Temperature-Dependent Hydration at Micellar Surface:  Activation Energy Barrier Crossing Model Revisited

Abstract

In recent years, the validity of the activation energy barrier crossing model at the micellar surface brings notable controversy (Sen, P.; Mukherjee, S.; Halder, A.; Bhattacharyya, K. Chem. Phys. Lett. 2004, 385, 357-361. Kumbhakar, M.; Pal, H. J. Phys. Chem. B 2004, 108, 19246-19254.) in the literature. In order to check the validity of the model by time-resolved solvation of a probe fluorophore, a wider range of temperature must be considered. At the same time, spatial heterogeneity (solubilization) of the probe and structural perturbation of the host micelle should carefully be avoided, which was not strictly maintained in the earlier studies. We report here the solvation dynamics of 4-(dicyanomethylene)-2-methyl-6(p-dimethylamino-styryl) 4H-pyran (DCM) in the SDS micelle at 298, 323, and 348 K. The probe DCM is completely insoluble in bulk water in this wide range of temperature. The size of the micelle at different temperatures using the dynamic light scattering (DLS) technique is found to have insignificant change. The hydration number of the micelle, determined by sound velocity measurements, decreases with increasing temperature. Time-resolved fluorescence anisotropy reveals the retention of the probe in the micellar interface within the temperature range. The average solvation time decreases with increasing temperature. The result of the solvation study has been analyzed in the light of energetics of bound to free water conversion at a constant size and decreasing hydration number at the micellar surface. The solvation process at the micellar surface has been found to be the activation energy barrier crossing type, in which interfacially bound type water molecules get converted into free type molecules. We have calculated E a to be 3.5 kcal mol -1 , which is in good agreement with that obtained by molecular dynamics simulation studies.

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

“…The same group has recently reported 13 the temperature-dependent solvation dynamics of coumarin dyes in aqueous triblock copolymer micelles in the temperature range 293-343 K and interpreted their results in view of structural and hydration studies. Temperature-dependent solvation dynamics of a fluorophore DCM (4-(dicyanomethylene)-2-methyl-6(p-(dimethylamino)-styryl)-4H-pyran) in SDS micelle have been recently reported from our group, 14 and the study revealed a good agreement with the Arrhenius type behavior within the framework of overall structural integrity of the micelle.…”

Interplay between Hydration and Electrostatic Attraction in Ligand Binding: Direct Observation of Hydration Barrier at Reverse Micellar Interface

“…The increased compressibility of hydration water at higher temperatures is also observed at the interface of anionic sodium dodecyl sulfate (SDS) micelles. 17 The observations suggest that both the rigidity and the number of bound waters in the interfacial hydration shell are lost at higher temperatures. The hydration shell, thus, becomes soft at higher temperatures.…”

“…This leaves us with the choice of biomimetics such as SDS micelles 17 and AOT/isooctane/water reverse micelles, 18 which retain their structural integrity over a wide range of temperatures 17 to serve as model biointerfaces. Instances of replacing a complicated biomolecule by more…”

Ligand binding strategies of human serum albumin: How can the cargo be utilized?

“…The recent advancement in the field of nanoscience requires the preparation of bioactive nanoparticles under different temperatures using protein molecules as templates. [86][87][88][89] The single-residue mutations could modify the stability of albumin and that the effects are more pronounced for mutations in sub domain IIA (sudlow site I) than for mutations in sub domain IIIA (sudlow site II). Similar approach was being made to characterize the simultaneous binding of the ligands in various temperaturedependent folded states of HSA for revealing the nature of binding of ligands.…”