A Generalized Unpaired Electron Spin Density Equation and Organic Hyperconjugation Mechanism

A large class of stereochemcial and related interactions in organic chemistry are repulsive and others are attractive, but the relative orientation of two methyl groups and the amount of energy required to twist one relative to the other (the hindered rotation energy barriers), or the alignment of such a group with respect to a conjugated ring to which it is attached (widely attributed to a mechanism called “hyperconjugation”) are estimated to be small in compared with the total energy of the molecule. We used theories of both isotropic and anisotropic proton hyperfine interactions in the π‐electron systems developed in the early sixties. They are approximated by the magnetic dipole nteractions between each proton and an electron spin magnetization that is distributed in 2s and 2p Slater atomic orbitals center on carbon atoms. We have extended these theories to the non‐planar olefinic cation radicals, which are very important in biochemistry as well as in petroleum catalysis. A three dimensional electron spin density equation has been developed in this paper to handle some Jahn‐Teller vibronic molecules. The new electron spin density equation related the observed proton hyperfine splittings to the non‐planar structures of the open‐chain alkene cation radicals generated by radiolysis and various chemical oxidation methods. The spin densities and the conformational calculations based on valence bond theory and symmetry principles are compared with some more elaborated molecular orbital calculations in the literature. The localized valence bond approaches are better in accord with our experimental results. The anomalous line‐width effect of the four methyl groups observed in the 2,3‐dimethyl‐2‐butene cation radicals also confirmed the positive sign of the electron‐proton hyperfine constant of hyper‐conjugation mechanism. A methyl substituent attached to a conjugated molecule often behaves as if it formed part of the region of conjugation; the charge appears to flow from the methyl group into the π electron system and it may also give rise to an appreciable dipole moment. Methylation also gives rise to an appreciable dipole moment, and the resultant red shift of electronic absorption bands is of some importance in the design of dye molecules.     

TGF-β1 induction of the adenine nucleotide translocator 1 in astrocytes occurs through Smads and Sp1 transcription factors

Background  

The adenine nucleotide translocator 1 (Ant1) is an inner mitochondrial membrane protein involved with energy mobilization during oxidative phosphorylation. We recently showed that rodent Ant1 is upregulated by transforming growth factor-beta (TGF-β) in reactive astrocytes following CNS injury. In the present study, we describe the molecular mechanisms by which TGF-β1 regulates Ant1 gene expression in cultured primary rodent astrocytes.     

Enhancing the enzymatic hydrolysis of cellulosic materials using simultaneous ball milling

One of the limiting factors restricting the effective and efficient bioconversion of softwood-derived lignocellulosic residues is the recalcitrance of the substrate following pretreatment. Consequently, the ensuing enzymatic process requires relatively high enzyme loadings to produce monomeric carbohydrates that are readily fermentable by ethanologenic microorganisms. In an attempt to circumvent the need for larger enzyme loadings, a simultaneous physical and enzymatic hydrolysis treatment was evaluated. A ball-mill reactor was used as the digestion vessel, and the extent and rate of hydrolysis were monitored. Concurrently, enzyme adsorption profiles and the rate of conversion during the course of hydrolysis were monitored. α-Cellulose, employed as a model substrate, and SO₂-impregnated steam-exploded Douglas-fir wood chips were assessed as the cellulosic substrates. The softwood-derived substrate was further posttreated with water and hot alkaline hydrogen peroxide to remove >90% of the original lignin. Experiments at different reaction conditions were evaluated, including substrate concentration, enzyme loading, reaction volumes, and number of ball beads employed during mechanical milling. It was apparent that the best conditions for the enzymatic hydrolysis of α-cellulose were attained using a higher number of beads, while the presence of air-liquid interface did not seem to affect the rate of saccharification. Similarly, when employing the lignocellulosic substrate, up to 100% hydrolysis could be achieved with a minimum enzyme loading (10 filter paper units/g of cellulose), at lower substrate concentrations and with a greater number of reaction beads during milling. It was apparent that the combined strategy of simultaneous ball milling and enzymatic hydrolysis could improve the rate of saccharification and/or reduce the enzyme loading required to attain total hydrolysis of the carbohydrate moieties.     

Synthesis of novel 1,3,4‐oxadiazinane‐2‐thiones derived from (1R,2S‐ephedrine, (1S,2S)‐pseudoephedrine and (1R,2S)‐norephedrine

A several novel 1,3,4‐oxadiazinan‐2‐thiones have been synthesized by the cyclization of β‐hydrazino‐alcohols with either carbon disulfide or 1,1′‐thiocarbonyldiimidazole (TCDI).     

Comparative study of electrochemically directed assembly versus conventional self-assembly of thioacetyl-terminated oligo(phenylene ethynylene)s on gold and platinum surfaces

The assembly of thioacetyl-terminated oligo(phenylene ethynylene)s (OPEs) on Au and Pt surfaces under an electric potential (electrochemical assembly, EA) was compared to assembly at an open circuit (conventional self-assembly, CSA). Cyclic voltammetry and ellipsometry were used to characterize the adsorption kinetics of self-assembled monolayers formed by these two techniques. The adsorption rate of the EA was remarkably faster at positive potentials but slower at negative potentials than that of the CSA, The EA at 400 mV proceeded about 800 times faster than the CSA when exposed to the same solution concentrations. The adsorption rates of both EA and CSA were found to be dependent on the molecular structures of OPEs. OPEs containing electron-donating groups assemble faster than those with electron-withdrawing groups. The amount of time that the thioacetyl-terminated OPE is in the presence of the base, for removal of the acetyl group to generate the thiolate, is called the deprotection time. Deprotection times play a critical role in achieving the maximum difference in adsorption rates between the EA and the CSA. The assembly must be initiated no later than 5 min after the basic deprotection is commenced so that the thiolate concentration remains low. The difference in the adsorption rates between EA and CSA might enable selective deposition of certain OPEs onto specific electrodes.     

