Electrospray ionization mass spectra of acyl carrier protein are insensitive to its solution phase conformation

Electrospray ionization mass spectrometry (ESI-MS) can be used to monitor conformational changes of proteins in solution based on the charge state distribution (CSD) of the corresponding gas-phase ions, although relatively few studies of acidic proteins have been reported. Here, we have compared the CSD and solution structure of recombinant Vibrio harveyi acyl carrier protein (rACP), a small acidic protein whose secondary and tertiary structure can be manipulated by pH, fatty acylation, and site-directed mutagenesis. Circular dichroism and intrinsic fluorescence demonstrated that apo-rACP adopts a folded helical conformation in aqueous solution below pH 6 or in 50% acetonitrile/0.1% formic acid, but is unfolded at neutral and basic pH values. A rACP mutant, in which seven conserved acidic residues were replaced with their corresponding neutral amides, was folded over the entire pH range of 5 to 9. However, under the same solvent conditions, both wild type and mutant ACPs exhibited similar CSDs (6(+)-9(+) species) at all pH values. Covalent attachment of myristic acid to the phosphopantetheine prosthetic group of rACP, which is known to stabilize a folded conformation in solution, also had little influence on its CSD in either positive or negative ion modes. Overall, our results are consistent with ACP as a "natively unfolded" protein in a dynamic conformational equilibrium, which allows access to (de)protonation events during the electrospray process.     

Trimmed linearizations for structured matrix polynomials

We discuss the eigenvalue problem for general and structured matrix polynomials which may be singular and may have eigenvalues at infinity. We derive condensed forms that allow (partial) deflation of the infinite eigenvalue and singular structure of the matrix polynomial. The remaining reduced order staircase form leads to new types of linearizations which determine the finite eigenvalues and corresponding eigenvectors. The new linearizations also simplify the construction of structure preserving linearizations.     

Iminobiotin Binding Induces Large Fluorescent Enhancements in Avidin and Streptavidin Fluorescent Conjugates and Exhibits Diverging pH-Dependent Binding Affinities

The pH-dependent binding affinity of either avidin or streptavidin for iminobiotin has been utilized in studies ranging from affinity binding chromatography to dynamic force spectroscopy. Regardless of which protein is used, the logarithmic dependence of the equilibrium dissociation constant (K_d) on pH is assumed conserved. However a discrepancy has emerged from a number of studies which have shown the binding affinity of streptavidin for iminobiotin in solution to be unexpectedly low, with the K_d at values usually associated with non-specific binding even at strongly basic pH levels. In this work we have utilized a Bodipy fluorescent conjugate of avidin and an Oregon Green fluorescent conjugate of streptavidin to determine the K_d of the complexes in solution in the pH range of 7.0 to 10.7. The study was made possible by the remarkable fluorescent enhancement of the two fluorescent conjugates (greater than 10 fold) upon saturation with iminobiotin. The streptavidin-iminobiotin interaction exhibited almost no pH dependence over the range studied, with K_d consistently on the order of 10⁻⁵ M. In contrast, under identical experimental conditions the avidin-iminobiotin interaction exhibited the expected logarithmic dependence on pH. We discuss the possible origins for why these two closely related proteins would diverge in their binding affinities for iminobiotin as a function of pH.     

Evidence that epoxide-opening cascades promoted by water are stepwise and become faster and more selective after the first cyclization

A detailed kinetic study of the endo-selective epoxide-opening cascade reaction of a diepoxy alcohol in neutral water was undertaken using (1)H NMR spectroscopy. The observation of monoepoxide intermediates resulting from initial endo and exo cyclization indicated that the cascade proceeds via a stepwise mechanism rather than through a concerted one. Independent synthesis and cyclization of these monoepoxide intermediates demonstrated that they are chemically and kinetically competent intermediates in the cascade. Analysis of each step of the reaction revealed that both the rate and regioselectivity of cyclization improve as the cascade reaction proceeds. In the second step, cyclization of an epoxy alcohol substrate templated by a fused diad of two tetrahydropyran rings proceeds with exceptionally high regioselectivity (endo:exo = 19:1), the highest we have measured in the opening of a simple trans-disubstituted epoxide. The origins of these observations are discussed.     

