A phenyl ester substrate and an imidazole catalyst that possess the azobenzene chromophore and are capable of bilayer formation were synthesized. Distribution of these amphiphiles in the dialkylammonium bilayer matrix was examined by using blue shifts due to cluster formation of the azobenzene chromophore. Formation of the substrate cluster was promoted by increased concentrations in the matrix and by the liquid crystal-to-crystal phase transition of the matrix. Formation of the catalyst cluster was promoted, in addition to these factors, by neutralization of the anionic histidine head group due to the change of the medium pH or due to complexation with Cu2+ ion. The rate of alkaline hydrolysis of the clustered substrate was smaller than that of the isolated (monomeric) substrate: 1/19 at 10 °C, pH 11.8. The Arrhenius plots show inflection regions near the phase transition of the matrix due to changing monomer-cluster ratios. The activation energy of the hydrolysis of p-nitrophenyl N-carbobenzoxy-L-phenylalaninate was 27 kcal/mol in the partially rigid bilayer matrix but decreased to 14 kcal/mol in the fluid matrix. This change was attributed to the formation of the catalyst cluster in the rigid matrix. The present study provides the first example of the reaction control by phase separation.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry