Factors affecting propagating fronts of addition polymerization: Velocity,front curvature,temperatue profile,conversion, and molecular weight distribution

Several properties of propagating fronts of addition polymerization were studied. A power function could be fit to the velocity dependence on initiator concentration, but not with the exponents predicted by current models or in agreement with other published work. Bubbles from the volatile by-products of initiator decomposition were found to affect the front velocity and curvature. The front velocity for triethylene glycol dimethacrylate polymerization was found to depend linearly on temperature over a moderate range. The conversion of methacrylic acid in fronts varied greatly with initiator type and concentration. Benzoyl peroxide produced much lower conversion than t-butyl peroxide, but fronts with tBPO propagated slower. A dual initiator system of BPO and tBPO produced rapidly propagating fronts with good conversion but the contribution of each initiator to the velocity was not additive. The possibility of chain branching was considered. The apparent molecular weight distributions were very broad, often trimodal, and found to depend on initiator type and concentration as well as the tube diameter. The temperature profiles were measured and found to be very sharp for BPO and broader for tBPO but both had front temperatures in excess of 200°C, indicating a high ceiling temperature. © 1995 John Wiley & Sons, Inc.     

Excitation-energy migration in self-assembled cyclic zinc(II)-porphyrin arrays: a close mimicry of a natural light-harvesting system

The excitation-energy-hopping (EEH) times within two-dimensional cyclic zinc(II)-porphyrin arrays 5 and 6, which were prepared by intermolecular coordination and ring-closing metathesis reaction of olefins, were deduced by modeling the EEH process based on the anisotropy depolarization as well as the exciton-exciton annihilation dynamics. Assuming the number of energy-hopping sites N = 5 and 6, the two different experimental observables, that is, anisotropy depolarization and exciton-excition annihilation times, consistently give the EEH times of 8.0 +/- 0.5 and 5.3 +/- 0.6 ps through the 1,3-phenylene linkages of 5 and 6, respectively. Accordingly, the self-assembled cyclic porphyrin arrays have proven to be well-defined two-dimensional models for natural light-harvesting complexes.     

Synthesis of Tricyclic beta-Methylene Spiro Lactones Related to Bakkenolides by Successive Radical Cyclization-High Pressure Diels-Alder Reactions

The synthesis of some novel bakkenolides and their epi-spiro analogues was achieved by a new approach. Photolysis of allyl 1-(phenylseleno)-2-oxocyclopentanecarboxylates 7-9 afforded the corresponding spiro lactones 10-12 by radical cyclization via group transfer of the phenylseleno group. Selenoxide elimination of 11 and 12 produced the corresponding beta-methylene lactones 14 and 15. Diels-Alder cycloaddition of lactone11 with piperylene failed at ambient pressure, but proceeded in generally good yield in the presence of various Lewis acids at pressures of ca. 16 kbar, to give mixtures of beta-exo, alpha-endo, and beta-endo cycloadducts 19, 21, and 23, respectively. The preponderance of endo products 21 and 23, formed via highly hindered, but more compact, transition states was attributed to the high pressure and resulted in trans-dimethyl configurations of the products. The facial selectivity was dependent upon the Lewis acids, and the greatest alpha:beta ratio was observed with catalysts of the type TiCl(2)(OR)(2). Epimerization of the C-4 methyl group in 21 and 23 to furnish the corresponding cis-dimethyl analogues was achieved via exo-epoxidation, regioselective reduction, oxidation to the corresponding 3-keto derivatives, and base-catalyzed equilibration, thereby affording (+/-)-3,6-dioxobakkenolide-A (39) and its epi-spiro derivative 28, respectively. When the radical cyclization step was performed subsequent to the Diels-Alder cycloaddition by photolysis of perhydrindane 43, only the epi-spiro product 44 was obtained.     

Fast folding of a four-helical bundle protein

The FK506-FKBP12 binding-domain of the kinase FRAP (FRB) forms a classic up-down four-helical bundle. The folding pathway of this protein has been investigated using a combination of equilibrium and kinetic studies. The native state of the protein is stable with respect to the unfolded state by some 7 kcal mol(-1) at pH 6.0, 10 degrees C. A kinetic analysis of unfolding and refolding rate constants as a function of chemical denaturant concentration suggests that an intermediate state may be populated during folding at low concentrations of denaturant. The presence of this intermediate state is confirmed by refolding experiments performed in the presence of the hydrophobic dye 8-anilinonaphthalene-1 sulfonate (ANS). ANS binds to the partially folded intermediate state populated during the folding of FRB and undergoes a large change in fluorescence that can be detected using stopped-flow techniques. Analysis of the kinetic data suggests that the intermediate state is compact and it may even be a misfolded species that has to partially unfold before it can reach the transition state. Folding and unfolding rate constants in water are approximately 150-200 s(-1) and 0.005-0.06 s(-1), respectively, at neutral pH and 10 degrees C. The folding of FRB is somewhat slower than for other all-helical proteins, probably as a consequence of the formation of a metastable intermediate state. The folding rate constant in the absence of any populated intermediate can be estimated to be 8800 s(-1). Despite the presence of an intermediate state, which effectively slows folding, the protein still folds rapidly with a half-life of 5 ms at 10 degrees C. The dependence of the rate constants on denaturant concentration indicates that the transition state for folding is compact with some 80% of the surface area exposed in the unfolded state buried in the transition state. Data presented for FRB is compared with kinetic data obtained for other all-helical proteins.     

