A mathematical description of retention times for several amino acids on a polyacrylamide resin containing immobilized Ag(I) is presented in this paper. Immobilization of Ag(I) ions onto a chromatography support allows for the expansion of immobilized metal affinity chromatography (IMAC) to include soft acid/soft base interactions. For the retention of methionine, histidine, and tryptophan, a mathematical model is presented that rationalizes the retention behavior of these amino acids on a Ag(I) column as a function of silver ion loading and pH. The pH effect is important in that methionine is retained longer than histidine at pH values below 5, while at pH 7 histidine is retained longer than methionine. The effects of nonspecific interactions with the modified polymer, electrostatic interactions, and soft acid/soft base interactions are also taken into account by the model. This approach may also be useful for modeling the retention times of amino acids on other types of IMAC columns.
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