ABSTRACT
Three poly(ethylene-co-acrylic) acid copolymers (-CH(2)CH(2)-)(x)[CH(2)CH(CO(2)H)-](y) with different chain lengths and mass fractions of acrylic acid were covalently immobilized as stationary phases on silica via two variants of spacer molecules (3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane). Different mobilities of the alkyl chains in the stationary phases were observed using (13)C solid-state NMR spectroscopy. The stationary phases with more rigid trans-ordered alkyl chains had better selectivity for geometric beta-carotene and xanthophyll isomers (provitamin A derivatives). Also, all the separations of the analytes were affected by polar interactions with the chromatographic sorbent. This was further proved by separating more polar cis/trans retinoic acid isomers (vitamin A derivatives). (13)C high-resolution/magic-angle spinning (HR/MAS) NMR measurements of the chromatographic sorbents suspended in the mobile phase confirmed a dependence of molecular shape recognition ability on alkyl chain conformation.
Subject(s)
Acrylic Resins/chemistry , Magnetic Resonance Spectroscopy/methods , Polyethylenes/chemistry , Silicon Dioxide/chemistry , Acrylic Resins/chemical synthesis , Chromatography, High Pressure Liquid/methods , Molecular Structure , Polyethylenes/chemical synthesis , Sensitivity and Specificity , Stereoisomerism , Surface PropertiesABSTRACT
The surface chemistry of a chiral stationary phase (CSP) with a (tert-butyl carbamoyl) quinine selector immobilized on thiol-modified silica has been characterized by (1)H HR/MAS NMR and (29)Si CP/MAS NMR spectroscopy. The mostly well-resolved (1)H signals could be assigned to stem from the surface-bound selector and the latter suggested a bi- and trifunctional silane linkage. Suspended-state NMR spectroscopy thus proved a well-characterized surface chemistry as proposed. To study chiral recognition phenomena in the presence of the CSP, (1)H HR/MAS 2D transfer NOESY investigations in methanol-d(4) have been undertaken with various solutes including N-3,5-dinitrobenzoyl derivatives of leucine (DNB-Leu) and N-acetyl phenylalanine (Ac-Phe). Both (R)- and (S)-enantiomers of DNB-Leu and Ac-Phe interacted with the tBuCQN-CSP as indicated by negative cross-peaks in the trNOESY spectra, while the 2D NOESY of the dissolved solutes in absence of the chiral stationary phase showed positive cross-peaks. The intensities of the trNOE cross-peaks were much stronger for the (S)-enantiomers. This stereoselectivity paralleled the experimental chromatographic behavior, where the (S)-enantiomers revealed stronger binding and retention on the tBuCQN-CSP as well. Hence, we were able to correlate the retention behavior to the trNOE NMR spectroscopic data in a qualitative manner.
Subject(s)
Dinitrobenzenes/chemistry , Leucine/analogs & derivatives , Leucine/chemistry , Phenylalanine/analogs & derivatives , Phenylalanine/chemistry , Chromatography, High Pressure Liquid , Dinitrobenzenes/isolation & purification , Leucine/isolation & purification , Molecular Structure , Nuclear Magnetic Resonance, Biomolecular , Phenylalanine/isolation & purification , Quinine/chemistry , StereoisomerismABSTRACT
A study of the retention behaviour of the enantiomers (R)- and (S)-1,1'-binaphthyl-2,2'-diol as well as (+) and t-)-O,O'-dibenzoyl-tartaric acid was performed using the two chiral stationary phases (CSPs) Kromasil-DMB and Kromasil-TBB. Detailed information about the interactions between the analytes and the two CSPs was obtained from suspended-state HR/MAS transferred NOESY NMR experiments as well as suspended-state HR/MAS 1H NMR titration experiments. Good correlation between the suspended-state HR/MAS NMR experiments and the corresponding HPLC experiments was obtained. This shows that suspended-state HR/MAS NMR as well as solid-state CP/MAS NMR spectroscopy can be used to investigate interactions between stationary phases and analytes under conditions that are similar to those used in HPLC.
