111, GR 269-270 and D333 found in hydrophilic loops were also found to be critical to Oac function. CONCLUSIONS: By studying the effect of the mutations on Oac's function and assembly, an insight into the possible roles played by the chosen amino acids in Oac was gained. The transmembrane serine-glycine motifs and hydrophilic residues (RK 110-111, GR 269-270 and D333) were shown to have an affect on Oac assembly which suggests a structural role for these motifs. The phenylalanine-proline and the tryptophan-threonine motifs affect Oac function which could suggest a catalytic role for these amino acids.     

Regioselective preferential nucleophilic addition of N-heterocycles onto haloarylalkynes over N-arylation of aryl halides

The study of preferential addition of heterocyclic amines onto halo-substituted arylalkynes over N-arylation under various catalytic conditions is described. The present work supports and confirms the mechanistic pathway of our recent work on the tandem synthesis of indolo- and pyrrolo-[2,1-a]isoquinolines via hydroamination followed by oxidative addition and not vice versa.     

Chemical composition of leaf and root essential oils of Boenninghausenia albiflora Reichb. from northern India

The leaf and root essential oil composition of Boenninghausenia albiflora Reichb and Meissner (family: Rutaceae), collected from Uttarakhand, India, was analysed by capillary gas chromatography and gas chromatography-mass spectrometry. The major constituents identified in the leaf essential oil were β-myrcene, (Z)-β-guaiene, (Z)-β-ocimene and β-caryophyllene, whereas bicyclogermacrene, α-terpinyl acetate, geijerene and β-copaene-4α-ol were identified as the major constituents of the root essential oil. This is the first time that the chemical compositions of leaf and root essential oils of B. albiflora have been investigated in detail. The results show significant qualitative and quantitative variations in leaf and root oil composition.     

Antituberculosis agent diaportheone B: synthesis, absolute configuration assignment, and anti-TB activity of its analogues

First synthesis of diaportheone B, an antituberculosis agent isolated from endophytic fungus Diaporthe sp. P133 is reported using two complementary routes, a one step and a three-step sequence. The absolute configuration of diaportheone B was determined by using X-ray crystal structure analysis of its dibromo derivative. In addition, we have prepared several close analogues of diaportheone B and determined their anti-TB potential using Alamar-blue assay (H(37)Rv).     

Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy

The electrical conductivity of aluminium doped zinc oxide (AZO, ZnO:Al) materials depends on doping induced defects and grain structure. This study aims at relating macroscopic electrical conductivity of AZO nanoparticles with their atomic structure, which is non-trivial because the derived materials are heavily disordered and heterogeneous in nature. For this purpose we synthesized AZO nanoparticles with different doping levels and narrow size distribution by a microwave assisted polyol method followed by drying and a reductive treatment with forming gas. From these particles electrically conductive, optically transparent films were obtained by spin-coating. Characterization involved energy-dispersive X-ray analysis, wet chemical analysis, X-ray diffraction, electron microscopy and dynamic light scattering, which provided a basis for a detailed structural solid-state NMR study. A multinuclear ((27)Al, (13)C, (1)H) spectroscopic investigation required a number of 1D MAS NMR and 2D MAS NMR techniques (T(1)-measurements, (27)Al-MQMAS, (27)Al-(1)H 2D-PRESTO-III heteronuclear correlation spectroscopy), which were corroborated by quantum chemical calculations with an embedded cluster method (EEIM) at the DFT level. From the combined data we conclude that only a small part of the provided Al is incorporated into the ZnO structure by substitution of Zn. The related (27)Al NMR signal undergoes a Knight shift when the material is subjected to a reductive treatment with forming gas. At higher (formal) doping levels Al forms insulating (Al, H and C containing) side-phases, which cover the surface of the ZnO:Al particles and increase the sheet resistivity of spin-coated material. Moreover, calculated (27)Al quadrupole coupling constants serve as a spectroscopic fingerprint by which previously suggested point-defects can be identified and in their great majority be ruled out.     

Qfold: a new modeling paradigm for the RNA folding problem

Journal of Heuristics - Ribonucleic acid (RNA) molecules play informational, structural, and metabolic roles in all living cells. RNAs are chains of nucleotides containing bases {A, C, G, U} that...     

Bifurcations and chaos in large-Prandtl number Rayleigh–Bénard convection

Rayleigh–Bénard convection with large-Prandtl number (P) is studied using a low-dimensional model constructed with the energetic modes of pseudospectral direct numerical simulations. A detailed bifurcation analysis of the non-linear response has been carried out for water at room temperature (P=6.8) as the working fluid. This analysis reveals a rich instability and chaos picture: steady rolls, time-periodicity, quasiperiodicity, phase locking, chaos, and crisis. Our low-dimensional model captures the reappearance of ordered states after chaos, as previously observed in experiments and simulations. We also observe multiple coexisting attractors consistent with previous experimental observations for a range of parameter values. The route to chaos in the model occurs through quasiperiodicity and phase locking, and attractor-merging crisis. Flow patterns spatially moving along the periodic direction have also been observed in our model.     

Thiourea dioxide promoted efficient organocatalytic one-pot synthesis of a library of novel heterocyclic compounds

The utility of thiourea dioxide as an efficient organocatalyst for the library synthesis of novel heterocyclic compounds via one-pot multicomponent coupling reactions is disclosed. Thiourea dioxide is an inexpensive and readily accessible catalyst, resulting in better product yields as compared to the corresponding thiourea as catalyst. Thiourea dioxide is found to be insoluble in various organic solvents and therefore at the end of the reaction products can be separated by extraction with diethyl ether and the recovered catalyst can be used several times with consistent catalytic activity.     

Base-mediated regio- and stereoselective intermolecular addition of alkynes to N-heterocycles

The regio- and stereoselective addition of N-heterocycles to alkynes using KOH is reported. Formation of (Z)-isomers and their conversion to (E)-products were found to be dependent upon time as well as the choice of base. Selective attack of N-heterocycles on a more electrophilic alkynyl carbon was supported by DFT calculations, and the stereochemistry of the products was established by X-ray crystallographic studies and intramolecular cyclization of ortho-haloalkynes in indolo-[2,1-a]isoquinolines.     

Onset of hydrogen bonded collective network of water in 1,4-dioxane

We have studied the evolution of water hydrogen bonded collective network dynamics in mixtures of 1,4-dioxane (Dx) as the mole fraction of water (X(w)) increases from 0.005 to 0.54. The inter- and intramolecular vibrations of water have been observed using terahertz time domain spectroscopy (THz-TDS) in the frequency range 0.4-1.4 THz (13-47 cm(-1)) and Fourier transform infrared (FTIR) spectroscopy in the far-infrared (30-650 cm(-1)) and mid-infrared (3000-3700 cm(-1)) regions. These results have been correlated with the reactivity of water in these mixtures as determined by kinetic studies of the solvolysis reaction of benzoyl chloride (BzCl). Our studies show an onset of intermolecular hydrogen bonded water network dynamics beyond X(w) ≥ 0.1. At the same concentration, we observe a rapid increase of the rate constant of solvolysis of BzCl in water-Dx mixtures. Our results establish a correlation between the onset of collective hydrogen bonded network with the solvation dynamics and the activity of clustered water.