Adsorbed polyethylenimine (PEI) of Mw 1,800 and 70,000 on silica (SiO2) dispersions produced flocculated slurry in the pH range of 5 to 12. Adsorbed citrate widens this flocculated pH regime.
It also increases the strength of the interparticle attractive force or the yield stress over the pH range of between 3 and
8. The stronger attractive force is due to particle bridging by the citrate anions bonding with positively charge sites of
the adsorbed PEI layer of the interacting particles at the closest point of interaction. The higher Mw PEI being more strongly attached to the silica particle produced a stronger attractive interparticle force with adsorbed
citrate anions. Via charge balance calculation using contributions from SiO2, PEI, and citrate, the pH of zero charge was found to correspond to the pH of zero zeta potential for PEI of Mw 70,000. This suggests 100% adsorption of PEI and citrate on SiO2. The bridging interaction was confirmed by a linear relationship between yield stress and the square of the limiting citrate
charge content. Adsorbed citrate was found for the first time to play the role of a bridging agent, a result of the positive
charges being located on a more flexible adsorbed layer rather than being fixed to a rigid surface. 相似文献
Transacetylation of diastereomeric pairs of sec. alcohols, derived from macrocyclic lactones of resorcylic acid; 7,β-trans-zearalenols (1,2, full names., (3S,7R and 3S,7S) trans- 3,4,5,6,9,10-octahydro-7,14,16-trihydroxy-3-methyl-1H-2-benzoxacyclotetradecine-1-ones), 7,β-cis-zearalenols (5,6),and 7,β-zearanols (9,10,full names., (3S,7R and 3S,7S) 3,4,5,6,9,10,11,12-decahydro-714,16-trihydroxy-3-methyl-1H-2-benzoxacyclotetradecine-1-ones) by vinylacetate, catalysed by Pseudomonas sp. and Pseudomonas fluorescens lipase in n-heptane and acetonitrile has been studied. It is highly stereoselective; diastereomeric excess of 7β-O-acetates was usually near 100%. KM, Vmax and specificity constant Vmax/KM do not significantly differ for 7β-stereoisomers 2, 6 and 10. However, they notably change on going from n-heptane to acetonitrile, and the specificity constant drops by factor ca 10-100. The enzyme-bound water is assumed to be partly released in the latter solvent, thus diminishing the interaction with the hydrophilic region of the substrates, and affecting the activity. but not the stereoselectivity of lipases. To correlate the structural and conformational properties of these substrates with unexpectedly high diastereoselectivity of enzymatic transacetylation, the solid state structures of 7β-isomers 2, 4, 6, 10 and 12 have been determined by X-ray analysis. The X-ray structure analysis has revealed that the 7 and 7β isomers possess notably different and in some cases almost mirror-image related absolute conformations around the reactive center. The importance of conformational chirality in the hydrophobic region of these substrates for stereoselection in transacylation by microbial lipases is discussed. 相似文献
The structure of cyclopentadienyl(duroquinone)cobalt dihydrate, (C5H5)Co-[(CH3)4C6O2]·2H2O, has been determined by three-dimensional X-ray analysis. The crystal structure consists of discrete cyclopentadienyl(duroquinone)cobalt molecules linked together by a complex network of hydrogen bonds between water molecules and duroquinone oxygen atoms. Each (C5H5)Co[(CH3)4C6O2] molecule consists of a cobalt atom sandwiched between a cyclopentadienyl ring and a duroquinone ring. A detailed comparison of the molecular parameters of this complex with those of closely related complexes is given. Crystallographic evidence that the metal---duroquinone interaction in cyclopentadienyl(duroquinone)cobalt dihydrate is considerably stronger than that in the electronically-equivalent 1,5-cyclooctadiene(duroquinone)nickel complex is given not only by the metal---C(olefin) distances being 0.12 Å (av) shorter in the duroquinone---cobalt complex [viz., 2.104(8) Å vs. 2.222(7) Å] but also by the much greater C2v-type distortion of the duroquinone ring from the planar D2h configuration in free duroquinone. The compound crystallizes with two formula species in a triclinic unit cell of symmetry P
and reduced cell dimensions á = 8.60 Å, b = 9.00 Å, c = 10.15 Å, = 87° 34′, β = 84° 10′, γ = 73° 44′. Least-squares refinement yielded final unweighted and weighted discrepancy factors of R1 = 10.8% and R2 = 12.0%, respectively, for 2481 independent diffraction maxima collected photographically. 相似文献
