Chromatographic study on stepwise aggregation patterns of zwitterionic derivatives of cholic acid

Abstract

The frontal chromatogram of the 2-hydroxy derivative (CHAPSO) of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) has been obtained on a Sephadex G-10 column at 298 K. From the centroid volume, V_c, of the chromatogram, the monomer concentration, the weight- and number-average aggregation numbers (n_w and n_n), the critical micelle concentration (c.m.c.) and the dimerization constant, k₂, are evaluated, and furthermore, stepwise aggregation constants, k_i, are estimated on the basis of several self-association models. At small aggregation numbers, an odd–even alternation of the aggregation number dependence of k_i is found and CHAPSO self-associates cooperatively, whereas it does so anti-cooperatively at high aggregation numbers. The c.m.c. depends on the treatment of the V_c data, and this result suggests that CHAPSO forms small micelles. The derivative chromatogram of CHAPSO and some characteristic values obtained therefrom are markedly different from those of a dye and typical surfactants, and reflect their aggregation patterns. The derivative chromatogram simulated on the basis of the above aggregation models is very similar to the observed one. The differences in limiting aggregation constant k and n_w/n_n between CHAPSO and CHAPS are explained on the basis of the fact that the head group of CHAPSO is more bulky than that of CHAPS. The micelle size distribution of CHAPSO is calculated on the basis of the above aggregation models. The concentrations of the monomer and each aggregate species will be useful for applications of CHAPSO and CHAPS to the solubilization of membrane proteins as well as for the investigation of the aggregation patterns of bile salts.