Controlling the Conformation of Oligocholate Foldamers by Surfactant Micelles

Abstract

Fluorescence resonance energy transfer (FRET) occurred readily in a cholate hexamer labeled with a naphthyl donor and a dansyl acceptor at the chain ends when the hexamer was solubilized by sodium dodecyl sulfate (SDS) micelles in water. Independence of the energy transfer efficiency over 1−70 mM SDS suggested that the energy transfer resulted from the folding of the hexamer instead of its intermolecular aggregation within the micelle. Upon addition of sodium chloride to the solution, energy transfer became less efficient, indicating unfolding of the oligocholate. In contrast, the oligocholate stayed folded in the micelle of nonionic Brij 30, in the presence or absence of NaCl. These results suggested that the oligocholate preferred to fold within the small spherical SDS micelles but unfold when the preference for spherical over rodlike micelles was not strong enough to overcome the tendency for the oligocholate to unfold. Disciplines Chemistry CommentsReprinted (adapted) Fluorescence resonance energy transfer (FRET) occurred readily in a cholate hexamer labeled with a naphthyl donor and a dansyl acceptor at the chain ends when the hexamer was solubilized by sodium dodecyl sulfate (SDS) micelles in water. Independence of the energy transfer efficiency over 1-70 mM SDS suggested that the energy transfer resulted from the folding of the hexamer instead of its intermolecular aggregation within the micelle. Upon addition of sodium chloride to the solution, energy transfer became less efficient, indicating unfolding of the oligocholate. In contrast, the oligocholate stayed folded in the micelle of nonionic Brij 30, in the presence or absence of NaCl. These results suggested that the oligocholate preferred to fold within the small spherical SDS micelles but unfold when the preference for spherical over rodlike micelles was not strong enough to overcome the tendency for the oligocholate to unfold.

Spacer-Dependent Folding and Aggregation of Oligocholates in SDS Micelles

“…The behavior contrasts that of the parent, more rigid cholate hexamer (6), which stays folded and unaggregated both below and above the CMC of SDS. 21 What makes the two types of oligocholates behave so differently in SDS solutions? Although the 4-aminobutyroyl spacers seem to be the obvious culprit, we have to rule out other factors, such -6 M. F 300 and F 340 represent the emissive intensity of dansyl at 500 nm in the excitation spectrum when λ ex is 300 and 340 nm, respectively.…”

“…After all, the main reason for the folding of 6 was attributed to its reduced size upon folding (which allows it to be "comfortably" encapsulated in a small spherical micelle). 21 To answer the above question, we synthesized donoracceptor-labeled tetramer 7 without the 4-aminobutyroyl spacers. Its emission in SDS solutions is different still from that of the flexible counterpart (1).…”

“…Similar SDSindependent emission was observed in rigid hexamer 6, which stays folded in all SDS solutions. 21 Is it possible that 7 might be folded within the SDS micelle as well, at least in <50 mM SDS?…”

“…21 This is probably also the reason why the hexamer folds better than the tetramer, as the size difference in the folded and unfolded tetramer is not as significant. Why do the flexible oligocholates prefer aggregation instead of folding?…”

“…Our previous work indicates that the naphthyl emission is weak under most conditions and FRET is better detected by the enhancement of the dansyl emission. 20,21,23 Naphthyl has the maximum absorption (λ max ) at 300 nm and emits around 360 nm.…”