Blending effects on adsorption and micellization of different membrane protein solubilizers: II. A thermodynamic study on a mixed system of CHAPS with a bile salt in pH 7.4 phosphate buffer solution

Abstract

For a mixed system of a typical membrane protein solubilizer CHAPS (a derivative of a bile acid cholic acid) combined with a bile salt (sodium salt of glycocholic acid, NaGC), which is also a candidate as a membrane protein solubilizer, micellization and adsorbed film formation in a phosphate buffer solution of pH 7.4 at 303 K were studied paying special attention to the synergistic effect upon mixing. The collection of sufficient data based on plots of surface tension ( γ) versus logarithmic concentration ( C t or m t) in total molality at discrete mole fractions ( X 2) in the mixture of surfactants 1 and 2 (where 1 and 2 correspond to CHAPS and NaGC, respectively) allowed us to accurately determine critical micelle concentration (CMC), minimum surface tension at CMC ( γ CMC), and the slope (d γ/d ln C t) from the γ–ln C t curves in the concentration range just below CMC. These data enabled us to estimate surface excess ( Γ t), and mean molecular area ( A m) in addition to such parameters as the minimum surface Gibbs energy ( G min ( S ) ), p C 20 and CMC/ C 20 related to synergism accompanied by blending. Applying the regular solution theory (RST), the relation of compositions of the singly dispersed phase ( X 2) and the micellar phase ( Y 2) as well as the interaction parameter ( ω R) (by using the Rubingh's equations) were estimated. The relation between the composition in the adsorbed film ( Z 2) and X 2 together with the interaction parameter ( ω A) in the adsorbed film was also estimated. The partial molecular area (PMA), γ CMC, and G min ( S ) were examined as functions of X 2 and/or Z 2. The resultant CMC– X 2 and CMC– Y 2 curves and ω R and ω A values have demonstrated that mixed micelles and adsorbed film formation are attained accompanying to some extent enhanced intermolecular interaction (with negative ω R and ω A values). Comparing with previous results for mixed systems of CHAPS with n-acyl (octanoly, nonanoyl, and decanoyl)- N-methylglucamides [MEGA- n's ( n = 8, 9, and 10)] and of sodium chenodeoxycholate (NaCDC) with sodium ursodeoxycholate (NaUDC), the synergism observed for the mixed system of CHAPS with NaGC lies between both combinations. However the expected properties as a membrane protein solubilizer are judged to be sufficient.