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PubMed1999

Covalent Three-Dimensional Titanium(IV)-Aryloxide Networks.

Ti((i)OPr)(4) was treated with 2.58 equiv of hydroquinone in THF to yield a red-orange powder formulated as [Ti(OC(6)H(4)O)(a)()(OC(6)H(4)OH)(3.34)(-)(1.83)(a)()(O(i)Pr)(0.66)(-)(0.17)(a)() (THF)(0.2)](n)() (1, (0.91 </= a </= 1.82)) based upon D(2)O/DCl quenching studies. Treatment of 1 with an excess of hydroquinone in Et(2)O or DME afforded burgundy [Ti(2)(&mgr;(1,4)-OC(6)H(4)O)(2)(&mgr;(1,4)-OC(6)H(4)OH)(2)(&mgr;-OC(6)H(4)OH)(2)](infinity) (2). Burgundy [Ti(2)(&mgr;(1,4)-OC(6)H(4)O)(2)(&mgr;(1,4):eta(2),eta(1)-OC(6)H(4)O)(2)(OH(2))(2).(H(2)O)(2).(H-OC(6)H(4)OH).(MeCN)](infinity) (4) was prepared from 1 and excess wet hydroquinone in CH(3)CN. The acetonitrile in 4 can be exchanged for THF or DME. Treatment of Ti(O(i)Pr)(4) with approximately 4 equiv of 2,7-dihydroxynaphthalene in Et(2)O at 100 degrees C (2 days) in a sealed tube yielded orange crystals of [Ti(2)(&mgr;(1,7)-OC(10)H(6)O)(2)(&mgr;(1,7):eta(2),eta(1)-OC(10)H(6)OH)(2)(O(i)Pr)(2)](infinity) (5). Diffraction studies were conducted at the Cornell High Energy Synchrotron Source (CHESS) because of the small crystal sizes. 2 (C(18)H(14)O(6)Ti, monoclinic, P2(1)/n, a = 9.624 (2), b = 11.283 (2), c = 14.916 (3), beta = 90.47(3) degrees, Z = 4), 4 (C(32)H(33)NO(14)Ti(2), monoclinic, P2(1)/n, a = 16.137 (3), b = 10.762 (2), c = 20.368 (4), beta = 111.65(3) degrees, Z = 4), and 5 (C(23)H(20)O(5)Ti, orthorhombic, Pbca, a = 11.095 (2), b = 17.970 (4), c = 19.484 (4), Z = 8) are 3-dimensional materials based on diaryloxide connectivity between geometrically similar edge shared (i.e., Ti(2)(&mgr;-OAr)(2)) bioctahedral dititanium building blocks. While 2 and 5 possess a roughly body-centered arrangement of dititanium units, 4 has a hexagonal secondary structural motif. The nature of crystallization through alcoholysis is also discussed.

Journal Article↗
PubMed2002

Rhodium-catalyzed asymmetric 1,4-addition of aryltitanium reagents generating chiral titanium enolates: isolation as silyl enol ethers.

The addition of aryltitanium triisopropoxide (ArTi(OPr-i )3) to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (94-99.8% ee) in the presence of 3 mol % of [Rh(OH)((S )-binap)]2 in THF at 20 degrees C to give high yields of the titanium enolates as 1,4-addition products. The titanium enolates were converted into silyl enol ethers by treatment with chlorotrimethylsilane and lithium isopropoxide.

Journal Article↗
PubMed2003

A new cine-substitution of alkenyl sulfones with aryltitanium reagents catalyzed by rhodium: mechanistic studies and catalytic asymmetric synthesis of allylarenes.

The reaction of alkenyl sulfones with aryltitanium triisopropoxide (ArTi(OPr-i )3) in the presence of 3 mol % of [Rh(OH)((S)-binap)]2 in THF at 40 degrees C gave high yield of cine-substitution products. The catalytic cycle was established by deuterium-labeling studies, and it was applied to catalytic asymmetric synthesis of allylarenes which proceeds with over 99% enantioselectivity.

Journal Article↗
PubMed2005

The radical trap in atom transfer radical polymerization need not be thermodynamically stable. A study of the MoX(3)(PMe(3))(3) catalysts.

