QUANTITATIVE STUDY OF THE HYDROPHOBIC INTERACTION MECHANISM BETWEEN UREA AND MOLECULAR PROBES USED IN SENSING SOME MICROHETEROGENEOUS MEDIA

The interactions of 2-phenyl-3,3-dimethyl-3H-indole (1), laminophenyl) -3,3-dimethyl-5-carboethoxy-3H-indole (3) and 2-(p-aminophenyl)-3,3-di methyl-5-cyano-3H-indole (4) with SDS micelles and of 3 and -aminophen yl)-3,3-dimethyl-5-carboethoxy-3H-indole (2) with aqueous solutions of beta-cyclodextrin CB-CD) in the absence and presence of urea, respect ively, were studied by absorption and steady-state fluorescence measur ements. It was found that the microviscosity of the interface of sodiu m dodecyl sulfate (SDS) micelle sensed by 1 increases greatly, while t he change in the micropolarity of the interface sensed by 4 is negligi ble with increasing the urea concentration. The estimated critical mic ellar concentration (cmc) of the SDS micelle also increases with the u rea concentration. It was also found that the binding constant (K-s) o f 3 with SDS micelle is reduced by the addition of urea. The steady-st ate fluorescence measurements of the stoichiometries of the guest (2 o r 3):beta-CD inclusion complexes indicate that two types of complexes, i.e., 1:1 and 1:2 types, are formed. The association constants, i.e., K-1 for the 1:1 complex and K-2 for the 1:2 complex, were obtained fo r 2 and 3 in the absence and presence of urea, respectively. The data reveal that the hydrophobic effect plays the major role in the stabili zation of 1:1 and 1:2 complexes. The addition of urea gives rise to a remarkable decrease in K-1 and a much greater decrease in K-2. For 2 i n the presence of 5 M urea, the formation of the 1:2 complex is inhibi ted completely. The effects of urea on K-s in SDS micelles and on K-1 and K-2 in aqueous solutions of beta-CD are described in terms of its hydrophobic interactions with 2 and 3. Some more direct evidences abou t this hydrophobic interaction in SDS micelles and the association con stant between urea and 3 are also presented. The above results of the hydrophobic interaction between urea and the 3H-indoles and of the ure a effect on the micellar interface strongly support the direct mechani sm of urea action, through which the mechanism of urea as a protein de naturant can be understood better.