The interactions between some acrylic and sulphonic polyanions and some protonated amines (diamines NH2-(CH2)x-NH2, x=2,…,10; linear tri-, tetra-, penta- and hexa-amines) were studied potentiometrically in aqueous solution, at 25°C. For both types of polyanions AL2Hi (L−, monomer of polyanion, A, amine) species are formed, with i=1,…,n (n=number of amino groups in the amine). The stability of these species is strictly dependent on the polyammonium cation charge, and fairly independent of the type of amine (in diamine species maximum stability is observed for x=4, 5). Acrylic and sulphonic polyanion complexes are considerably stronger than analogous species formed by low molecular weight anions. Mean stability can be expressed as log K=2.87ζ2/3, for polyacrylic anions and log K=2.42ζ2/3 for polysulphonic anions (ζ=absolute value for charge product of reactants). 相似文献
The surface of AISI 316 grade stainless steel (SS) was modified with a layer of poly(ethylene glycol) (PEG) (molecular weight 5000) with the aim of preventing protein adsorption and bacterial adhesion. Model SS substrates were first modified to introduce a very high density of reactive amine groups by the adsorption of branched poly(ethylenimine) (PEI) from water. Methoxy-terminated aldehyde-poly(ethylene glycol) (M-PEG-CHO) was then grafted onto the PEI layers using reductive amination at the lower critical solution temperature (LCST) of the PEG in order to optimize the graft density of the linear PEG chains. The chemical composition and uniformity of the surfaces were determined using X-ray photoelectron spectroscopy (XPS) and time-of-flight static secondary ion mass spectrometry (ToF-SSIMS) in the imaging mode. The effects of PEI concentration and different substrate pre-cleaning methods on the structure and stability of the final PEG layer was examined. Piranha solution proved to be the most effective method for removing adventitious hydrocarbon contamination, compared to cleaning with ultrasonication in organic solvents, and was the SS substrate that produced the most stable and thickest PEI layer. The surface density of PEI was shown to increase with increasing PEI concentration (up to 30 mg/ml), as determined from XPS measurements, and subsequently produced the PEG layer with the highest density of attached chains. In model experiments using β-lactoglobulin no protein adsorption was detected on the optimized PEG surface as determined by XPS and ToF-SSIMS analysis. However, neither the adhesion of a Gram-negative (Pseudomonas sp.) nor a Gram-positive (Listeria monocytogenes) bacterium was affected by the coating as equal numbers adhered to all surfaces tested. Our results show that preventing protein adsorption is not a prerequisite stopping bacterial adhesion, and that other mechanisms most likely play a role. 相似文献
The complex Fe(η6-C5H5CMe3)2 crystallizes in the centrosymmetric triclinic space group P
(Ci1; No. 2) with unit cell dimensions of a 8.770(1) Å, b 8.878(1) Å, c 11.991(1) Å, 107.56(1)°, β 90.85(1)°, γ 90.13(1)°, V 890.0(2) Å3 and Z = 2. A full sphere of data was collected on a four-circle diffractometer. The structure was solved and refined to R 7.93% for all 3155 independent reflections and R 4.98% for those 2002 data with | F0 | > 6σ. | F0 |. The molecules lie on crystallographic inversion centers at 0, 0, 0 and 1/2, 0, 1/2; the crystallographic asymmetric unit therefore consists of two independent half molecules. The molecule centered at 0, 0, 0 (molecule “A”) is ordered and well-defined; that centered on 1/2, 0, 1/2 (molecule “B”)is probably disordered, as indicated by larger “thermal parameters” and a greater range of apparent interatomic distances. Discussion em phasizes the geometry of molecule A, which has precise Ci symmetry with Fe(1A)-B(1A) 2.297(4) Å and Fe(1A)-C(ring) distances ranging from 2.057(6) Å to 2.138(4) Å. 相似文献
Ion-exclusion chromatography–cation-exchange chromatography was developed for the simultaneous separation of common inorganic anions and cations (Cl−, NO3− and SO42−; Na+, NH4+, K+, Mg2+ and Ca2+) on a weakly acidic cation-exchange column by elution with weak acid. Generally, the resolution among these monovalent cations was only moderate, thereby hindering the determination of these analytes in natural-water samples. Therefore, 18-crown-6 was added to the eluent to improve the resolution. A good separation of these anions and cations on a weakly acidic cation-exchange column was achieved in 30 min by elution with 5 mM tartaric acid/6 mM 18-crown-6/methanol–water (7.5:92.5). The ion-exclusion chromatography–cation-exchange chromatography method developed here was successfully applied to the separation of major anions and cations in an environmental water sample. 相似文献
