A new Co(2)(CO8-mediated tandem [5 + 1]/[2 + 2 + 1]-cycloaddition reaction: a one-pot synthesis of tricyclic delta-lactones from cis-epoxy ene-ynes

In the presence of Co2(CO)8 and CO, cis-epoxyalkynes bearing a tether olefin undergo a tandem [5 + 1]/[2 + 2 + 1]-cycloaddition to give tricyclic delta-lactones efficiently in a one-pot operation. The reaction mechanism is proposed to involve a cobalt-coordinated cyclic allene species.     

Synthesis and anti-HIV activity of 3-alkylamido-3-deoxy-betulinic acid derivatives

3-Alkylamido-3-deoxy-betulinic acids were synthesized and evaluated for anti-HIV activity as part of the structure-activity relationship study of the potent anti-HIV agent 3-O-(3',3'-dimethyl)-succinyl-betulinic acid (DSB) (2). 3Alpha-diglycorylamide-3-deoxy-betulinic acid demonstrated relatively potent anti-HIV activity (EC50 0.24 microm, TI 728). However, replacing the ester group at C-3 in 2 and its analogues with an amido group yielded inactive or much less potent compounds against HIV replication, indicating that the ester group at C-3 in 2-4 is essential for potent anti-HIV activity.     

Role for bound water and CH-pi aromatic interactions in photosynthetic electron transfer

Photosystem I (PSI) is one of two photosynthetic reaction centers present in plants, algae, and cyanobacteria and catalyzes the reduction of ferredoxin and the oxidation of cytochrome c or plastocyanin. The PSI primary chlorophyll donor, which is oxidized in the primary electron-transfer events, is a heterodimer of chl a and a' called P700. It has been suggested that protein relaxation accompanies light-induced electron transfer in this reaction center (Dashdorj, N.; Xu, W.; Martinsson, P.; Chitnis, P. R.; Savikhin, S. Biophys. J. 2004, 86, 3121. Kim, S.; Sacksteder, C. A.; Bixby, K. A.; Barry, B. A. Biochemistry 2001, 40, 15384). To investigate the details of electron transfer and relaxation events in PSI, we have employed several experimental approaches. First, we report a pH-dependent viscosity effect on P700+ reduction; this result suggests a role for proton transfer in the PSI electron-transfer reactions. Second, we find that changes in hydration alter the rate of P700+ reduction and the interactions of P700 with the protein environment. This result suggests a role for bound water in electron transfer to P700+. Third, we present evidence that deuteration of the tyrosine aromatic side chain perturbs the vibrational spectrum, associated with P700+ reduction. We attribute this result to a linkage between CH-pi interactions and electron transfer to P700+.     

Migration behavior and enantioseparation of hydrobenzoin and structurally related compounds in capillary zone electrophoresis with a dual cyclodextrin system consisting of heptakis-(2,3-dihydroxy-6-O-sulfo)-beta-cyclodextrin and beta-cyclodextrin

Migration behavior and enantioseparation of racemic hydrobenzoin and structurally related compounds, including benzoin and benzoin methyl ether, in CZE with a dual CD system consisting of heptakis-(2,3-dihydroxy-6-O-sulfo)-beta-CD (SI-S-beta-CD) and beta-CD as chiral selectors in the presence and absence of borate complexation at pH 9.0 were investigated. The results indicate that enantioseparation of hydrobenzoin is mainly governed by CD complexation of hydrobenzoin-borate complexes with SI-S-beta-CD when SI-S-beta-CD concentration is relatively high. Whereas CD complexation of hydrobenzoin-borate complexes with beta-CD plays a significant role in enantioseparation when SI-S-beta-CD concentration is comparatively low. The (S,S)-enantiomer of the hydrobenzoin-borate complex was found to interact more strongly than the corresponding (R,R)-enantiomer with both SI-S-beta-CD and beta-CD. These two types of CD show the same chiral recognition pattern, but they exhibit opposite effects on the mobility of the enantiomers of hydrobenzoin-borate complexes. Enantiomer migration reversal of hydrobenzoin occurred in the presence of borate complexation when varying the concentration of beta-CD, while keeping SI-S-beta-CD at a relatively low concentration. Binding constants of the enantiomers of benzoin-related compounds to beta-CD and those of hydrobenzoin-borate complexes to SI-beta-CD were evaluated; the mobility contributions of all complex species to the effective mobility of the enantiomers of hydrobenzoin as a function of beta-CD concentration in a borate buffer were analyzed. In addition, comparative studies on the enantioseparation of benzoin-related compounds with SI-S-beta-CD and with randomly sulfate-substituted beta-CD were made.     

