Sigma* anion

SDS-PAGE of purified CinnAE (10% polyacrylamide gel). Samples in lanes as follows Lane 1 6 /tg CinnAE after anion-exchange chromatography lane 2, high molecular weight markers from Sigma. 

The Chemistry and Anionic Sigma Complexes by E. Buncel, M.R. Crampton,... 

F. Terrier, Eiectron Deficient Aromatic and Heteroaromatic Base Interactions. The Chemistry of Anionic Sigma Complexes, pp. 78-85. Elsevier, Amsterdam, 1984. 

Material. 3 X crystallized, dialyzed and lyophilized Grade 1 Hen Egg White Lysoz3mie was obtained from Sigma Chemical Company, and used without further purification. Purity of the protein was verified by both gel electrophoresis on acrylamide and chromatography on a column packed with anion exchange resin (Cl form) Sephadex DEAE. The sample showed a single peak in these analyses. 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

The sp2 hybridized carbanion 496 can also be viewed as an sp3 hybridized anion and can therefore look like 497 or 498. In 487, the electron pair is antiperiplanar to the two C —0 bonds of the dioxane ring, so that the carbanion orbital can be delocalized by an overlap with the antibonding orbitals of the two C —0 sigma bonds (n-o interaction). On that basis, carbanion 496 would be closer to 497 than 498, and the equatorial approach of the electrophile is thus readily understood. Banks has however given a different explanation based on the work of Klein (152, 153). 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

Lypozyme IM-20 (Mucor miehei lipase immobilized on a weak anion-exchange resin) was kindly supplied by Novo Nordisk (Bagsvaerd, Denmark). The gas chromatography (GC) standards (mono-, di-, and trilaurin) were obtained from Sigma (St. Louis, MO). Analytical-grade glycerol, lauric acid (99.9%), n-hexane, ethyl acetate, acetone, and ethanol were purchased from Merck (Darmstadt, Germany). 

Although several different rate laws have been drawn for different substrates, the kinetic equations are generally consistent with a mechanism involving an electrophilic attack by I2 on an anionic imidazole ring system, followed by proton abstraction from the sigma complex as the rate-determining step. 

D. A conjugated molecule is a molecule with double bonds on adjacent atoms such as the molecule shown in A. Choice B and C give the definition of sigma and pi molecular orbitals. D is false because a resonance form is one of multiple equivalent Lewis structures, but these structures do not describe the actual state of the molecule. The anion will exist in a state between the two forms. 

As this chapter develops you will see other examples of the versatility of sulfur. You will see how it takes part readily in rearrangements from the simple cationic to the sigma tropic. You will see that it can be removed from organic compounds in either an oxidative or a reductive fashion. You will see that it can stabilize anions or cations on adjacent carbon atoms, and the stabilization of anions is the first main section of the chapter. 

Sigma-bonded anionic ligands, usually chelating see Chelating Ligands) ones such as /3-diketenimides. 

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