Separation of tocopherol isomers using capillary electrochromatography: comparison of monomeric and polymeric C30 stationary phases.

The comparison of tocopherol isomer separation achieved using different stationary-phase alkyl chain lengths (i.e., C18 vs C30) and modes of alkyl phase attachment to the silica-based supports (i.e., polymeric vs monomeric synthesis) using capillary electrochromatography (CEC) has been demonstrated. The smaller alkyl chain does not exhibit the shape selectivity needed to resolve all of the tocopherol isomers. Conversely, both polymeric and monomeric C30 phases show increased tocopherol isomer selectivity. Changing the elutropic strength of the mobile phase had dramatic effects on the selectivity, with small additions of water to a methanol mobile phase yielding the best selectivity. The complete baseline separation of the tocopherol isomers was achieved using the monomerically bound C30 stationary phase and a methanol/ water mobile phase. The differences in stationary-phase selectivity were examined using a NIST standard reference material for determining column selectivity in LC. The results indicated that the monomeric C30 actually had "intermediate" phase characteristics (i.e., high phase loading, end capping, etc). This new CEC-based separation was also used to separate the tocopherols in a vitamin E supplement sample.

NMR and molecular mechanics study of pyrethrins I and II.

Bioassay-directed fractionation of the organic extract of the Kenyan pyrethrum flowers (Chrysanthemum cinerariaefolium Vissiani) resulted in the isolation of two natural pyrethrin esters, pyrethrin I (PI) and pyrethrin II (PII) as the major constituents. These esters elicited inhibition of the multiple drug resistant (MDR) Mycobacterium tuberculosis. The high-field (1)H and (13)C nuclear magnetic resonance (NMR) chemical shifts of PI and PII were unequivocally assigned using modern two-dimensional (2D) proton-detected heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple-bond correlation (HMBC) experiments. The conformations of both esters were deduced from (1)H-(1)H vicinal coupling constants and confirmed by 2D nuclear Overhauser effect spectroscopy (NOESY). Computer molecular modeling (MM) studies revealed that PI and PII molecules adopt a "love-seat" conformation in chloroform (CDCl(3)) solution.