The molybdenum(III) coordination complexes MoX(3)(PMe(3))(3) (X = Cl, Br, and I) are capable of controlling styrene polymerization under typical atom transfer radical polymerization (ATRP) conditions, in conjunction with 2-bromoethylbenzene (BEB) as an initiator. The process is accelerated by the presence of Al(OPr(i))(3) as a cocatalyst. Electrochemical and synthetic studies aimed at identifying the nature of the spin trap have been carried out. The cyclic voltammogram of MoX(3)(PMe(3))(3) (X = Cl, Br, I) shows partial reversibility (increasing in the order Cl < Br < I) for the one-electron oxidation wave. Addition of X(-) changes the voltammogram, indicating the formation of MoX(4)(PMe(3))(3) for X = Cl and Br. On the other hand, I(-) is more easily oxidized than the MoI(3)(PMe(3))(3) complex; thus, the putative MoI(4)(PMe(3))(3) complex is redox unstable. Electrochemical studies of MoI(3)(PMe(3))(3) in the presence of X(-) (X = Cl or Br) reveal the occurrence of facile halide-exchange processes, leading to the conclusion that the MoI(3)X(PMe(3))(3) products are also redox unstable. The oxidation of MoX(3)(PMe(3))(3) with (1)/(2)Br(2) yields MoX(3)Br(PMe(3))(3) (X = Cl, Br), whose molecular nature is confirmed by single-crystal X-ray analyses. On the other hand, the oxidation of MoI(3)(PMe(3))(3) by I(2) slowly yields a tetraiodomolybdate(III) salt of iodotrimethylphosphonium, [Me(3)PI][MoI(4)(PMe(3))(3)], as confirmed by an X-ray study. This product has no controlling ability in radical polymerization. The redox instability of MoI(3)X(PMe(3))(3) can be reconciled with its involvement as a radical trapping species in the MoI(3)(PMe(3))(3)-catalyzed ATRP, given the second-order nature of its decomposition rate.

Journal Article↗
PubMed2004

Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties.

The p-methoxy-substituted pincer-ligated iridium complexes, (MeO-(tBu)PCP)IrH(4) ((R)PCP = kappa(3)-C(6)H(3)-2,6-(CH(2)PR(2))(2)) and (MeO-(iPr)PCP)IrH(4), are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite "POCOP" ((R)POCOP = kappa(3)-C(6)H(3)-2,6-(OPR(2))(2)) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of nu(CO) values of the respective CO adducts, the MeO-PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO-PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition of C-H and H-H bonds and CO. In general, both ligands favor (relative to unsubstituted PCP) addition to the 14e (pincer)Ir fragments but disfavor addition to the 16e complexes (pincer)IrH(2) or (pincer)Ir(CO). These kinetic and thermodynamic effects are all largely attributable to the same electronic feature: O --> C(aryl) pi-donation, from the methoxy or phosphinito groups of the respective ligands. DFT calculations also indicate that the kinetics (but not the thermodynamics) of C-H addition to (pincer)Ir are favored by sigma-withdrawal from the phosphorus atoms. The high nu(CO) value of (POCOP)Ir(CO) is attributable to electrostatic effects, rather than decreased Ir-CO pi-donation or increased OC-Ir sigma-donation.

Journal Article↗
PubMed2002

Mono-N-methylation of primary amines with alkyl methyl carbonates over Y faujasites. 2. Kinetics and selectivity.

In the presence of a Na-exchanged Y faujasite, the reaction of primary aromatic amines 1 with 2-(2-methoxyethoxy)methylethyl carbonate [MeO(CH(2))(2)O(CH(2))(2)OCO(2)Me, 2a] yields the corresponding mono-N-methyl derivatives ArNHMe with selectivity up to 95%, at substantially quantitative conversions. At 130 degrees C, the reaction can be run under diffusion-free conditions and is strongly affected by the solvent polarity: for instance, in going from xylene (epsilon(r) = 2.40) to triglyme (epsilon(r) = 7.62) as the solvent, the pseudo-first-order rate constant for the aniline (1a) disappearance shows a 5-fold decrease. In DMF (epsilon(r) = 38.25), the same reaction does not occur at all. Competitive adsorption of the solvent and the substrate onto the catalytic sites accounts for this result. The behavior of alkyl-substituted anilines ZC(6)H(4)NH(2) [Z = p-Me, p-Et, p-Pr, p-(n-Bu) (1b-e); Z = 3,5-di-tert-butyl- and 2,4,6-tri-tert-butylanilines (1f,g)] and p-alkoxyanilines p-ZC(6)H(4)NH(2) [Z = OMe, OEt, OPr, O-n-Bu (1b'-e')] clearly indicates a steric effect of ring substituents: as diffusion of the amine into the catalytic pores is hindered, the reaction hardly proceeds and the mono-N-methyl selectivity (S(M/D)) drops as well. Moreover, the strength of adsorption of the amine onto the catalyst influences the rate and the selectivity as well: the reaction of p-anisidine and p-toluidine-despite the higher nucleophilicity of these compounds-is slower and even less selective with respect to aniline. From a mechanistic viewpoint, the intermediacy of carbamates ArN(Me)CO(2)R [R = MeO(CH(2))(2)O(CH(2))(2)] is suggested. At 90 degrees C, the reaction of benzylamine (7)-a model for aliphatic amines-with dimethyl carbonate shows that the reaction outcome can be improved by tuning the amphoteric properties of the catalyst: in going from CsY to the more acidic LiY zeolite, methylation is not only more selective (S(M/D) ratio increases from 77% to 84%) but even much faster (CsY, conversion of 36% after 22 h; LiY, conversion of 43% after 7 h).