Level of some minor and trace elements in Bangladeshi meat products

An investigation on the level of some minor and trace elements in some varieties of meat (beef, mutton and chicken) consumed in Bangladesh is reported. In this study, protoninduced X-ray emission (PIXE) and radioisotope-induced X-ray fluorescence (XRF) techniques were used for analytical measurements. In PIXE measurements, the samples were exposed to the proton beam in air as 1 mm thick pellets and irradiated with 2.0 MeV protons having the beam intensity of 30 nA for characteristic X-ray excitation, whereas in XRF, the samples were excited for 5000 seconds with a 10 mCi¹⁰⁹Cd annular X-ray source. The elemental concentrations of Ca, Mn, Fe, Ni, Cu, Zn, As, Se, Br, Rb and Sr were determined in the samples by comparison with X-ray yield curves constructed from IAEA and NBS standard reference materials. The significance of the results is dicussed in relation to human health and diseases.This research received financial support from the International Atomic Energy Agency, Vienna, under the research contract RC-4265/RB.     

Lithiation of (tBuNH)3PNSiMe3 and formation of tetraimidophosphate complexes containing M3O3 rings (M = Li, K): X-ray structure of the stable radical [(Me3SiN)P(mu3-N(t)Bu)3[mu3-Li(THF)]3(OtBu))

The reaction of ((t)BuNH)(3)PNSiMe(3) (1) with 1 equiv of (n)BuLi results in the formation of Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] (2); treatment of 2 with a second equivalent of (n)BuLi produces the dilithium salt Li(2)[P(NH(t)Bu)(N(t)Bu)(2)(NSiMe(3))] (3). Similarly, the reaction of 1 and (n)BuLi in a 1:3 stoichiometry produces the trilithiated species Li(3)[P(N(t)Bu)(3)(NSiMe(3))] (4). These three complexes represent imido analogues of dihydrogen phosphate [H(2)PO(4)](-), hydrogen phosphate [HPO(4)](2)(-), and orthophosphate [PO(4)](3)(-), respectively. Reaction of 4 with alkali metal alkoxides MOR (M = Li, R = SiMe(3); M = K, R = (t)Bu) generates the imido-alkoxy complexes [Li(3)[P(N(t)Bu)(3)(NSiMe(3))](MOR)(3)] (8, M = Li; 9, M = K). These compounds were characterized by multinuclear ((1)H, (7)Li, (13)C, and (31)P) NMR spectroscopy and, in the cases of 2, 8, and 9.3THF, by X-ray crystallography. In the solid state, 2 exists as a dimer with Li-N contacts serving to link the two Li[P(NH(t)Bu)(2)(N(t)Bu)(NSiMe(3))] units. The monomeric compounds 8 and 9.3THF consist of a rare M(3)O(3) ring coordinated to the (LiN)(3) unit of 4. The unexpected formation of the stable radical [(Me(3)SiN)P(mu(3)-N(t)Bu)(3)[mu(3)-Li(THF)](3)(O(t)Bu)] (10) is also reported. X-ray crystallography indicated that 10 has a distorted cubic structure consisting of the radical dianion [P(N(t)Bu)(3)(NSiMe(3))](.2)(-), two lithium cations, and a molecule of LiO(t)Bu in the solid state. In dilute THF solution, the cube is disrupted to give the radical monoanion [(Me(3)SiN)((t)BuN)P(mu-N(t)Bu)(2)Li(THF)(2)](.-), which was identified by EPR spectroscopy.     

Effects of Nonionic and Mixed Cationic-Nonionic Micelles on the Rate of Alkaline Hydrolysis of 4-Nitrophthalimide

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of 4-nitrophthalimide show a monotonic decrease with increase in [C₁₂E₂₃]_T (total concentration of Brij 35) at constant [CH₃CN] and [NaOH]. This micellar effect is explained in terms of a pseudophase micelle model. The rate of hydrolysis becomes too slow to monitor at [C₁₂E₂₃]_T≥0.03 M in the absence of cetyltrimethylammonium bromide (CTABr) and at [C₁₂E₂₃]_T≥0.04 M in the presence of 0.006–0.02 M CTABr at 0.01 M NaOH. The plots of k_obs versus [C₁₂E₂₃]_T show minima at 0.006 and 0.01 M CTABr, while such a minimum is not visible at 0.02 M CTABr.     

A reliable treatment of Abel’s second kind singular integral equations

The central idea of this paper is to construct a new mechanism for the solution of Abel’s type singular integral equations that is to say the two-step Laplace decomposition algorithm. The two-step Laplace decomposition algorithm (TSLDA) is an innovative adjustment in the Laplace decomposition algorithm (LDA) and makes the calculation much simpler. In this piece of writing, we merge the Laplace transform and decomposition method and present a novel move toward solving Abel’s singular integral equations.     

Corrosion inhibition of mild steel in hydrochloric acid using extract of glycine max leaves

Research on Chemical Intermediates - The activity of glycine max leaves extract against corrosion of mild steel in 0.5 M HCl solution has been investigated at different temperatures...