Solvent isotope effects on the interaction between the hyperbranched cationic polyelectrolyte, polyethylene imine (PEI), and the anionic surfactant sodium dodecyl sulfate (SDS) were investigated using potentiometric titration and eletrophoretic mobility measurements. In the basic pH range, a significantly higher fraction of the amine groups was found to be protonated when the PEI was dissolved in D2O compared to H2O at the same pH/pD. The difference in polymer charge in the two solvents decreases gradually with decreasing pH, and it completely diminishes at around pH = 4. Electrophoretic mobility measurements of PEI/SDS complexes at different pH values correlated very well with these observations. At pH/pD approximately 9 a much higher mobility of the PEI/SDS complexes was found in D2O than in H2O at low surfactant concentrations, and the charge neutralization point shifted to a considerably larger surfactant concentration in heavy water. These results can be explained by the significantly higher charge density of the PEI in D2O compared to H2O. However, at the natural pH/pD as well as at pH = 4 and pD = 4 conditions the PEI molecules have roughly equal charge densities, which result in very similar charged characteristics (mobilities) of the PEI/SDS complexes as well as the same charge neutralization SDS concentration. It can be concluded that extreme care must be taken in the general analysis of those experiments in which weak polyelectrolyte/surfactant aggregates are investigated in heavy water, and then these observations are correlated with structures of the same system in water. 相似文献
Irradiation of the 30-electron Mo2(η5-C5Me5)2(CO)4 and Re2(CO)10 in toluene solution (containing H2O) afforded (in 1–2% yields) a novel triangular metal cluster, (η5-C5Me5)3Mo3(CO)4(η2-H)(η3-O) (1), which was characterized by a single-crystal X-ray diffraction study. Compound 1, of pseudo Cs-m symmetry, has a triangulo-Mo3(η3-O) core with composite Mo---H---Mo and Mo---Mo electron-pair bonds along one unusually short edge (2.660(1) Å) and Mo--- electron-pair bonds along the other two edges (2.916(1) and 2.917(1) Å). The edge-bridged hydride ligand, which displays a characteristic high-field proton NMR resonance at δ −17.79 ppm, was not found from the crystallographic determination but was located via a quantitative potential-energy-minimization method. This procedure unambiguously established that the optimized hydrogen position, which corresponds to a distinct coordination site with identical Mo---H distances of 1.85 Å, is the only one that can be sterically occupied by a metal-bound hydride ligand. This 46-electron species is the first electron-deficient trimolybdenum cluster containing a monoprotonated Mo---Mo double bond; its existence is attributed to ligand overcrowding due to the bulky pentamethylcyclopentadienyl rings. Black (η5- C5Me5)3Mo3(CO)4(η2-H)(η3-O) · 1/2THF crystallizes with two formula species in a triclinic unit cell of P1 symmetry with a 8.603(4), b 11.115(4), c 19.412(11) Å, 80.69(4)°, β 101.10(4)°, and γ 98.88(3)° at −40° C. Least-squares refinement (RAELS with 221 variables) of one independent Mo3 molecule and a centrosymmetrically-disordered THF molecule converged at R1(F) 5.62%, R2(F 6.88% for 8460 independent diffractometry data (I0 ρ 3σ(I0 collected at −40° C with Mo-K radiation 相似文献
An electrodialysis method is described for the off-line neutralization of strongly alkaline samples containing trace levels of common inorganic anions. This method uses an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes. These compartments comprise an anode compartment housing, a platinum wire anode and 10 ml of a suitable hydrogen ion donating medium, a sample compartment which contains 1 ml of the alkaline sample, and a cathode compartment housing a platinum wire cathode and a dilute solution of sodium hydroxide. During electrodialysis at either constant applied current or constant applied power, hydrogen ions from the anode compartment displace sodium ions from the sample, thereby effecting neutralization.
