Utilization of Soft Acid/Base Interactions in Low Molecular Weight Biochemical Separations

Categorization of acid/base interactions using hard soft acid base (HSAB) theory suggested that sulfur-containing low molecular weight biological molecules could be specifically targeted for reversible complexation using soft metal ions. A viable method of employing soft metal ions for bioseparations is to immobilize Ag(I) and Pt(II) ions using a soft ligand such as thiourea. This immobilization chemistry allows for the use of Ag(I) columns that are stable in the presence of chloride and phosphate ions in the mobile phase, and it enhances the complexation chemistry of Ag(I) and Pt(II) ions toward solutes which are soft bases. Because chloride ions are soft bases, NaCl can be used for competitive elution. However, in amino acid separations, electrostatic and hydrophobic interactions influence the selectivity and capacity of Ag(I) and Pt(II) columns. A detailed study of the effects of Ag(I) ion loading and pH on the retention time of methionine, histidine, and tryptophan illustrates the need for accounting for Lewis acid/base, electrostatic, and hydrophobic interactions in biological molecule separations.