Imprinted polymers for chiral resolution of (+/-)-ephedrine: understanding the pre-polymerisation equilibrium and the action of different mobile phase modifiers.
A thorough study has been made of the ephedrine-methacrylic acid (MAA) system for molecular imprinting, involving NMR studies of the pre-polymerisation equilibria, modelling and HPLC enantioseparations with different mobile phases. When dimerisation of MAA is accounted for, NMR titrations demonstrate there is a very strong ('stoichiometric', K approximately 10000 M(-1)) interaction between ephedrine and a single MAA monomer. Polymers prepared with a 1:1 monomer:template ratio are capable of enantioseparation, indicating, in combination with the NMR results, that the 1:1 interaction probably involves the carboxylic acid acting as a chelating monomer, forming hydrogen bonds to both the template amine and hydroxyl moieties. Higher monomer:template ratios cause further changes in the NMR signals, suggesting at least one further MAA can interact with the amine group, with a weaker association constant (K approximately 80 M(-1)). Polymers prepared with a 4:1 monomer:template ratio are thus proposed to contain a mixture of 1-monomer binding sites and 2-monomer binding sites, the latter being of enhanced acidity. In HPLC, better results are obtained with the 4:1 polymer using acetic acid as a modifier, while better results are obtained for the 1:1 polymer using butylamine as a modifier. We propose a model whereby acetic acid exerts its effect by reducing binding to the 1-monomer sites, while butylamine works largely through blocking the most acidic, 2-monomer sites. For preparative chromatography we suggest the 1:1 polymer, with a larger population of weaker but more uniform binding sites, is most promising.
Ansell
Richard J
RJ
School of Chemistry, University of Leeds, Leeds, UK. r.ansell@chem.leeds.ac.uk
Kuah
Kim L
KL
D019342:Acetic Acid; D001665:Binding Sites; D002082:Butylamines; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D004809:Ephedrine / Q000032:analysis*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008689:Methacrylates; D011108:Polymers; D013237:Stereoisomerism
D002082:Butylamines; D008689:Methacrylates; D011108:Polymers; C008384:methacrylic acid; D004809:Ephedrine; D019342:Acetic Acid