Total synthesis and stereochemistry of the antitumor antibiotic PD 113,271

[Structure: see text] A total synthesis of PD 113,271, an antitumor fostriecin analogue isolated from Streptomyces pulveraceus, was achieved by the chiral pool approach starting with D-galactose and L-tartaric acid. The synthesis of PD 113,271 led to unambiguous assignment of the relative and absolute stereochemistry of its stereocenters.     

Critical metrics of the Schouten functional

Given a Riemannian metric on a compact smooth manifold, we consider its Schouten tensor, which is a tensor field of type (0, 2) arising in the remainder of the Weyl part in the standard decomposition of the curvature tensor of the metric. We study extremal properties of the Schouten functional, defined to be the scaling-invariant L ²-norm of the Schouten tensor. It is proved, for instance, that space form metrics are characterized as critical points of the Schouten functional among conformally flat metrics.     

A circular embedding of a graph in Euclidean 3-space

A spatial embedding of a graph G is an embedding of G into the 3-dimensional Euclidean space . J.H. Conway and C.McA. Gordon proved that every spatial embedding of the complete graph on 7 vertices contains a nontrivial knot. A linear spatial embedding of a graph is an embedding which maps each edge to a single straight line segment. In this paper, we construct a linear spatial embedding of the complete graph on 2n−1 (or 2n) vertices which contains the torus knot T(2n−5,2) (n4). A circular spatial embedding of a graph is an embedding which maps each edge to a round arc. We define the circular number of a knot as the minimal number of round arcs in among such embeddings of the knot. We show that a knot has circular number 3 if and only if the knot is a trefoil knot, and the figure-eight knot has circular number 4.     

Structure conversions of cellulose IIII crystal models in solution state: a molecular dynamics study

This paper re-examines our previous molecular dynamics (MD) study on cellulose III_I crystal models with finite dimensions solvated in explicit water molecules. Eight crystal models, differing in a constituent lattice plane and dimensions, were studied. One calculation allowed for O–H and C–H bond stretching, and had a small time step of 0.5 fs. The other calculation adopted non-scaling factors of the 1–4 non-bonded interactions. As in our previous study, in the former MD calculations, six of the eight crystal models exhibited structure conversion with cooperative chain slippages generated by a progressive fiber bend. This converted the initial non-staggered chain packing of cellulose III_I into a near one-quarter staggering and gave the crystal model a triclinic-like configuration. In contrast, in the non-1–4 scaling MD calculations, all of the eight crystal models retained the initial cellulose III_I crystal structure. Another series of non-1–4 scaling MD calculations were performed for the four crystal models containing chains with a degree of polymerization (DP) of 40 at 370 K, which simulated hot water treatment to convert cellulose III_I to Iβ. Some of the hydroxymethyl groups irreversibly rotated from gt into tg conformation. This accompanied exchange of the intrasheet hydrogen bonding scheme along the (1 ?1 0) lattice plane from O2–O6 to O3–O6. The original corrugated (1 ?1 0) chain sheet was partly converted into a cellulose I-like flat chain sheet.     

Dopamine selective molecularly imprinted polymers via post-imprinting modification

A novel synthetic dopamine receptor bearing bidentate binding sites were prepared by covalent imprinting using a disulfide linkage which is cleaved and oxidized to a non-covalent sulfoxide recognition group. The used templates have dopamine-like structures connected to an allyl moiety via a disulfide and to a 4-vinylphenyl group via a cyclic boronic diester. After the polymerization, the ester bonds were hydrolyzed and the disulfide bond was reduced to remove the template moiety from the polymer matrix, followed by the oxidation to transform the thiol residues into sulfonic acid (post imprinted process). The imprinted polymer adsorbed dopamine selectively in aqueous solution with the two-point interaction, i.e. the formation of cyclic boronic diester and electrostatic interaction with the sulfonic acid residue.     

