Solvation dynamics in bile salt aggregates

Abstract

The solvation dynamics inside an aggregate of a bile salt, sodium deoxycholate (NaDC), in aqueous solution has been studied using picosecond time-resolved emission spectroscopy and two fluorescent probes, 4-(dicyanomethylene)-2-methyl-(p-dimethylaminostyryl)-4H-pyran (DCM) and 2,6-p-toluidinonaphthalenesulfonate (TNS). Addition of NaDC to an aqueous solution causes a nearly 110-fold increase in the emission quantum yield (ff) of TNS. From the variation of ff of TNS, two critical micellar concentrations of NaDC have been detected at around 7 and 60 mM, respectively. The solvation dynamics of TNS in 100 mM NaDC solution is described by three components, 95 ps (17%), 380 ps (16%), and 1.9 ns (67%). The solvation dynamics of DCM bound to 100 mM NaDC is also found to be triexponential with components of 110 ps (19%), 700 ps (17%), and 2.75 ns (64%). The substantially slow solvation dynamics of water in the vicinity of NaDC indicates restricted solvation dynamics of water inside the bile salt aggregate.