Combined use of NMR relaxation measurements and hydrodynamic calculations to study protein association. Evidence for tetramers of low molecular weight protein tyrosine phosphatase in solution

We describe a novel method for determining weak association constants of oligomeric protein complexes formed transiently under equilibrium conditions. This type of equilibrium process is recognized as being biologically important, but generally hard to study. Heteronuclear spin relaxation rates measured at multiple protein concentrations are analyzed using relaxation rates predicted from hydrodynamic calculations, yielding equilibrium constants and structural characterization of the protein complexes. The method was used to study the oligomerization equilibrium of bovine low molecular weight protein tyrosine phosphatase. X-ray structures of monomeric and dimeric forms of the protein have been reported previously. Using longitudinal and transverse (15)N relaxation rates measured at four different protein concentrations, we detected the monomer, dimer, and a previously unknown tetramer and measured the dissociation constants of the equilibria involving these species. A comparison of experimental and predicted relaxation rates for individual backbone amide (15)N spins enabled delineation of the tetramerization interface. The results suggest a novel concept for substrate modulation of enzymatic activity based on a "supramolecular proenzyme". The fast and reversible switching of the "supramolecular proenzyme" would have obvious advantages for the regulation of enzymes involved in cell signaling pathways.     

Emulsion stability based on phase behavior in sodium naphthenates containing systems: gels with a high organic solvent content

Addition of heptane to a sodium naphthenates/toluene/water system at 25 degrees C reduces the lamellar liquid-crystal phase range and increases the microemulsion phase range. Both of these effects result in the extension of the composition range where emulsions have low stability. This effect is even stronger at 40 degrees C. Heptane addition also results in the formation of very stable emulsions within the overlapping phase-existence ranges of aqueous (L1) and organic (L2) phases. Stable non-birefringent gel observed in equilibrium with L1 and L2 phases contains only a small percentage of water and sodium naphthenates. The swelling behavior of an unstable gel, an emulsion previously compressed by centrifugation, appears to be due to a stepwise thickening of the thin liquid films between the droplets.     

Influence of secondary preparative parameters and aging effects on PLGA particle size distribution: a sedimentation field flow fractionation investigation

Poly(lactic-co-glycolic acid) particles in the 200–400-nm size range were formulated through nanoprecipitation and solvent evaporation methods. Different concentrations of the polymer and stabilizer (Pluronic® F 68) were tested in order to identify the best conditions for making poly(lactic-co-glycolic acid) particles of suitable size, stable in time, and to be used as carriers for brain-targeting drugs. The particles with the best characteristics for delivery system design were those formulated by nanoprecipitation with an organic/water phase ratio of 2:30, a polymer concentration of 25 mg/mL, and a surfactant concentration of 0.83 mg/mL; their surface charge was reasonably negative (approximately -27 mV) and the average size of the almost monodisperse population was roughly 250 nm. Particle characterization was obtained through ζ-potential measurements, scanning electron microscope observations, and particle size distribution determinations; the latter achieved by both photon-correlation spectroscopy and sedimentation field flow fractionation. Sedimentation field flow fractionation, which is considered more reliable than photon-correlation spectroscopy in describing the possible particle size distribution modifications, was used to investigate the effects of 3 months of storage at 4 °C had on the lyophilized particles.

Simultaneous determination of sixteen underivatized biogenic amines in human urine by HPLC-MS/MS

The broad group of biogenic amines includes polyamines and catecholamines, whose presence in human tissues and biological fluids can give important diagnostic information and act as marker of many pathologies. In particular, polyamines are involved in cancer cell growth while catecholamines act as neurotransmitters and hormones. Their simultaneous determination in biological tissues and fluids is therefore an important task. A high-performance liquid chromatography tandem mass spectrometry method is presented here for the simultaneous determination in urine of 16 biogenic amines: adrenaline (epinephrine), agmatine, cadaverine, dopamine, histamine, 3-methoxytyramine, noradrenaline (norepinephrine), norephedrine, octopamine, 2-phenylethylamine, putrescine, serotonin, spermidine, spermine, tryptamine, and tyramine. The method does not require any derivatization step. To guarantee the maximum of sensitivity, the mass spectrometer works in selected reaction monitoring mode, monitoring for each analyte the two most intensive transitions. Method validation includes the evaluation of limits of detection (that range from 0.3 to 6.6 μg L^?1), limits of quantitation (that range from 1.0 to 21.9 μg L^?1), linearity range (three orders of magnitude), recovery, intra- and inter-day precision on both concentration, and retention time. Recovery (R) is shown not to depend on the analyte concentration: the average R percent ranges from 72.9 to 100.0 %. Particular attention is devoted to the matrix effect and the correlated phenomena of ion enhancement or suppression in mass spectrometry detection.