Journal Article↗
PubMed1999

Synthesis of 2'beta-Deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine: Unusual Annulation Conditions To Assemble the Purine Core.

Synthesis of 2'beta-Deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine has been accomplished using a five-step process which employs a novel annulation of [(13)C]formamide [N-(4-[(15)N]amino-6-chloro-5-pyrimidinyl)] 2. To effect this dehydration, a complex of triethyl phosphite and TiCl(2)(i-OPr)(2) was used. 6-Chloro-[8-(13)C; 9-(15)N]purine was then converted in two steps to 2'beta-deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine.

Journal Article↗
PubMed1999

Four Enantiomerization Routes of 1,2,2-Trimesitylvinyl Acetate. Enantioselective Liquid Chromatography of (E)- and (Z)-2-m-Methoxymesityl-1,2-dimesitylvinyl Acetates.

The vinyl propellers (E)- and (Z)-2-m-methoxymesityl-1,2-dimesitylvinyl acetates (3c and 3d) were prepared and their geometries assigned. The stereoisomerization barriers of the trimesityl vinyl acetate system were determined by DNMR and by enantioselective LC resolution and polarimetric monitoring of the behavior of the two diastereomeric racemates of 3c and 3d. Combined with data for trimesitylvinyl-OAc 3a and its 1-m-methoxymesityl analogue 3b, the following order of barriers DeltaG() is obtained: alphabeta-2-ring flip > alphabeta'-2-ring flip > betabeta'-2-ring flip > alphabetabeta'-3-ring flip (the threshold enantiomerization barrier). This order which differs from the previously found orders for trimesitylvinyl-X, X = H, OPr-i was rationalized and discussed.

Journal Article↗
PubMedDate not supplied

Global analysis of the membrane subproteome of Pseudomonas aeruginosa using liquid chromatography-tandem mass spectrometry.

Pseudomonas aeruginosa is one of the most significant opportunistic bacterial pathogens in humans causing infections and premature death in patients with cystic fibrosis, AIDS, severe burns, organ transplants, or cancer. Liquid chromatography coupled online with tandem mass spectrometry was used for the large-scale proteomic analysis of the P. aeruginosa membrane subproteome. Concomitantly, an affinity labeling technique, using iodoacetyl-PEO biotin to tag cysteinyl-containing proteins, permitted the enrichment and detection of lower abundance membrane proteins. The application of these approaches resulted in the identification of 786 proteins. A total of 333 proteins (42%) had a minimum of one transmembrane domain (ranging from 1 to14) and 195 proteins were classified as hydrophobic based on their positive GRAVY values (ranging from 0.01 to 1.32). Key integral inner and outer membrane proteins involved in adaptation and antibiotic resistance were conclusively identified, including the detection of 53% of all predicted opr-type porins (outer integral membrane proteins) and all the components of the mexA-mexB-oprM transmembrane protein complex. This work represents one of the most comprehensive proteomic analyses of the membrane subproteome of P. aeruginosa and for prokaryotes in general.

Affinity Labels↗
PubMed2003

2:1 Ba/Ti(IV) heterobimetallic complex based on two calix[6]arenes.

Reaction of p-Bu(t)-calix[6]arene with barium metal in methanol then [Ti(OPr(i))4] affords a heterobimetallic complex with a central Ti(IV) attached to two calix[6]arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium centre.

Journal Article↗
PubMed2003

A paradigm shift in the construction of heterobimetallic complexes: synthesis of group 2 & 4 metal-calix[6]arene complexes.

