Slow solvation dynamics in supramolecular systems based on bile salts: Role of structural rigidity of bile salt aggregates

Abstract

The dynamical aspects of solvation has been explored in supramolecular biomimetic systems of bile salt aggregates, using an intramolecular charge transfer (ICT) dye 5-(4″-dimethylaminophenyl)-2-(4′-sulfophenyl)oxazole, sodium salt (DMO). The solvation response is bimodal, with a fast time constant of a few tens of picoseconds due to relaxation of the hydrophilic surface-bound water molecules, and a slower component of few nanoseconds, originating from the coupling of the structural flexibility of the bile salt aggregates with their hydration dynamics. The dynamics of solvation is mainly governed by the slower nanosecond relaxation component, due to its dominant contribution (68–90%), and is significantly retarded (average solvation time ∼4.9 ns) in the aggregates of sodium deoxycholate (NaDC) relative to sodium taurocholate (NaTC) or sodium cholate (NaCh), (∼2.2–2.7 ns). From the time-resolved anisotropy decay measurements, a strong correlation between the structural rigidity of the bile salt aggregates and the slower relaxation component is observed, which clearly demonstrates the presence of a strong dynamical coupling between the structural flexibility of the bile salt aggregates and their hydration water. The local structure of the dye environment is significantly more rigid in the bile salt aggregates of NaDC than NaCh or NaTC, owing to higher rigidity of the former, which directly influences the neighboring water-network flexibility and hence, coupled water-bile salt fluctuations. In consequence, a remarkably slower dynamics of solvation is manifested in the more rigid aggregates of NaDC than the more flexible aggregates of NaCh or NaTC.