Spacer-Dependent Folding and Aggregation of Oligocholates in SDS Micelles

Insertion of flexible, 4-aminobutyroyl spacers in between the cholate repeat units had been found previously to enhance the folding of cholate oligomers in homogeneous solution (Zhao, Y. J. Org. Chem. 2009, 74, 834-843). The opposite effect was observed when the oligomers were solubilized in aqueous solutions of sodium dodecyl sulfate (SDS). The spacers enabled formation of tight aggregates of the oligocholates in SDS solutions when the surfactant was below its critical micelle concentration (CMC). Above the CMC, SDS micelles formed and dissociated the oligocholate aggregates. The parent oligocholates (without spacers in between the repeat units) also aggregated when they were too short to fold (e.g., dimer). The longer tetramer and hexamer preferred to fold, as their rigid, awkwardly shaped backbones prevented tight packing needed in the formation of stable aggregates. Folding was favored both below and above the CMC of SDS and was enhanced by an increase in the chain length.