Molecular shape selectivity through multiple carbonyl-pi interactions with noncrystalline solid phase for RP-HPLC.
A new approach for the synthesis of double-alkylated L-glutamide-derived stationary phases to use in RP-HPLC is described. TEM observation of lipid distearylglutamide (DSG) showed the formation of fibrous aggregates in methanol or in chloroform through intermolecular hydrogen bonding among the amide moieties while dibutylglutamide (DBG) cannot aggregate in aqueous or organic media due to its lower order of short alkyl chain. DSG and DBG were covalently bonded to silica via amino-propyl linkages. Lipid membrane analogues (e.g., DSG) attached to the silica surface have been found in noncrystalline and solid states and can form supramolecular assemblies with specific properties based on their highly ordered structures in aqueous and organic media. 13C CP/MAS NMR and suspension (in methanol)-state 1H NMR, elemental analysis, and DSC measurements were used to characterize Sil-DSG and were compared with the three other octadecyl phases, i.e., monomeric C18, polymeric C18, and silica grafted poly(octadecyl acrylate) Sil-ODA25. The chromatographic behavior of the new RP material was investigated using detailed retention studies of planar and nonplanar polyaromatic hydrocarbons (PAHs) and nonpolar aromatic positional isomers. Aspects of shape selectivity were also evaluated with Standard Reference Materials 869a, Column Selectivity Test Mixture for Liquid Chromatography. Detailed chromatographic study revealed that Sil-DSG showed extremely enhanced molecular shape selectivity compared with the other phases investigated. The higher molecular shape selectivity obtained by Sil-DSG can be explained by a carbonyl pi (present in lipid-grafted stationary phases)-benzene pi (present in guest PAHs) interaction mechanism, and these interactions are more effective for ordered carbonyl groups.