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ABSTRACT: Bile salt micelles can be employed as a pseudostationary phase in micellar electrokinetic capillary chromatography (MEKC) separations of chiral analytes. To improve MEKC separations of chiral analytes, a molecular level understanding of micelle aggregation in the presence of analyte is needed. Here, aggregation of sodium cholate has been observed by exploiting the presence of a model analyte molecule. The 31P and 1H nuclear magnetic resonance spectroscopy (NMR) chemical shifts of (R,S)-1,1'-binaphthyl-2,2'-diylhydrogenphosphate ((R,S)-BNDHP), a model analyte in chiral MEKC separations, are demonstrated to be very sensitive to the aggregation state of the bile salt sodium cholate. In addition to probing micellar aggregation, the NMR spectral resolution of enantiomeric species is also stronglycorrelated with chiral separations in MEKC. In this work, the aggregation of sodium cholate in basic solutions (pH 12) has been observed over the concentration range 0-100 mM. The primary critical micelle concentration (cmc) was found to be 14 +/- 1 mM for basic solutions of sodium cholate. In addition, a primitive aggregate is clearly observed to form at 7 +/- 1 mM sodium cholate. The data also show pseudo-cmc behavior for secondary aggregation observed in the regime of 50-60 mM cholate. Finally, the H5-H7 edge of BNDHP is shown to be sensitive to chirally selective interactions with primary cholate micelles.
Langmuir 12/2008; 24(24):13866-74. · 4.19 Impact Factor
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ABSTRACT: Solid-state NMR spectroscopy and ab initio computational chemistry are used to determine the structure of the complex formed upon adsorption of the mononucleotide 2'-deoxyadenosine 5'-monophosphate (dAMP) to the surface of a mesoporous alumina. In this multi-technique approach, rotational-echo double-resonance NMR results reveal that the phosphate group of dAMP interacts predominantly with octahedrally coordinated aluminum species at the surface, and therefore, adsorption is modeled with both mono- and bidentate sorption of the nucleotide phosphate group with octahedral aluminum. 31P chemical shielding tensors are calculated from the structure of the lowest energy conformations, and these results are compared to tensor values extracted from analysis of spinning-sideband patterns in the experimental 31P cross-polarization magic-angle-spinning NMR spectrum. The chemical shift anisotropy and asymmetry parameter indicate that the binding is via a monodentate, inner-sphere complex.
Langmuir 11/2006; 22(22):9281-6. · 4.19 Impact Factor
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ABSTRACT: Solid-state 19F nuclear magnetic resonance (NMR) spectroscopy is used for the quantitative investigation of accessible hydroxyl sites on low surface area glass fibers. Samples with surface areas as low as 0.2 m2/g are investigated through covalent binding of (3,3,3-trifluoropropyl)dimethylchlorosilane. 19F is an ideal nucleus for solid-state NMR, as it has a nuclear spin of 1/2 and a natural isotopic abundance of 100%. High-speed MAS techniques (with rotor spinning frequencies greater than 15 kHz) sufficiently average the CSA and any strong dipolar couplings to allow for superior resolution, especially from terminal -CF3 groups. Studies of two model silica gels with higher surface area, but different pore sizes, provide chemical shift and spin-lattice relaxation rate parameters for probe molecules bound within different environments: pores approaching the size of the probe molecule and pores much larger than the molecular size where intermolecular interactions are assumed to be at a minimum. Resonances assignable to both types of binding environments are found in the spectra of similarly functionalized low surface area fibers. Accessible hydroxyl coverages in the range of 0.8-1.3 OH/nm2 have been measured, and an initial discussion of fiber surface roughness and microporosity is advanced.
Journal of the American Chemical Society 04/2003; 125(9):2378-9. · 9.91 Impact Factor
