Impact of the aggregation behaviour of sodium cholate and sodium deoxycholate on aqueous solution structure and dynamics: A combined time resolved fluorescence and dielectric relaxation spectroscopic study

The impact of aggregation behaviour of sodium cholate (SC) and sodium deoxycholate (SDC) on aqueous solution structure and dynamics has been studied and compared by performing dielectric relaxation (DR) and time-resolved fluorescence (TRF) measurements at three different concentrations, 30 mM, 100 mM and 300 mM at similar to 298 K. For DR studies, measurements have been carried out in the frequency range, 0.2 <=nu(GHz)<= 50. TRF measurements, on the other hand, employ a fluorescent probe, coumarin 153 (C153), as a local reporter. Both DR and TRF measurements confirm presence of surface water molecules that are characterized by sub-nanosecond timescale. This is further supported by the sub-melting features in the differential scanning calorimetric measurements. DR, dynamic Stokes shift and fluorescence anisotropy experiments indicate SDC solutions are dynamically slower than SC solutions. In addition, excitation energy dependence of steady state fluorescence emission measurements suggests SDC solutions are more heterogeneous. All these results indicate that absence of only one hydroxyl (-OH) group in SDC introduces significant difference in the aggregation behaviour between SC and SDC, affecting substantially the solution structure and dynamics. (C) 2016 Elsevier B.V. All rights reserved.