Conformation of oligocholate foldamers with 4-aminobutyroyl spacers

Extended oligocholates with 4-aminobutyroyl groups in between the cholate units were labeled with a naphthyl and a dansyl at the chain ends. Fluorescence resonance energy transfer (FRET) from the naphthyl to the dansyl was observed in 2:1 hexane/ethyl acetate (EA) containing a few percent of methanol. An increase of methanol in the solvent caused unfolding of the extended oligocholates, diminishing the energy-transfer efficiency. The 4-aminobutyroyl spacers strongly influenced the conformation of the oligocholates. Whereas the parent oligocholates (with no spacing groups in between the cholates) require at least five cholate groups to fold cooperatively, the 4-aminobutyroyl-spaced oligocholates could do so in more competitive solvents with as few as three or four cholates.