Experimental parameters, such as the magnitude of the applied current or power, the type of cation-exchange membrane used and the design of the cell have been studied and optimum results were obtained using a Neosepta CM-2 membrane, of area 616 mm2 supported between two perspex discs, with an applied current of 150 mA or applied power of 3 W. Under these conditions, a 1 ml sample of 1 M sodium hydroxide could be neutralized in 11 min. The most effective hydrogen ion donating medium consisted of a 2:1 (w/v) slurry of BioRad AG 50W-X8 (200–400 mesh, H+ form) cation-exchange resin in 1 mM octanesulfonic acid. Recoveries of solute anions (3–10 μg/ml) from the dialysed solution were close to quantitative, except for fluoride and nitrite, which gave recoveries of less than 60%. It is suggested that low recoveries for these ions are due to formation of neutral, protonated species within the membrane with subsequent loss by diffusion. 相似文献
The dielectric permittivity tensor components, εII and ε⊥, in the nematic phase of 6CB (4-n-hexyl-4'-cyanobiphenyl) were measured in the pressure range 0.1-130 MPa and the temperature range 12-58°C. The dielectric anisotropy, Δε(p, V, T) = εII - ε⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates. 相似文献
Protein A affinity chromatography is often employed as a capture step to meet the purity, yield, and throughput requirements for pharmaceutical antibody purification. However, a trade-off exists between step performance and price. Protein A resin removes 99.9% of feed stream impurities; however, its price is significantly greater than those of non-affinity media. With many therapeutic indications for antibodies requiring high doses and/or chronic administration, the consideration of process economics is critical. We have systematically evaluated the purification performance of cation-exchange, anion-exchange, hydroxyapatite, hydrophobic interaction, hydrophobic charge induction, and small-molecule ligand resins in each step of a three-step chromatographic purification process for a CHO-derived monoclonal antibody. Host cell proteins were removed to less-than-detectable for three processes (cation-exchange-anion-exchange-hydrophobic interaction chromatography, cation-exchange-anion-exchange-mixed cation-exchange chromatography, and cation-exchange-mixed cation-exchange-anion-exchange chromatography). The order of the process steps affected purification performance significantly. 相似文献
The X-ray crystal structure and absolute configuration of (−)436-(η5-C5H5)Fe(CO)(CH3CO)[Ph2PNHCH(Me)(Ph)] have been determined from single crystal diffraction data. The compound crystallizes in the monoclinic space group P21 with two molecules in a unit cell of dimensions a = 10.676(4), b = 8.913(7), c = 13.275(9) Å, and β = 91.36°. The structure was solved by the Patterson method and refined to a final R value of 4.7% using 2299 independent data. The iron atom has distorted octahedral coordination, and the configuration at the iron is found to be (S) for the (−)436 diastereoisomer. The Fe---Cp distances average 2.131 Å, with an Fe-(ring centroid)distance of 1.76 Å. The Fe-acetyl distance is virtually identical to that found in another iron/acetyl complex, but shows substantial variation from other compounds where the nature of the C(=O)R group is changed. Comparison to the Mo-alkyl/Mo-acetyl series is made, and the argument for back-donation in transition metal acyls is strengthened.
The orientation of the acetyl group is determined by a strong NHO intra-molecular hydrogen bond having an NO separation of only 2.86 ». The phosphine ligand has a very short Fe---P bond which could be in part caused by the role of the adjacent nitrogen in hydrogen bonding. The remaining ligand geometry is the same as that found in a recently reported ruthenium structure, although the absolute configurations at the chiral carbons are reversed, with the current compound being designated (S) at this site. 相似文献
The interaction between Mo2(O2CCH3)4, Me3SiI and I2 in THF resulted in oxygen abstraction from the solvent and formation of [Mo2(μ-O)(μ-I)(μ-O2CCH3) I2(THF)4]+[MoOI4(THF)]− and I---(CH2)4---I. The molybdenum complex has been characterized by X-ray diffractometry. Crystal data: triclinic, space group P
, a = 13.827(3) Å; b = 15.803(7) Å; c = 9.950(3) Å; = 93.34(4)°; β = 102.40(2)°; γ = 90.09(2)°; V = 2120(2) Å3; Z = 2; dcalc = 2.559 g cm−3; R = 0.0476 (Rw = 0.0613) for 370 parameters and 3938 data with F02> 3σ(F02). The metal-metal distance in the cation is 2.527(2) Å and indicates a strong interaction. The magnetic behavior is consistent with the assignment of one unpaired electron to the Mo27+ core of the cation and one to the d1 Mo(V) center of the anion. The interaction between Mo(CO)6 and I2 in THF also results in the formation of 1,4-diiodobutane. 相似文献
Three synthesis lots of linear poly(ethyleneimine) (PEI) are compared to a fully hydrolyzed linear PEI (commercially available as PEI “Max”) regarding structure, polyplex formation with plasmid DNA, and transfection of suspension‐adapted HEK‐293E cells. PEI “Max” binds DNA more efficiently than the other PEIs, but it is the least effective in terms of transient recombinant protein yield. One PEI lot is fractionated by means of SEC. The fractions of high‐$\overline {M} _{{\rm n}} $ PEI are the most efficient for complex formation and transfection. Nevertheless, the highest transient recombinant protein yields are achieved with unfractionated PEI. The results demonstrate that the polydispersity and charge density of linear PEI are important parameters for gene delivery to suspension‐adapted HEK‐293E cells.
1-Aza-5-stanna-5-chlorotricyclo[3.3.3.0]undecane crystallizes in the space group P63. The unit cell, with a 8.435(3), c 9.243(2) Å, V 569.5(3) Å3, contains two molecules. The structure was refined to a final R value of 0.057. The molecules CISn(CH2CH2CH2)3N belong to the point group 3, and the Sn, Cl and N atoms lie on the crystallographic 3-axis. The tin atom has a trigonal bipyramidal environment with the nitrogen and chlorine atoms in apical positions. The tin-nitrogen intramolecular interaction is 2.372(29) Å. 相似文献