Evaluation of separation and purification processes in the antibiotic industry

Antibiotic production is a complex capital intensive process, which divides naturally into two segments, fermentation and separation/purification. The separation and purification section is very large as a result of the number of processing steps required (up to 60) and the need to purify and recycle large quantities of organic solvents. Separation and purification is not generic within the antibiotic industry. Not only does each individual antibiotic require a different separation process, but also there are many different separation schemes in use for the same antibiotic. Much research is currently in progress on three relatively new separation techniques on a commercial level, which may lead to substantial reductions in the complexity of the process; chromatography (both conventional preparative HPLC and annular chromatography), supercritical extraction, and various membrane processes.     

Light Reflected from Colored Mulches to Growing Turnip Leaves Affects Glucosinolate and Sugar Contents of Edible Roots

Abstract— Plastic mulches are widely used to conserve water and control weeds with less applied herbicides in production of food crops. Both yield and quality are important and can be affected by reflected blue (B), red (R) and far-red (FR) light combinations received during growth and development. Photosynthate allocation among growing plant parts and flavor of edible roots were studied in turnip ( Brassica rapa L.) grown in trickle-irrigated field plots with blue, green and white mulches. The blue and green mulches reflected different amounts of B, but they both reflected FR/R ratios higher than the ratio in incoming sunlight. The white mulch reflected more photosyn-thetic light and a lower FR/R ratio than the blue or green mulches. Plants grown with blue and green mulches did not differ significantly in leaf length, root size and shoot/ root biomass ratio. Those grown with white had shorter leaves and larger roots. Taste testers found that plants grown with blue mulch developed roots with a sharp flavor, and roots from plants grown with green mulch had a mild flavor. Those grown with white had a less distinct flavor. Roots grown with blue mulch had the greatest concentrations of total glucosinolates (GSL) and ascorbic acid. Reducing sugar concentrations were higher in roots grown with green than in those grown with blue mulches. The comparison of chemical composition of roots from plants grown with blue versus green mulches is important because the main difference was the amount of reflected B, suggesting that B influenced an enzyme involved in the pathway from glucose to GSL. We conclude that the spectrum of light reflected from mulch on the soil surface can influence not only shoot/root biomass ratio but also flavor-related chemical composition of field-grown food crop plants.     

The use of pH to influence regio- and chemoselectivity in the asymmetric aminohydroxylation of styrenes

[reaction: see text] The pH-controlled Sharpless asymmetric aminohydroxylation (AA) of styrenes provides 1-aryl-2-amino ethanols (regioisomer B) with high enantio-, chemo-, and regioselectivity. As existing AA protocols typically give regioisomer A as the major reaction product when using carbamate nitrogen sources, this method is a convenient alternative for the selective production of regioisomer B.     

The perturbation theory of the extended Hartree–Fock approximation for two-electron atoms

The first-order 1/Z perturbation theory of the extended Hartree–Fock approximation for two-electron atoms is described. A number of unexpected features emerge: (a) it is proved that the orbitals must be expanded in powers of Z^?1/2, rather than in Z^?1 as expected; (b) it is shown that the restricted Hartree–Fock and correlation parts of the orbitals can be uncoupled to first order, so that second-order energies are additive; (c) the equation describing the first-order correlation orbital has an infinite number of solutions of all angular symmetries in general, rather than only one of a single symmetry as expected; (d) the first-order correlation equation is a homogeneous linear eigenvalue-type equation with a non-local potential. It involves a parameter μ and an eigenvalue ω(μ) which may be interpreted as the probability amplitude and energy of a virtual correlation state. The second-order correlation energy is 2μ²ω. Numerical solutions for the first-order correlation orbitals, obtained variationally, are presented. The approximate second-order correlation energy is nearly 90% of the exact value. The first-order 1/Z perturbation theory of the natural-orbital expansion is described, and the coupled first-order integro-differential perturbation equations are obtained. The close relationship between the first-order extended Hartree–Fock correlation orbitals and the first-order natural correlation orbitals is discussed. A comparison of the numerical results with those of Kutzelnigg confirms the similarity.