Redox equilibria of iron oxides in aqueous-based magnetite dispersions: effect of pH and redox potential

The effect of pH and redox potential on the redox equilibria of iron oxides in aqueous-based magnetite dispersions was investigated. The ionic activities of each dissolved iron species in equilibrium with magnetite nanoparticles were determined and contoured within the Eh-pH framework of a composite stability diagram. Both standard redox potentials and equilibrium constants for all major iron oxide redox equilibria in magnetite dispersions were found to differ from values reported for noncolloidal systems. The "triple point" position of redox equilibrium among Fe(II) ions, magnetite, and hematite shifted to a higher standard redox potential and an equilibrium constant which was several orders of magnitude higher. The predominant area of magnetite stability was enlarged to cover a wider range of both pH and redox potentials as compared to that of a noncolloidal magnetite system.     

Compatibility and fungal degradation of poly[(R)-3-hydroxybutyrate]/aliphatic copolyester blend

Poly[(R)-3-hydroxybutyrate] (PHB) was blended with an aliphatic copolyester, which was synthesized by the esterification of adipic acid, ethylene glycol, and lactic acid. The blend showed a single T_g, which varied systematically but convexly upwards with the composition. The growth rate of PHB spherulites, the crystallization temperature, and the equilibrium melting temperature of the blend were decreased as the amount of the copolyester was increased. Therefore, the blend system was determined to be compatible. However, the degree of crystallinity, and the enthalpies of crystallization and fusion of PHB in the blend remained almost constant, regardless of the compositional change, although the crystallization rate was decreased upon blending. No chemical change such as transesterification was observed as a result of the blending, yet there was a slight change in the crystalline morphology of PHB. The rate of fungal degradation was lowered with an increase in the copolyester content of the blend. © 1996 John Wiley & Sons, Inc.     

Investigation of viscoelastic focusing of particles and cells in a zigzag microchannel

Microfluidic particle focusing has been a vital prerequisite step in sample preparation for downstream particle separation, counting, detection, or analysis, and has attracted broad applications in biomedical and chemical areas. Besides all the active and passive focusing methods in Newtonian fluids, particle focusing in viscoelastic fluids has been attracting increasing interest because of its advantages induced by intrinsic fluid property. However, to achieve a well-defined focusing position, there is a need to extend channel lengths when focusing micrometer-sized or sub-microsized particles, which would result in the size increase of the microfluidic devices. This work investigated the sheathless viscoelastic focusing of particles and cells in a zigzag microfluidic channel. Benefit from the zigzag structure of the channel, the channel length and the footprint of the device can be reduced without sacrificing the focusing performance. In this work, the viscoelastic focusing, including the focusing of 10 μm polystyrene particles, 5 μm polystyrene particles, 5 μm magnetic particles, white blood cells (WBCs), red blood cells (RBCs), and cancer cells, were all demonstrated. Moreover, magnetophoretic separation of magnetic and nonmagnetic particles after viscoelastic pre-focusing was shown. This focusing technique has the potential to be used in a range of biomedical applications.     

Chemistry of constrained dioxocyclam ligands with Co(III): unusual examples of C-H and C-N bond cleavage

The reactions between H(2)dc3 and Co(acac)(3) have been studied in the presence and absence of base. In the presence of base, a complex with an intramolecular Co-C bond, Co(dc3-C-(8))(H(2)O), 1, is formed, presumably through heterolytic C-H bond activation. An X-ray crystallographic study demonstrates the presence of a Co-C bond and shows that the diazacyclooctane (daco) subunit adopts the chair-boat conformation with respect to the metal. The cobalt-carbon bond induces strain in the macrocycle as demonstrated by bond angles significantly deviating from tetrahedral. The (13)C NMR resonance of the carbon atom bound to cobalt (-10.5 ppm) suggests significant ionic character in the cobalt-carbon bond. However, we were unable to cleave this bond in the presence of strong acid. In the absence of base, the reaction of Co(acac)(3) with H(2)dc3 resulted in C-N cleavage of the ligand and the formation of a complex of dioxocyclam, Co(dc)(acac), 2. This complex has subsequently been prepared in high yield by the reaction of Co(acac)(3) with dioxocyclam. An X-ray crystallographic study demonstrates that dioxocyclam adopts the heretofore unreported cis configuration, having folded along a N-Co-N axis that is perpendicular to the Co-acac plane.     

The accurate calculation of dipole moments and dipole polarizabilities using Gaussian-based density functional methods

Dipole moments and static dipole polarizabilities have been calculated for a number of small molecules using the linear combination of Gaussian-type orbitals–local spin density method. The effect of augmenting standard orbital basis sets with polarization functions has been investigated. A set of optimum ζ_d, for use in calculating polarizabilities, has been derived for the first-row atoms C, N, O, and F. The results of this optimized doubly polarized double-zeta basis set compare well with results obtained using a double-zeta basis set augmented by four even-tempered ζ_d polarization functions. The results of the optimized basis set, and a basis set augmented with only a single ζ_d polarization function derived from it, compare very favorably with those obtained from Møller–Plesset perturbation theory and with experimental data. They show a marked improvement on results obtained using standard Hartree–Fock self-consistent-field molecular orbital methods where no treatment of electron-correlation is included.     