Multicomponent dynamical nucleation theory and sensitivity analysis

Vapor to liquid multicomponent nucleation is a dynamical process governed by a delicate interplay between condensation and evaporation. Since the population of the vapor phase is dominated by monomers at reasonable supersaturations, the formation of clusters is governed by monomer association and dissociation reactions. Although there is no intrinsic barrier in the interaction potential along the minimum energy path for the association process, the formation of a cluster is impeded by a free energy barrier. Dynamical nucleation theory provides a framework in which equilibrium evaporation rate constants can be calculated and the corresponding condensation rate constants determined from detailed balance. The nucleation rate can then be obtained by solving the kinetic equations. The rate constants governing the multistep kinetics of multicomponent nucleation including sensitivity analysis and the potential influence of contaminants will be presented and discussed.     

Toward the development of a structurally novel class of chiral auxiliaries: diastereoselective aldol reactions of a (1R,2S)-ephedrine-based 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one

[reaction: see text] Asymmetric aldol addition reactions have been conducted with (1R,2S)-ephedrine-derived 3,4,5,6-tetrahydro-2H-1,3,4-oxadiazin-2-one (2). Diastereoselectivities range from 75:25 to 99:1 for the formation of the crude non-Evans syn adducts 8a-h. The facial selectivity of the enolate is directed by the stereogenic N(4)-methyl substituent. Aldol adduct 8a is readily cleaved by acid hydrolysis to afford (2S,3S)-3-hydroxy-2-methyl-3-phenylpropionic acid (9) in >95% ee.     

A convenient one-pot synthesis of 1,8-naphthyridones

In this paper, we disclose an efficient one-pot procedure for the preparation of substituted 1,8-naphthyridin-4-one analogues. Previous efforts to effect this type of transformation were complicated by the formation of benzene tricarboxylate. Via the use of excess base, the impurity formation was completely inhibited. This allowed for the clean preparation of the desired intermediate and subsequent formation of naphthyridone analogues in a single flask, which could then be crystallized directly from the reaction mixture in good yield and high purity.     

Cyclopentadiene annulated polycyclic aromatic hydrocarbons: investigations of electron affinities

The adiabatic electron affinities of cyclopentadiene and 10 associated benzannelated derivatives have been predicted with both density functional and Hartree-Fock theory. These systems can also be regarded as benzenoid polycyclic aromatic hydrocarbons (PAHs) augmented with five-membered rings. Like the PAHs, the electron affinities of the present systems generally increase with the number of rings. To unequivocally bind an electron, cyclopentadiene must have at least two conventionally fused benzene rings. 1H-Benz[f]indene, a naphthalene-annulated cyclopentadiene, is predicted to have a zero-point energy corrected adiabatic electron affinity of 0.13 eV. Since the experimental E(A) of naphthalene is negative (-0.19 eV), the five-membered ring appendage contributes to the stability of the naphthalene-derived 1H-benz[f]indene radical anion significantly. The key to binding the electron is a contiguous sequence of fused benzenes, since fluorene, the isomer of 1H-benz[f]indene, with separated six-membered rings, has an electron affinity of -0.07 eV. Each additional benzene ring in the sequence fused to cyclopentadiene increases the electron affinity by 0.15-0.65 eV: the most reliable predictions are cyclopentadiene (-0.63 eV), indene (-0.49 eV), fluorene (-0.07 eV), 1H-benz[f]indene (0.13 eV), 1,2-benzofluorene (0.25 eV), 2,3-benzofluorene (0.26 eV), 12H-dibenzo[b,h]fluorene (0.65 eV), 13H-indeno[1,2-b]anthracene (0.82 eV), and 1H-cyclopenta[b]naphthacene (1.10 eV). In contrast, if the six-membered ring-fusion is across the C(2)-C(3) cyclopentadiene single bond, only a single benzene is needed to bind an electron. The theoretical electron affinity of the resulting molecule, isoindene, is 0.49 eV, and this increases to 1.22 eV for 2H-benz[f]indene. The degree of aromaticity is responsible for this behavior. While the radical anions are stabilized by conjugation, which increases with the size of the system, the regular indenes, like PAHs in general, suffer from the loss of aromatic stabilization in forming their radical anions. While indene is 21 kcal mol(-1) more stable than isoindene, the corresponding radical anion isomers have almost the same energy. Nucleus-independent chemical shift calculations show that the highly aromatic molecules lose almost all aromaticity when an extra electron is present. The radical anions of cyclopentadiene and all of its annulated derivatives have remarkably low C-H bond dissociation energies (only 18-34 kcal mol(-1) for the mono-, bi-, and tricyclics considered). Hydrogen atom loss leads to the restoration of aromaticity in the highly stabilized cyclopentadienyl anion congeners.