Small-angle scattering study of structural changes in the microfibril network of nanocellulose during enzymatic hydrolysis

The hydrolysis of nanofibrillated cellulose (NFC), consisting of individual cellulose fibrils, was followed using small-angle scattering techniques in order to reveal changes in the substrate structure caused by cellulose degrading enzymes. In particular, the nanoscale structure of the network of cellulose fibrils was characterized with the combination of small-angle neutron scattering and small-angle x-ray scattering. In the nanocellulose with higher xylan content, the interfibrillar distance was shown to remain unchanged during enzymatic degradation, whereas the distance increased in the nanocellulose with lower xylan content. The limiting effect of xylan on the hydrolysis and a faster hydrolysis of the more thoroughly fibrillated segments of the NFC network could be observed. Despite the extensive fibrillation of the raw material, however, the hydrolysis was eventually limited by the aggregated and heterogeneous structure of the substrate.     

Palladium-catalyzed coupling of allyl acetates with aldehyde and imine electrophiles in the presence of Bis(pinacolato)diboron

[reaction: see text] An efficient one-pot procedure was developed for palladium-catalyzed electrophilic substitution of allyl acetates (2a-h) in the presence of bis(pinacolato)diboron (1). These reactions proceed with an excellent regioselectivity and with a remarkably high stereoselectivity. The catalytic transformations take place via palladium-catalyzed formation of allyl boronates, which subsequently react with aldehyde (3) and sulfon-imine (4) electrophiles to afford homoallylic alcohols (5a-h) and amines (6a-d), respectively. A particularly interesting mechanistic feature is that the allylic substitution of the transient allyl boronate with sulfon-imine requires palladium catalysis. This finding indicates that the formation of the homoallylic amine derivatives (6a-d) involves bis-allylpalladium intermediates.     

Polyamine-stabilized sodium aryloxides: simple initiators for the ring-opening polymerization of rac-lactide

Metalation of 2,4,6-tri(methyl)phenol ((Me)ArOH) and 2,6-di(tert-butyl)-4-methylphenol ((Bu)ArOH) with NaN(SiMe(3))(2) in toluene and in the presence of stoichiometric amounts of the polydentate amines N,N,N',N'-tetramethylethylenediamine (TMEDA) and N,N,N',N',N'-pentamethyldiethylenetriamine (PMDETA) affords three new sodium aryloxide complexes [Na(μ-OAr(Bu))(TMEDA)](2) (3), [Na(μ-OAr(Me))(PMDETA)](2) (4), and [Na(OAr(Bu))(PMDETA)] (5). Complexes 3 to 5 have been isolated as crystalline materials in reasonable yields and characterized in the solid state by X-ray crystallography and in solution by NMR spectroscopy. Complexes 3 to 5 and the related [tris(2-dimethylaminoethyl)amine] (Me(6)TREN) derivatives [Na(OAr(Me))(HOAr(Me))(Me(6)TREN)] (1) and [Na(OAr(Bu))(Me(6)TREN)] (2), recently prepared in our group, are shown to be active as initiators for the ring-opening polymerization (ROP) of rac-lactide with benzyl alcohol as a co-initiator. However, during the course of the polymerization reactions intrachain and stereorandom transesterification side-reactions were observed under some of the experimental conditions tested.     

Poly[[tetra‐μ3‐acetato‐hexa‐μ2‐acetato‐diaqua‐μ2‐oxalato‐tetrapraseodymium(III)] dihydrate]

The title complex, {[Pr₄(C₂H₃O₂)₁₀(C₂O₄)(H₂O)₂]·2H₂O}_n, was synthesized under hydrothermal conditions from praseodymium acetate and the ionic liquid 1‐butyl‐3‐methylimidazolium chloride via an in situ oxalate‐ligand synthesis. The compound is a two‐dimensional polymer and in the structure presents tightly bound planes parallel to (100), which are in turn linked into a three‐dimensional network by hydrogen bonds involving both coordinated and solvent water molecules. The oxalate anion lies across an inversion centre and acts as a bridge between pairs of Pr atoms within a tetranuclear segment of the polymer.     

Tautomers of cytosine and their excited electronic states: a matrix isolation spectroscopic and quantum chemical study

We have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated ab initio at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form.