在Al-P-Ti-Si-O催化剂上由甲醇和邻苯二酚合成愈创木酚的研究

采用非均匀沉淀法制备了Al1P1.30TiaSib(a=0-0.51,b=0-0.90）复合催化剂,研究了邻苯二酚与甲醇在该催化剂上的气相o-甲基化反应。结果表明,催化剂组成、相结构和表面性质对催化剂的活性和选择性有很大影响。其中Al1P1.30Ti0.30si0.17催化剂的愈创木酚收率最高,在反应温度为553K时,邻苯二酚转化率和愈创木酚收率分别为92%和86%。     

Biosynthesis of Streptolidine Involved Two Unexpected Intermediates Produced by a Dihydroxylase and a Cyclase through Unusual Mechanisms

Streptothricin‐F (STT‐F), one of the early‐discovered antibiotics, consists of three components, a β‐lysine homopolymer, an aminosugar D ‐gulosamine, and an unusual bicyclic streptolidine. The biosynthesis of streptolidine is a long‐lasting but unresolved puzzle. Herein, a combination of genetic/biochemical/structural approaches was used to unravel this problem. The STT gene cluster was first sequenced from a Streptomyces variant BCRC 12163, wherein two gene products OrfP and OrfR were characterized in vitro to be a dihydroxylase and a cyclase, respectively. Thirteen high‐resolution crystal structures for both enzymes in different reaction intermediate states were snapshotted to help elucidate their catalytic mechanisms. OrfP catalyzes an Fe^II‐dependent double hydroxylation reaction converting L ‐Arg into (3R,4R)‐(OH)₂‐L ‐Arg via (3S)‐OH‐L ‐Arg, while OrfR catalyzes an unusual PLP‐dependent elimination/addition reaction cyclizing (3R,4R)‐(OH)₂‐L ‐Arg to the six‐membered (4R)‐OH‐capreomycidine. The biosynthetic mystery finally comes to light as the latter product was incorporation into STT‐F by a feeding experiment.     

Lithiation of a Silyl Ether: Formation of an ortho‐Fries Hydroxyketone

A hydroxy‐directed alkylation of an N,N‐diethylarylamide using CIPE‐assisted α‐silyl carbanions (CIPE=complex‐induced proximity effect) has been developed using a simple reagent combination of LDA (lithium diisopropylamide) and chlorosilane. A study of the mechanism, and the application of the procedure to an anionic Snieckus–Fries rearrangement for a highly efficient synthesis of the potent phosphatidylinositol 3‐kinase (PI3K) inhibitor LY294002, are reported.     

Transition‐Metal‐Catalyzed CS Bond Coupling Reaction

Sulfur‐containing molecules such as thioethers are commonly found in chemical biology, organic synthesis, and materials chemistry. While many reliable methods have been developed for preparing these compounds, harsh reaction conditions are usually required in the traditional methods. The transition metals have been applied in this field, and the palladium‐catalyzed coupling of thiols with aryl halides and pseudo halides is one of the most important methods in the synthesis of thioethers. Other metals have also been used for the same purpose. Here, we summarize recent efforts in metal‐catalyzed C? S bond cross‐coupling reactions, focusing especially on the coupling of thiols with aryl‐ and vinyl halides based on different metals.     

Hexanuclear Gold(I) Phosphide Complexes as Platforms for Multiple Redox‐Active Ferrocenyl Units

The synthesis, X‐ray crystal structures, electrochemical, and spectroscopic studies of a series of hexanuclear gold(I) μ₃‐ferrocenylmethylphosphido complexes stabilized by bridging phosphine ligands, [Au₆(P?P)_n(Fc‐CH₂‐P)₂][PF₆]₂ (n=3, P?P=dppm (bis(diphenylphosphino)methane) ( 1 ), dppe (1,2‐bis(diphenylphosphino)ethane) ( 2 ), dppp (1,3‐bis(diphenylphosphino)propane) ( 3 ), Ph₂PN(C₃H₇)‐PPh₂ ( 4 ), Ph₂PN(Ph‐CH₃‐p)PPh₂ ( 5 ), dppf (1,1′‐bis(diphenylphosphino)ferrocene) ( 6 ); n=2, P?P=dpepp (bis(2‐diphenylphosphinoethyl)phenylphosphine) ( 7 )), as platforms for multiple redox‐active ferrocenyl units, are reported. The investigation of the structural changes of the clusters has been probed by introducing different bridging phosphine ligands. This class of gold(I) μ₃‐ferrocenylmethylphosphido complexes has been found to exhibit one reversible oxidation couple, suggestive of the absence of electronic communication between the ferrocene units through the Au₆P₂ cluster core, providing an understanding of the electronic properties of the hexanuclear Au^I cluster linkage. The present complexes also serve as an ideal system for the design of multi‐electron reservoir and molecular battery systems.