Stereoselective Lewis acid-catalyzed alpha-acylvinyl additions

Silyloxyallenes derived from alpha-hydroxypropargylsilanes undergo efficient addition to aldehydes with catalytic amounts of Lewis acids. The allenes are accessed from the corresponding propargylsilanes in a base-catalyzed 1,2-Brook rearrangement/SE2' process. Enantioenriched propargylsilanes are synthesized by a new zinc-promoted addition of alkynes to acylsilanes in up to 74% ee. This work demonstrates that conversion to the silyloxyallenes occurs with minimal erosion in optical activity. The use of a chiral chromium(III) Lewis acid effects the catalytic asymmetric addition of racemic silyloxyallenes to aromatic aldehydes in up to 92% ee. The overall reaction is broad in scope and accommodates a wide variety of aromatic and aliphatic substituents on both the propargylsilane and aldehyde.     

Spectroscopic studies of the corrinoid/iron-sulfur protein from Moorella thermoacetica

Methyl transfer reactions are important in a number of biochemical pathways. An important class of methyltransferases uses the cobalt cofactor cobalamin, which receives a methyl group from an appropriate methyl donor protein to form an intermediate organometallic methyl-Co bond that subsequently is cleaved by a methyl acceptor. Control of the axial ligation state of cobalamin influences both the mode (i.e., homolytic vs heterolytic) and the rate of Co-C bond cleavage. Here we have studied the axial ligation of a corrinoid iron-sulfur protein (CFeSP) that plays a key role in energy generation and cell carbon synthesis by anaerobic microbes, such as methanogenic archaea and acetogenic bacteria. This protein accepts a methyl group from methyltetrahydrofolate forming Me-Co(3+)CFeSP that then donates a methyl cation (Me) from Me-Co(3+)CFeSP to a nickel site on acetyl-CoA synthase. To unambiguously establish the binding scheme of the corrinoid cofactor in the CFeSP, we have combined resonance Raman, magnetic circular dichroism, and EPR spectroscopic methods with computational chemistry. Our results clearly demonstrate that the Me-Co3+ and Co2+ states of the CFeSP have an axial water ligand like the free MeCbi+ and Co(2+)Cbi+ cofactors; however, the Co-OH2 bond length is lengthened by about 0.2 angstroms for the protein-bound cofactor. Elongation of the Co-OH2 bond of the CFeSP-bound cofactor is proposed to make the cobalt center more "Co1+-like", a requirement to facilitate heterolytic Co-C bond cleavage.     

New bioluminogenic substrates for monoamine oxidase assays

Novel bioluminogenic substrates were designed for probing monoamine oxidase (MAO) activity based on a simple and effective beta-elimination strategy. By modifying the amino group and the central core of luciferin derivatives, we have developed a series of substrates useful for assays of MAO A or B, or both. One of these substrates, exhibiting low Km values and high signal-to-background ratios with both isozymes, was shown to accurately measure the Ki values of known MAO inhibitors. This substrate is a key component in the development of a highly sensitive homogeneous MAO assay for high-throughput screening (HTS) of compounds in drug discovery and for monitoring MAO activity in complex biological systems. This design strategy should be applicable to fluorogenic MAO substrates and could broaden the structural requirements of substrates for other enzyme assays.     

Solid-state (55)Mn NMR spectroscopy of bis(μ-oxo)dimanganese(IV) [Mn(2)O(2)(salpn)(2)], a model for the oxygen evolving complex in photosystem II

We have examined the antiferromagneticly coupled bis(μ-oxo)dimanganese(IV) complex [Mn(2)O(2)(salpn)(2)] (1) with (55)Mn solid-state NMR at cryogenic temperatures and first-principle theory. The extracted values of the (55)Mn quadrupole coupling constant, C(Q), and its asymmetry parameter, η(Q), for 1 are 24.7 MHz and 0.43, respectively. Further, there was a large anisotropic contribution to the shielding of each Mn(4+), i.e. a Δσ of 3375 ppm. Utilizing broken symmetry density functional theory, the predicted values of the electric field gradient (EFG) or equivalently the C(Q) and η(Q) at ZORA, PBE QZ4P all electron level of theory are 23.4 MHz and 0.68, respectively, in good agreement with experimental observations.     

