Catalytic Efficiency of Functionalized Vesicles in the Transamination of Pyridoxal-5′-phosphate with a Hydrophobic Amino Acid

The transamination reaction of pyridoxal-5′-phosphate (PLP) with N-dodecyl-l-alaninamide (AlaC12) was investigated in an aqueous phosphate–borate buffer at pH 7.0, μ 0.10 (KCl), and 30.0±0.1 °C in the presence of single-walled vesicles of N,N-ditetradecyl-Nα-(6-trimethylamnioniohexanoyl)-l-histidinamide bromide(N+C5His2C14). The electrostatic and hydrophobic interactions between the vesicles and the reactants resulted in incorporation of PLP and AlaC12 into polar and hydrophobic domains of the vesicles, respectively, in the Schiff-base formation process. The isomerization of the aldimine Schiff-base to the corresponding ketimine Schiff-base was confirmed to be the rate-determining step in the transamination process. The reaction site in the vesicular system was found to be equivalent in polarity to dioxane–water (7:3 v/v). However, the overall reaction rate in the vesicles was enhanced 230-fold relative to that in dioxane–water (7:3 v/v). A hydrophobic and suitably polar microenvironment constructed at the reaction site is responsible for such a marked rate-enhancement. In addition, each vesicle of N+C5His2C14 provided functional (imidazolyl) groups in its hydrogen-belt domain to catalyze the intramolecular pro to tropic shift to yield the ketimine Schiff-base. The microenvironmental effects of molecular assemblies of N,N-ditetradecyl-Nα-(6-trimethylammoniohexanoyl)-l-alaninamide bromide and CTAB on the overall transamination were also discussed.