Deprotonation of calix[6]arenes with barium in methanol followed by the addition of [Ti(OPr(i))(4)] or [Zr(OBu(n))(4)] is effective in the formation of novel dimeric 2:1 barium-titanium(IV)/zirconium(IV) calix[6]arene complexes. In these complexes a central Ti(IV)/Zr(IV) coordinated in the exo-position connects the two calix[6]arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium/zirconium centre and participating in cation-pi interactions. A homometallic barium calix[6]arene dimer was also prepared wherein the calix[6]arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix[6]arenes through bridging methanol ligands. In this and the heterometallic complexes, cation-pi complexation of the Ba(2+) ion within the 1,3 alternate conformation of calix[6]arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr(2+) ion did not form similar complexes in methanol, and the formation of an analogous 2:1 strontium-titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation-pi interaction. The results show (i) that a subtle interplay of solvation strength, coordination array type and cavity/cation size influences the accessibility of heterobimetallic complexes based on calix[6]arenes, and (ii) a synergistic endo-exo binding behaviour.

Journal Article↗
PubMed2002

Serotyping and RAPD profiles of Salmonella enterica isolates from Mauritius.

AIMS: The genus Salmonella is a common agent of gastroenteritis in Mauritius, generating more cases of the disease during summer than during winter. The aims of this study were to assess the genetic diversity of isolates of Salmonella enterica by RAPD fingerprinting, and to establish the relationship between human and chicken isolates. METHODS: Twenty-six isolates were obtained from hospital laboratories and commercial poultry producers locally. RESULTS: The RAPD profiles, biochemical and serological analyses showed that two of the chicken isolates were mistakenly identified as Salmonella. The genetic diversity of the remaining 24 isolates (five chicken and 19 human), confirmed as Salmonella, was analysed using four arbitrary primers, OPA-10, OPR-03, OPI-06 and OPJ-09, chosen from an initial set of 10 decamers. Seventy RAPD markers were generated in four individual DNA profiles. SIGNIFICANCE AND IMPACT OF THE STUDY: Cluster analysis (UPGMA) performed using the NTSYS-pc V 1.8 computer software, confirmed that some strains of Salmonella isolated from chicken were genetically similar to those isolated from humans. Furthermore, a 1 kbp band amplified using primer OPA-10 was specific for the Salmonella genus as it was not amplified in any of the control bacteria.

Animals↗
PubMed2004

Mechanistic studies on the catalytic cycle of rhodium-catalyzed asymmetric 1,4-addition of aryltitanate reagents to alpha,beta-unsaturated ketones.

Addition of lithium aryl(tetraisopropoxy)titanates [ArTi(OPr-i)(4)(-)Li(+)] to alpha,beta-unsaturated ketones proceeded with high enantioselectivity (up to 99% ee) in the presence of an excess amount of chlorotrimethylsilane and a rhodium catalyst (3 mol % Rh), generated from [RhCl(C(2)H(4))(2)](2) and (S)-binap, in tetrahydrofuran at 20 degrees C to give high yields of the corresponding silyl enolates as 1,4-addition products. The presence of chlorotrimethylsilane is essential for the 1,4-addition to take place. (31)P NMR spectroscopic studies revealed that the catalytic cycle consists of three transformations, that is, (i) insertion of an enone into arylrhodium species forming (oxa-pi-allyl)rhodium intermediate, (ii) silylation of the (oxa-pi-allyl)rhodium with chlorotrimethylsilane giving silyl enolate and a chloro-rhodium complex, and (iii) transmetalation of aryl group from aryltitanate to the chloro-rhodium regenerating the aryl-rhodium.

Journal Article↗
PubMed2000

The Arabidopsis male-sterile mutant, opr3, lacks the 12-oxophytodienoic acid reductase required for jasmonate synthesis.

Jasmonic acid (JA) and its precursor 12-oxophytodienoic acid (OPDA) act as plant growth regulators and mediate responses to environmental cues. To investigate the role of these oxylipins in anther and pollen development, we characterized a T-DNA-tagged, male-sterile mutant of Arabidopsis, opr3. The opr3 mutant plants are sterile but can be rendered fertile by exogenous JA but not by OPDA. Cloning of the mutant locus indicates that it encodes an isozyme of 12-oxophytodienoate reductase, designated OPR3. All of the defects in opr3 are alleviated by transformation of the mutant with an OPR3 cDNA. Our results indicate that JA and not OPDA is the signaling molecule that induces and coordinates the elongation of the anther filament, the opening of the stomium at anthesis, and the production of viable pollen. Just as importantly, our data demonstrate that OPR3 is the only isoform of OPR capable of reducing the correct stereoisomer of OPDA to produce JA required for male gametophyte development.

