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

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 (gamma) 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 (gamma(CMC)), and the slope (dgamma/dlnC(t)) from the gamma-lnC(t) curves in the concentration range just below CMC. These data enabled us to estimate surface excess (Gamma(t)), and mean molecular area (A(m)) in addition to such parameters as the minimum surface Gibbs energy (G(min)((S))), pC(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 (omega(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 (omega(A)) in the adsorbed film was also estimated. The partial molecular area (PMA), gamma(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 omega(R) and omega(A) values have demonstrated that mixed micelles and adsorbed film formation are attained accompanying to some extent enhanced intermolecular interaction (with negative omega(R) and omega(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.     

On the role of tetramethylammonium cation and effects of solvent dynamics on the stability of the cage-like silicates Si6O156- and Si8O208- in aqueous solution. A molecular dynamics study

We have undertaken explicit solvent molecular dynamics simulations to investigate the preferential stabilization of the silicate octamer Si(8)O(20)(8-) over the hexamer Si(6)O(15)(6-) in relation with the ability of tetramethylammonium (TMA) to form an adsorption layer around these cage-like polyions. We have found that the hexamer cannot support such a layer and as a result is vulnerable to hydrolysis. The dynamics of TMA desorption off the surface of the hexamer is investigated in connection with the solvent dynamics. We have studied the energetics of this preferential stabilization by calculating the relative change in the free energies of formation between the complexes Si(8)O(20)(8-).8TMA and Si(6)O(15)(6-).6TMA and found the former to be more stable by 70 kcal/mol. We also find that the energetics are consistent with experimental data, suggesting that the hexamer is a long-lived metastable species. Furthermore, we have studied the solvent structure and dynamics in the vicinity of both the bare polyions and their complexes with TMA. We have found that, as anticipated, both the octamer and the hexamer participate in hydrogen bonds with the water molecules, regardless of whether a TMA adsorption layer exists or not. In fact, we find that the presence of a TMA adsorption layer has a rather profound effect on the stability of these hydrogen bonds-it increases their lifetime by at least a factor of 2 relative to that of the hydrogen bonds between water and the bare polyions.     

Kinetic, spectral and redox properties of the transients formed on one-electron reduction of 3, 3, 6, 6-tetramethyl-3, 4, 6, 7, 9-pentahydro-10-phenyl (1, 8) (2H, 5H) acridinedione in aqueous-organic mixed solvent system: A pulse radiolysis study

The phenyl substituted acridine-1,8-dione (AD) dye reacts with (CH₃)₂*COH radicals with a bimolecular rate constant of 0.6 × 10⁸ dm³ mol⁻¹ s⁻¹ in acidic aqueous-organic mixed solvent system. The transient optical absorption band (λ_max = 465 nm, ɛ = 6.8 × 10² dm³ mol⁻¹ cm⁻¹) is assigned to ADH* formed on protonation of the radical anion. In basic solutions, (CH₃)₂*COH radicals react with a bimolecular rate constant of 4.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and the transient optical absorption band (λ_max = 490 nm, ɛ = 10.4 × 10³ dm³ mol⁻¹ cm⁻¹) is assigned to radical anion, AD*⁻, which has a pK_a value of 8.0. The reduction potential value of the AD/AD*⁻ couple is estimated to be between −0.99 and −1.15 V vs NHE by pulse radiolysis studies. The cyclic voltammetric studies showed the peak potential close to −1.2 V vs Ag/AgCl.