Quantitative Study of the Hydrophobic Interaction Mechanism between Urea and Molecular Probes Used in Sensing Some Microheterogeneous Media

Abstract

The interactions of 2-phenyl-3,3-dimethyl-3H-indole (1), 2-(p-dimethylaminophenyl)-3,3-dimethyl-5-carboethoxy-3H-indole (3) and 2-(p-aminophenyl)-3,3-dimethyl-5-cyano-3H-indole (4) with SDS micelles and of 3 and 2-(p-aminophenyl)-3,3-dimethyl-5-carboethoxy-3H-indole (2) with aqueous solutions of β-cyclodextrin (β-CD) in the absence and presence of urea, respectively, were studied by absorption and steady-state fluorescence measurements. It was found that the microviscosity of the interface of sodium dodecyl sulfate (SDS) micelle sensed by 1 increases greatly, while the change in the micropolarity of the interface sensed by 4 is negligible with increasing the urea concentration. The estimated critical micellar concentration (cmc) of the SDS micelle also increases with the urea concentration. It was also found that the binding constant (Ks) of 3 with SDS micelle is reduced by the addition of urea. The steady-state fluorescence measurements of the stoïchiometries of the guest (2 or 3):β-CD inclusion complexes indicate that two types of complexes, i.e., 1:1 and 1:2 types, are formed. The association constants, i.e., K1 for the 1:1 complex and K2 for the 1:2 complex, were obtained for 2 and 3 in the absence and presence of urea, respectively. The data reveal that the hydrophobic effect plays the major role in the stabilization of 1:1 and 1:2 complexes. The addition of urea gives rise to a remarkable decrease in K1 and a much greater decrease in K2. For 2 in the presence of 5 M urea, the formation of the 1:2 complex is inhibited completely. The effects of urea on Ks in SDS micelles and on K1 and K2 in aqueous solutions of β-CD are described in terms of its hydrophobic interactions with 2 and 3. Some more direct evidences about this hydrophobic interaction in SDS micelles and the association constant between urea and 3 are also presented. The above results of the hydrophobic interaction between urea and the 3H-indoles and of the urea effect on the micellar interface strongly support the direct mechanism of urea action, through which the mechanism of urea as a protein denaturant can be understood better.