Evolution of strategies to achieve baseline separation of ten anionic, water-soluble sulfated estrogens via achiral packed column supercritical fluid chromatography.

Near baseline separation of ten sulfated sodium salts of various structurally related estrogens employing a variety of bonded stationary phase packed columns was obtained using a conventional supercritical fluid chromatograph coupled with UV detection. Critical pairs 2/3 (8,9-dehydroestrone/17ß-dihydroequilin) and 6/7 (17α-estradiol or 17α-dihydroequilin/estrone), however, failed to baseline separate. In all preliminary separations, 10mM ammonium acetate and variable percentages of H2O were initially used as co-additives in conjunction with methanol as a modifier. Different modifier programs and temperatures were employed to optimize the separation in a timely manner. A 2-ethylpyridine column provided the best separation compared to bare silica, diol, and cyano-based bonded phase columns. The employment of both salt and water as additives to the methanol-modified CO2 mobile phase suggested a mixed mode separation mechanism involving both ion pairing of each anionic sulfated estrogen with ammonium ion and hydrophilic interaction facilitated by partitioning of analyte between the aqueous solvated stationary phase and the aqueous component of the mobile phase. Upon more extensive study with either iso-propylamine or formic acid-ammonium formate buffer, the critical anionic pairs were 95% baseline resolved.

Binding of ethidium to DNA measured using a 2D diffusion-modulated gradient COSY NMR experiment.

The binding of ethidium bromide to a DNA hairpin (dU(5)-hairpin) was investigated using a novel 2D diffusion-modulated gradient correlation spectroscopy (DMG-COSY) experiment to evaluate the applicability of this technique for studying the binding of drugs to DNA. The DMG-COSY experiment includes a preparation period during which coherent magnetization is attenuated due to molecular self-diffusion. Magnetization then evolves due to scalar coupling during an evolution delay, and is detected using gradient pulses for coherence selection. The time-domain data are processed in an analogous manner as for gradient-selected COSY experiments. The diffusion coefficient for uridine in DMSO solution was determined from the H5-H6 crosspeak intensities for a series of 2D DMG-COSY experiments that differed in the magnitude of the gradient pulses applied during the preparation period of the DMG-COSY experiment. The diffusion coefficient for uridine calculated from the DMG-COSY experiments was identical (within experimental error) to that determined from 1D diffusion experiments (5.24x10(-6) cm(2)/s at 26 degrees C). The diffusion coefficients for ethidium bromide and for the dU(5)-hairpin were first measured separately using the DMG-COSY experiment, and then measured in the putative complex. The diffusion coefficient for free ethidium bromide (4.15x10(-6) cm(2)/s at 26 degrees C) was considerably larger than for the dU(5)-hairpin (1. 60x10(-6) cm(2)/s at 26 degrees C), as expected for the smaller molecule. The diffusion coefficient for ethidium was markedly decreased upon addition of the dU(5)-hairpin, consistent with complex formation (1.22x10(-6) cm(2)/s at 26 degrees C). Complex formation of 1:1 stoichiometry between ethidium and the stem of the dU(5)-hairpin was verified independently by fluorescence spectroscopy. These results demonstrate the utility of the DMG-COSY experiment for investigating the binding of drugs to DNA in aqueous solution.

Synthesis and structural characterization of mixed aluminum-gallium-offretites.

A new synthesis procedure is reported for the preparation of mixed Al,Ga-offretites over the entire solid solution range 0 < or = Ga/(Ga + Al) < or = 1. The resulting materials are characterized by X-ray powder diffraction, adsorption microcalorimetry and multinuclear solid state NMR. The 29Si MAS-NMR data are consistent with statistical occupancy of the T1 and T2 sites by aluminum and gallium, and also show no positive evidence for preferential siting effects between both framework metals. Isotropic chemical shifts and nuclear electric quadrupolar coupling constants for 27Al and 71Ga have been obtained from a field-dependent analysis of the center of gravity in the MAS-NMR spectra. H-Al,Ga-offretites produced by ammonium exchange and subsequent calcination reveal evidence of partial demetallation of the framework with formation of extra-lattice metal species.

Spectral editing in MAS NMR of aprotic solids. 31P-113Cd cross-polarization and heteronuclear double-quantum filtering studies in II-IV-V2 semiconductor alloys.

Magic-angle-spinning NMR spectra of aprotic solids, ceramics and glasses frequently suffer from poor site resolution due to wide chemical shift distribution effects. In such cases, cross-polarization and heteronuclear double-quantum filtering experiments involving nuclei other than 1H offer unique spectral editing capabilities. The utility of such assignment techniques for examining site populations in semiconductor alloys is demonstrated for the chalcopyrite systems CdGeAs2-xPx, CdSiAs2-xPx and ZnxCd1-xGeP2. The results permit a distinction between local and non-local effects on experimental chemical shift trends and reveal that compositional dependences observed in these alloys are dominated by non-local chemical shift contributions.

Heteronuclear X-Y double quantum MAS NMR in crystalline inorganic solids. Applications for indirect detection and spectral editing of rare-spin resonances.

Heteronuclear double quantum MAS NMR experiments in the solid state, involving pairs of highly abundant 31P spins and the rare spins 113Cd, 77Se, and 29Si, are described for the three crystalline compounds CdSiP2, CdGeP2, and Ag7PSe6. These experiments offer the opportunity of indirectly detecting the rare-spin resonance via chemical shift evolution of heteronuclear double quantum coherence. For suitable cases, this method results in significantly enhanced detection sensitivities. Criteria for favorable indirect detection experiments are established. Besides offering high sensitivity, the pulse sequence also acts as a heteronuclear double quantum filter, hence providing heteronuclear correlation and useful spectral editing for peak assignments.