Effect of the Supporting Electrolyte on the Adsorption of Octanoic Acid at the Mercury/Electrolyte Interface

The adsorption and the changes in the interfacial composition of octanoic acid at the mercury/electrolyte interface was studied by measuring the differential capacitance at different concentrations of the supporting electrolyte, at various supporting electrolyte systems and at various temperatures. The adsorption was followed by means of capacity-potential curves in the short-term region and capacity-time transients in the long-term region at selected potentials, in all the potential ranges. A decrease of the capacitance with time was observed in most cases, followed either by a constant capacitance value or by its increase. In the short-term region, anion-surfactant complexes are formed, where the anions act as bridges between the perpendicularly oriented surfactant molecules. The larger is the negative charge of the anion, the more negative will be the charge of the anion-surfactant complex leading to a shift of the potential of maximal adsorption to more positive values. The formation of metastable condensed films is best when the hydration of the anion and its size are not too large. In the long-term region the observed increase of the capacity with time can be explained as an exchange of the metastable condensed film by a hemimicellar surface state. Here, the anions act as cores of the hemimicelles, and the hydrophilic acid groups of the amphiphiles contact the solution. Two contrary effects determine the formation of the hemimicelles. The greater is the specific adsorption of the anions, the larger is the formation of hemimicelles and the increase of the capacity. With an increase in the ability of the anions to break the water structure (lyotropic or Hofmeister series), the formation of hemimicelles will be decreased. Copyright 2000 Academic Press.     

Relative komplexe quotienten

Let XS be a holomorphic map, and let RX×SX be an equivalence relation. The restriction of R to the fibre ^–1(S) is denoted by R_s. The quotient X/R is called a relative complex quotient, if the quotient map XX/R is holomorphic over S. Two cases are studied: (C) All fibres of are locally R_s-separable (relative Cartan quotient); (R) All fibres of are holomorphically convex, and R_s is given by tke holomorphic functions on ^–1 (s) (relative Remmert quotient).     

Dissolution of cellulose in ethylene diamine/salt solvent systems

Investigation of the dissolution of cellulose in Ethylene Diamine (EDA)/Potassium thiocyanate (KSCN) solutions by infrared spectroscopy (FTIR) and thermal analysis (DSC) indicated that changes to the solvent during freeze thaw cycling of mixtures was consistent with increased interaction between cellulose and solvent. Thermal transitions in the system, however, occurred at temperatures outside the range used in thermal cycling to promote dissolution. Further exploration of the dissolution and mixing process indicated that mixing was the limiting step in solution formation. The dissolution of two types of cellulose with different molecular weights (Degree of Polymerization (DP)=210 and >1000) was studied using EDA/KSCN solution as the solvent. The solubility and the dissolution rate of cellulose depended on both the solvent composition and cellulose molecular weight. Cellulose could dissolve faster in the solvent with lower salt concentration but the highest cellulose concentration was obtained in the solvent with 30~35% KSCN. Rheological measurements showed that cellulose solutions exhibited viscous solution behavior at low KSCN concentration but primarily elastic behavior at high salt concentration.     

On packing constraints in the formation of surface micelles of amphiphiles at the metal/electrolyte interface-the necessity of introducing a generalized packing parameter

The long-term capacity-time dependence of 1-decanol was measured at the mercury/electrolyte interface for the electrolytes KCl, NaCl, LiBr, and K(2)SO(4), each of concentration 0.1 M, in the temperature range 20 to 50 degrees C and at 35 degrees C for electrolyte concentrations of NaCl in the region 0.05 to 0.4 M. All capacity-time dependences exhibited a slow increase of the capacity after a sudden decrease in the short-term range. The corresponding long-term time dependence of the degree of coverage can be theoretically well described with a first-order surface reaction. The ratios of final equilibrium capacity to minimal capacity can be explained with the formation of hemispherical or bilayer surface micelles from an initial monolayer of perpendicularly adsorbed molecules. A generalized packing parameter that involves the increase or decrease of the head groups' distance due to electrolyte ions or other surfactants was introduced. In the case of bilayers of parallel adsorbed molecules, obviously a new type of transition, which will be discussed in detail, occurs.