Effects of Microheterogeneous Environments of SDS, TX-100, and Tween 20 on the Electron Transfer Reaction between L-Leucine and AuCl4-/AuCl3(OH)-

The effect of an ionic (SDS) and two nonionic (TX-100 and Tween 20) surfactants on the electron transfer reaction between L-leucine and gold(III) complexes has been investigated in acetate buffer. H+, Cl-, and decreasing dielectric constant of the medium have inhibiting effects on the reaction rate. The reaction product has been identified as isovaleraldehyde by H-1 NMR. Two different Au(III) species and the zwitterion form of the amino acid react where Au(III) undergoes a one step two-electron transfer process. In the presence of surfactants, the postmicellar kinetics has been explained in the light of Berezin's model where both the oxidant and the substrate are solubilized in the micellar pseudophase and then react. The leucine molecules occupy the hydrophobic core of the micelle with the polar amino and acid groups projected in the Stern/palisade layer among the surfactant head-groups and the oxidant species. The binding constants for substrate micelle association and the corresponding enthalpy changes have been evaluated. The hydrophobic interaction of leucine with SDS micelles is greater than those with Tween 20 and TX-100 micelles. The compensation between substrate water interaction and substrate micelle interaction determines the enthalpy change for the substrate micelle association. Entropy change for (Leu)(W) -> (Leu)(M) controls substrate micelle binding.