Molecularly imprinted polymers for the isolation of bioactive naphthoquinones from plant extracts

• Molecularly Imprinted Polymers for isolation of shikonin from plant extracts were prepared.
• Strong association of shikonin with both acidic and basic monomers was found by UV–vis titrations.
• Polymerisation inhibition in the presence of basic monomers was investigated by solid state NMR.
• Recoveries up to 72% from commercial shikonin samples were achieved using DEAEMA based imprints.

Molecularly Imprinted Polymers (MIPs) targeting shikonin, a potent antioxidant and wound healing agent, have been prepared using methacrylic acid (MAA) and 2-diethylaminoethyl methacrylate (DEAEMA) as functional monomers. An investigation of solution association between shikonin and both acidic and basic functional monomers by UV–vis titrations, suggested stronger affinity towards the basic functionality. Strong inhibition of the co-polymerisation reaction of such basic monomers was observed, but was overcome by reduction of the amount of template used during polymer synthesis. Polymer morphology was severely impacted by the template's radical scavenging behaviour as demonstrated by solid state NMR spectroscopy measurements. HPLC evaluation of the final materials in polar conditions revealed limited imprinting effects and selectivity, with the MAA polymers exhibiting marginally better performance. During application of the polymers as MI-SPE sorbents in non-polar solvents it was found that the DEAEMA based polymer was more selective towards shikonin compared to the MAA counterpart, while shikonin recoveries of up to 72% were achieved from hexane solutions of a commercial sample of shikonin, hexane extract of Alkanna tinctoria roots and a commercial pharmaceutical ointment.