Host–guest complexation‐triggered chiroptical change of poly(phenylacetylene)s bearing binaphthocrown ether moieties on the main chain

Diacetylene monomers with respective lengths of the oxyethylene chains were cyclopolymerized with a rhodium catalyst to produce novel poly(phenylacetylene)s bearing a different cavity size of the chiral crown ether in the repeating units ( 2a – c ). In the circular dichroism spectra of the resulting polymers, characteristic Cotton effects were observed in the range from 350 to 500 nm corresponding to the absorption of the conjugated polymer backbone, indicating that the polymers possessed a helical structure with an excess single screw sense induced by the covalently bonded binaphthyl units. The host–guest complexation of 2a – c with achiral guests produced a chiroptical change based on the fluctuation in the main chain conformation. The behavior of the complexation‐induced chiroptical change was essentially dictated by the cavity size of the binaphthocrown ether units. Additionally, a chirality‐responsive helicity change was observed in the case of the complexation of 2a – c with chiral guests, which also depended on the crown ether size. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1197–1206, 2010     

Lewis acid-catalyzed nucleophilic substitutions of propargylic and allylic silyl ethers with enol silyl ethers

Nucleophilic reactions of various enol silyl ethers with carbocation species generated from propargyl silyl ethers by the action of a Lewis acid have been developed. The present method is highly useful for the introduction of allenic or enyne functionalities into the alpha-position of substituted ketones. [reaction--see text]     

Top-down analysis of protein isoprenylation by electrospray ionization hybrid quadrupole time-of-flight tandem mass spectrometry; the mouse Tgamma protein

Protein isoprenylation, an important post-translational modification with a lipid, involves the selective attachment of two types of isoprenoids, farnesyl (C15) and geranylgeranyl (C20). The isoprenoid is linked via a thioether bond to the C-terminal cysteine residue of a variety of cellular proteins, including the heterotrimeric G protein gamma-subunits. One member of the G protein family, transducin (Talpha/Tbetagamma), plays a central role in visual transduction, and the structure-function relationship has been extensively studied with purified proteins, predominantly with bovine transducin that was shown to be farnesylated at the C-terminal cysteine residue of the gamma-subunit (Tgamma). We report here the structure of the C-terminal modification of mouse Tgamma, which has not yet been elucidated owing to the low amount of protein that can be isolated from the mouse retina. Electrospray ionization mass spectrometry (ESI-MS) of the high-performance liquid chromatography (HPLC)-purified Tgamma was in good agreement with the calculated mass of the farnesylated and methylated form of mouse Tgamma (Pro1-Cys70). A 'top-down' analysis of intact Tgamma using an ESI hybrid quadrupole time-of-flight (TOF) tandem mass spectrometer provided isoprenyl-specific ions that were observed to produce ions separated by 204 Da from the conventional (unmodified) precursor ion or the C-terminal sequence ions. Such characteristic fragmentation on an isoprenoid observed in top-down analysis could be useful in general for determining the type of isoprenylation as well as probing the site of modification in the protein sequence.     

Synthetic polymers adsorbing bisphenol A and its analogues prepared by covalent molecular imprinting using bisphenol A dimethacrylate as a template molecule

Synthetic polymers which can adsorb bisphenol A (BPA) and related compounds were prepared by a covalent molecular imprinting technique. BPA dimethacrylate, used as template molecule, was polymerized with a crosslinker, triethylene glycol dimethacrylate (TEGDMA) or trimethylol propane trimethacrylate (TRIM). After the polymerization treatment with dilute NaOH was used to cleave BPA from the polymers. For high recovery of BPA with low polymer matrix degradation, the hydrolysis conditions were determined to be treatment with 1.0 mol L^–1 NaOH for 48 h. The binding sites generated by the hydrolysis were evaluated by determination of the retentivity of BPA, BPA analogues, and other endocrine disruptors. The polymers strongly adsorbed compounds with two hydroxyl groups at the 4,4-positions. Generally the TEGDMA-based polymers had stronger affinity than the TRIM-based polymers, although the TRIM-based polymer adsorbed steroidal hormones with two hydroxyl groups, for example 17-estradiol and 17-estradiol, more strongly than the TEGDMA-based polymer, meaning that the crosslinkers affected the properties of the binding sites and, depending upon the target molecules, suitable crosslinkers should be chosen in this system.