Tributylgermanium hydride as a replacement for tributyltin hydride in radical reactions

Tributylgermanium hydride (Bu(3)GeH) can be used as an alternative to tributyltin hydride (Bu(3)SnH) as a radical generating reagent with a wide range of radical substrates. Tributylgermanium hydride has several practical advantages over tributyltin hydride, e.g. low toxicity, good stability and much easier work-up of reactions. The reagent can be easily prepared in good yield and stored indefinitely. Suitable substrates include iodides, bromides, activated chlorides, phenyl selenides, tert-nitroalkanes, thiocarbonylimidazolides and Barton esters. Alkyl, vinyl and aryl radicals can be generated in radical reactions including reduction and cyclisation processes. Common radical initiators such as ACCN and triethylborane can be used. The slower rate of hydrogen abstraction by carbon-centred radicals from Bu(3)GeH as compared to Bu(3)SnH facilitates improved cyclisation yields. Polarity reversal catalysis (PRC) with phenylthiol can be used in reactions which generate stable radical intermediates which will not abstract hydrogen from Bu(3)GeH.     

Poly(choloyl)-based amphiphiles as pore-forming agents: transport-active monomers by design

This paper describes the design and synthesis of a family of pore-forming amphiphiles. Two of these amphiphiles, which are derived from cholic acid, lysine, and p-phenylenediamine, can produce pores in lipid bilayers as individual molecules. In sharp contrast, analogous amphiphiles that do not contain a rigid 1,4-phenylenediamide moiety favor the formation of dimer-based pores. Kinetic evidence in support of monomer- and dimer-based pores has been obtained from Na+ transport measurements across bilayers made from 1-palmitoyl-2-oleoyl-2-sn-glycero-3-phosphocholine (POPC). Structure-activity studies that have been carried out with pore-forming, dimer-based amphiphiles have also revealed a significant activity dependence on their overall compactness. The practical potential of pore-forming amphiphiles with controllable supramolecular properties is briefly discussed.     

Diamond and diamond-like carbon from a preceramic polymer

The synthesis of poly(hydridocarbyne), one of a class of carbon-based random network polymers and a structural isomer of polyacetlyene, is reported. The network backbone of this polymer is primarily composed of tetrahedrally hybridized carbon atoms, each bearing one hydride substituent and linked via three carbon-carbon single bonds into a three-dimensional random network of fused rings. This atomic-level carbon network backbone confers unusual properties on the polymer, including facile thermal decomposition to form diamond or diamond-like carbon high-quality films at atmospheric pressure, by direct deposition or by chemical vapor deposition (CVD), without the use of hydrogen or any other reagent.     

Interference between the hydrogen bonds to the two rings of nicotine

The biochemical transport and binding of nicotine depends on the hydrogen bonding between water and binding site residues to the pyridine ring and the protonated pyrrolidinium ring. To test the independence of these two moderately separated hydrogen-bonding sites, we have calculated the structures of clusters of protonated nicotine with water and a bicarbonate anion, benzene, indole, or a second water molecule. Unprotonated nicotine-water clusters have also been studied for contrast. The potential energy surfaces are first explored with an intermolecular anisotropic atom-atom model potential. Full geometry optimizations are then carried out using density functional theory to include nonadditive terms in the interaction energies. The presence of the charge on the pyrrolidine nitrogen removes the conventional hydrogen-bonding site on the pyridine ring. The hydrogen-bond ability of this site is nearly recovered when the protonated pyrrolidinium ring is bound to a bicarbonate anion, whereas its interaction with benzene shows a much smaller effect. Indole appears to partially restore the hydrogen-bond ability of the pyridine nitrogen, although indole and benzene both pi-bond to the pyrrolidinium ring. A second hydrogen-bonding water produces a significant conformational distortion of the nicotine. This demonstrates the limitations of the conventional qualitative predictions of hydrogen bonding based on the independence of molecular fragments. It also provides benchmarks for the development of atomistic modeling of biochemical systems.     

Quantitation of the human basal ganglia with positron emission tomography: a phantom study of the effect of contrast and axial positioning

The accurate measurement of the concentration of a radioisotope in small structures with positron emission tomography (PET) requires a correction for quantitation loss due to the partial volume effect and the effect of scattered radiation. To evaluate errors associated with measures in the human basal ganglia (BG), a unilateral model of the BG has been inserted in a 20-cm cylinder. The recovery coefficient (RC=measured activity/true activity) for the BG phantom has been measured on a CTI tomograph with different background concentrations (contrast) and at different axial locations in the gantry. The BG was visualized on four or five slices, depending on its position in the gantry and on the contrast used. The RC was 0.75 with no background (contrast equal to 1.0). Increasing the relative radioactivity concentration in the background increased the RC from 0.75 to 2.00 when the contrast was -0.7 (BG相似文献