Amino Acid Sequence↗
PubMed1999

A homolog of old yellow enzyme in tomato. Spectral properties and substrate specificity of the recombinant protein.

A cDNA was isolated and characterized from a tomato shoot cDNA library, the deduced amino acid sequence of which exhibited similarity with yeast Old Yellow Enzymes (OYEs) and related enzymes of bacterial and plant origin. Sequence identity was particularly high with 12-oxophytodienoate 10,11-reductase (OPR) from Arabidopsis thaliana. The cDNA-encoded protein was expressed as a glutathione S-transferase fusion protein in Escherichia coli and was purified from bacterial extracts. The protein was found to be a flavoprotein catalyzing the NADPH-dependent reduction of the olefinic bond of alpha,beta-unsaturated carbonyl compounds, including 12-oxophytodienoic acid. Thus, the tomato enzyme was termed LeOPR. The catalytic efficiency of LeOPR was highest with N-ethylmaleimide followed by 12-oxophytodienoic acid and maleic acid as substrates. Photoreduction of the LeOPR-bound FMN resulted in the formation of a red, anionic semiquinone prior to the formation of the fully reduced flavin dihydroquinone. Spectroscopic characterization of LeOPR revealed the formation of charge transfer complexes upon titration with para-substituted phenolic compounds, a distinctive feature of the enzymes of the OYE family. The ligand binding properties were compared between LeOPR and OYE, and the findings are discussed with respect to structural differences between the active sites of OYE and LeOPR.

Amino Acid Sequence↗
PubMed2004

Solution structure of atypical protein kinase C PB1 domain and its mode of interaction with ZIP/p62 and MEK5.

Atypical protein kinase C (aPKC) has been implicated in several signaling pathways such as cell polarity, cell survival, and cell differentiation. In contrast to other PKCs, aPKC is unique in having the PB1 (Phox and Bem 1) domain in the N terminus. The aPKC PB1 domain binds with ZIP/p62, Par6, or MEK5 through a PB1-PB1 domain interaction that controls the localization of aPKC. Here, we determined the three-dimensional structure of the PB1 domain of PKCiota by NMR and found that the PB1 domain adopts a ubiquitin fold. The OPCA (OPR, PC, and AID) motif inserted into the ubiquitin fold was presented as a betabetaalpha fold in which the side chains of conserved Asp residues were oriented to the same direction to form an acidic surface. This structural feature suggested that the acidic surface of the PKCiota PB1 domain interacted with the basic surface of the target PB1 domains, and this was confirmed in the case of the PKCiota-ZIP/p62 complex by mutational analysis. Interestingly, in the PKCiota PB1 domain a conserved lysine residue was located on the side opposite to the OPCA motif-presenting surface, suggesting dual roles for the PKCiota PB1 domain in that it could interact with either the conserved lysine residue or the acidic residues on the OPCA motif of the target PB1 domains.

Amino Acid Sequence↗
PubMed2004

Structure of a cell polarity regulator, a complex between atypical PKC and Par6 PB1 domains.

A complex of atypical PKC and Par6 is a common regulator for cell polarity-related processes, which is an essential clue to evolutionary conserved cell polarity regulation. Here, we determined the crystal structure of the complex of PKCiota and Par6alpha PB1 domains to a resolution of 1.5 A. Both PB1 domains adopt a ubiquitin fold. PKCiota PB1 presents an OPR, PC, and AID (OPCA) motif, 28 amino acid residues with acidic and hydrophobic residues, which interacts with the conserved lysine residue of Par6alpha PB1 in a front and back manner. On the interface, several salt bridges are formed including the conserved acidic residues on the OPCA motif of PKCiota PB1 and the conserved lysine residue on the Par6alpha PB1. Structural comparison of the PKCiota and Par6alpha PB1 complex with the p40phox and p67phox PB1 domain complex, subunits of neutrophil NADPH oxidase, reveals that the specific interaction is achieved by tilting the interface so that the insertion or extension in the sequence is engaged in the specificity determinant. The PB1 domain develops the interaction surface on the ubiquitin fold to increase the versatility of molecular interaction.

Adaptor Proteins, Signal Transducing↗