Weak Affinity Chromatography Using Organic Solvents on Cyanopropyl- and Arene-Modified Silica-Gel Columns (R) Structural Demands for Host and Guest Moieties and Effect of Solvents

Weak affinity chromatography (WAC) is effective for isolat-ing target compounds from analogous compounds with similar functional groups. Previously, a few chromatographic behav-iors based on WAC using organic solvents were observed in a series of cyclic multiporphyrin systems on cyanopropyl-modified silica gel (CN-MS). Here, three cyclic porphyrin trimers with various rigidity were examined on CN-MS to understand the mechanism of the specific interactions between porphyrin derivatives and functional groups on modified silica gel. In addition to CN-MS, six modified silica-gel columns were tested to compare their retention abilities for a cyclic nickel porphyrin dimer (C4Ni2MsCP2). We examined the cosolvent effects of the pyridine eluents for C4Ni2MsCP2. Apparent dissociation constants of C4Ni2MsCP2 with func-tional groups on the MS columns and effective amounts of the functional groups were estimated by frontal affinity chroma-tography (FAC). 1H NMR titrations of acetonitrile and nitro-benzene to C4Ni2MsCP2 were conducted to compare their association constants with movable guest molecules to the dissociation constants with immobilized functional groups obtained in FAC. We found rigidity of cyclic porphyrin deriv-atives and immobilization of functional groups on silica gel is necessary for significant retentions using WAC. The affinity interaction does not occur at the center of C4Ni2MsCP2, but probably occurs on the surface composed of a bipyridyl moiety and the adjacent edges of the two porphyrins. Polar solvents, such as nitrobenzene, acetonitrile, and methanol, weakened the interaction. Although C4Ni2MsCP2 dissolves well in chloro-form, the interactions between C4Ni2MsCP2 and the MS col-umns are considerably strengthened in the presence of chloro-form. The competitiveness of solvents and cosolvents with the interaction of the porphyrin on WAC is independent of the solubility of the analyte.