Evaluation of 4-substituted styrenes as functional monomers for the synthesis of theophylline-specific molecularly imprinted polymers

Abstract

Six novel functional monomers: 4-(4-vinylphenyl)pyridine (M1), 4′-vinylbiphenyl-4-ol (M2), N,N-dimethyl-4′-vinylbiphenyl-3-amine (M3), (4′-vinylbiphenyl-4-yl)methanol (M4), 4′-vinylbiphenyl-4-carboxylic acid (M5) and 4-hydroxy-5-methyl-4′-vinylbiphenyl-3-carboxylic acid (M6), were examined for their ability to imprint theophylline (1). Using a molecular modelling-NMR titration approach, M2 and M6 were predicted to give rise to the most specific molecularly imprinted polymers (MIPs). Rebinding analysis suggests that no imprinting effect resulted from the polymerisation of monomers M1, M5 and M6, but modest to good levels of imprinting were evident from monomers M2, M3 and M4 with IF values ranging from 1.1 (MIP_M3, 20 mg) to 45 (MIP_M2, 10 mg). The selective recognition of 1 varied as a function of polymer mass used. At low polymer loadings MIP_M2 gave the very high IF of 45, reducing to IF = 4.1–2.3 at 20–40 mg polymer loading. With monomer M2, microwave synthesised MIP (MW-MIP_M2) was examined. The MW-MIP_M2 displayed lower specific rebinding than its conventionally produced counterpart (MIP_M2) with IF values ranging from 1.6–2.3 (cf., MIP_M2 IF 2.3–45), but significantly higher levels of rebinding with 25–52% of 1 rebound from a 0.080 mM CH₃CN solution of 1 (cf., MIP_M2 5–25%). MW-MIP_M2 displayed a lower BET surface area than MIP_M2 (185 m² g⁻¹vs. 240 m² g⁻¹), and lower surface (zeta) potential (−13.1 ± 8.22 mV vs. −31.4 ± 4.84 mV). Freundlich isotherm analysis revealed that MW-MIP_M2 possessed higher affinity binding sites for 1 than MIP_M2 with K_d values of 1.38 and 2.31 respectively. In addition, MW-MIP_M2 also exhibits a higher number of binding sites (N_T) compared to MW-NIP_M2 (0.72 and 0.41 mg g⁻¹, respectively). In specificity studies using caffeine (2), MIP_M2 displayed a two-fold preference for rebinding of 1 and MW-MIP_M2 a five-fold preference for 1 over 2. The quantity of 2 bound in both cases was consistent with non-specific binding events. In competitive rebinding experiments, increased discrimination in favour of 1 over 2 was observed.