Designing higher surface area metal-organic frameworks: are triple bonds better than phenyls?

We have synthesized, characterized, and computationally validated the high Brunauer-Emmett-Teller surface area and hydrogen uptake of a new, noncatenating metal-organic framework (MOF) material, NU-111. Our results imply that replacing the phenyl spacers of organic linkers with triple-bond spacers is an effective strategy for boosting molecule-accessible gravimetric surface areas of MOFs and related high-porosity materials.     

Structure-property relationships in poly(n-vinyl pyrrolidone)-phenoxy-water gels

Miscible 80/20 and 90/10 by weight blends of poly (n-vinyl pyrrolidone) [PNVP] and phenoxy polymer were swollen with 73–85% water to produce rubbery hydrogels with shear moduli as high as 10³dyne/cm². Small-angle x-ray scattering, rheological, and calorimetric measurements showed that the gel consisted of tie chains, most probably of PNVP embedded in glassy phenoxy particles with radius of gyration 50–200 Å. A ternary phase diagram was calculated assuming athermal mixing between PNVP–H₂O and PNVP and the known endothermic interaction between H₂O–phenoxy. Phase separation into a phenoxy phase containing minimal H₂O and PNVP and a water-swollen PNVP phase was predicted in accordance with experimental results. Phase separation was suspected as forming first an interconnected phenoxy phase which was later dispersed by swelling forces transmitted through PNVP tie chains. The presence of localized internal stress in the swollen network was thought to be responsible for the irreversible decrease in the shear moduli observed at shear strain above 10%.     

Screening for proteolytic activities in snake venom by means of a multiplexing electrospray ionization mass spectrometry assay scheme

A multiplexed mass spectrometry based assay scheme for the simultaneous determination of five different substrate/product pairs was developed as a tool for screening of proteolytic activities in snake venom fractions from Bothrops moojeni. The assay scheme was employed in the functional characterization of eight model proteases. Time-resolved reaction profiles were generated and the relative reaction progress at each time point was determined. These were used to semi-quantitatively sort the catalytic activities of each enzyme towards the respective substrates into six classes. The resulting activity pattern served as an activity fingerprint for each enzyme. The multiplex assay scheme was then applied to a screening for proteolytic activities in fractions of the pre-separated venom from B. moojeni. Activity patterns of each fraction were generated and used to sort the fractions into three different categories of activity. By comparison of the fingerprint activity patterns of the venom fractions and the model enzymes, a compound with proteolytic properties similar to activated protein C was detected.     

A silica gel orthogonal high-performance liquid chromatography method for the analyses of steviol glycosides: novel tetra-glucopyranosyl steviol

A silica gel orthogonal method using acetonitrile: water was developed for the analyses of fractions rich in very polar steviol glycosides and resolve regions of co-elution of these compounds in reversed-phase. Additionally, we also used this normal phase analytical method to scale up the purification process of steviol glycosides. Using these approaches, one novel minor tetra-glucopyranosyl diterpene glycosides together with three known compounds were purified from a commercial Stevia rebaudiana leaf extract. Compound 1 was unambiguously elucidated as 13-[(2-O-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy]ent-kaur-16-en-19-oic acid-(6-O-β-D-glucopyranosyl-β-D-glucopyranosyl) ester (rebaudioside Y) based on high-performance liquid chromatography retention times, tandem mass spectrometry dissociation pattern and 1D and 2D NMR experiments. Known compounds were isolated in gram quantities and identified as rebaudioside D, E and M.     

Comparison of chromatographic band profiles obtained under microwave irradiated and non-irradiated reversed-phase liquid chromatography column

The possible influence of the application of microwave energy to a reversed-phase liquid chromatography column on the mass transfer kinetics and the thermodynamics of equilibrium between mobile and stationary phases was examined. Chromatograms of propylbenzene and phenol were recorded under the same experimental conditions, on the same column, successively irradiated and not. The effect of microwave irradiation on the mass transfer kinetics was determined by measuring the second moment of small pulses of propylbenzene in a 70:30 (v/v) solution of methanol in water and microwave outputs of 15 and 30 W. The effect of microwave irradiation on the equilibrium thermodynamics was determined by measuring the elution time of breakthrough curves of phenol at high concentrations in a 20:80 (v/v) solution of methanol and water and microwave outputs of 15, 50, and 150 W. A qualitative comparison of the profiles of the propylbenzene peaks obtained with and without irradiation suggests that this irradiation affects significantly the peak shapes. However, a qualitative comparison of the profiles of the breakthrough curves of phenol obtained with and without irradiation suggests that this irradiation has no significant effect on their shapes. The peak sharpening observed may be due to an increase in the diffusivity, resulting from the dielectric polarization under microwave irradiation. This effect is directly related to an increase of the rate of mass transfers in the column. In contrast, the similarity of the overloaded band profiles at high concentrations suggests that the equilibrium thermodynamics is unaffected by microwave irradiation. This may be explained by the transparence of the stationary phase to microwaves at 2.45 GHz. The column temperature was measured at the column outlet under irradiation powers of 15, 30, 50, and 150 W. It increases with increasing power, the corresponding effluent temperatures being 25+/-1, 30+/-1, 35+/-1, and 45+/-1 degrees C, respectively.     

Short-time mobility of spherical particles in concentrated aqueous dispersions determined by diffusing wave spectroscopy

Summary The diffusion of optically dense, charge stabilized, polystyrene latex spheres of 500 nm radius in water at 25°C was determined by diffusing wave spectroscopy in the backscattering mode. The diffusion coefficient of the latex spheres had a value of 4.8×10^–13 m² s^–1 at infinite dilution and monotonically decreased as the number density of latex spheres increased. The diffusion coefficient dropped by a factor of 3 at 38 volume % latex and a factor of 20 at 59 volume % latex. Comparison was made to diffusion of sterically stabilized poly(methyl methacrylate) latex in refractive index matched mixed organic fluids as determined by dynamic light scattering. The concentration dependence determined by diffusing wave spectroscopy matched closely to the short-time diffusion determined by dynamic scattering and not to the long-time diffusion. Although the difference in repulsive potential between the charge and the sterically stabilized lattices must be large, it has a surprisingly small effect on the translational diffusion of the latex over distances small compared to the intersphere distance.