Thermodynamic studies on solvent effects in molecularly imprinted polymers. 2. Concentration of the organic modifier

The effects of the organic modifier concentration on the isotherm parameters of the two enantiomers of Fmoc-tryptophan (Fmoc-L,D-Trp) on an Fmoc-L-Trp-imprinted polymer were investigated over a wide concentration range (0.005-100 mM), using frontal analysis. The modifier was acetic acid; concentrations of 0.2, 0.9, 1.7, and 3.7 M in an acetonitrile-based mobile phase were studied. At each concentration, adsorption isotherm data were acquired for each enantiomer. From these data, the isotherm parameters of each compound were derived from nonlinear isotherm fitting and the affinity energy distributions were calculated independently. We found that three types of sites coexist for Fmoc-L-Trp but only two types of sites for Fmoc-D-Trp, except at the lowest acetic acid concentration (0.2 M), at which three types of sites coexist. Increasing the acetic acid concentration decreases the selectivity and the overall affinity of both enantiomers. The overall affinity of Fmoc-L-Trp is dominated by the contribution of the low-density highest energy sites while that of Fmoc-D-Trp is dominated by the most abundant, low-energy sites. For the low-energy sites, increasing the acetic acid concentration affects the association constant of the enantiomers more than the number of corresponding sites. In contrast, for the highest energy sites (sites that exist only for Fmoc-L-Trp), increasing the concentration of acetic acid affects significantly the number of sites but hardly changes the association constant.