Search

Partitioning of 4-nitrophenol in a reverse micellar system consisting of aerosol-OT (AOT)–H2O–isooctane was monitored spectrophotometrically. At pH 10.0, the ionized form 4-nitrophenolate in the water pool absorbs visible light with a maximum peak at 402 nm. However, that partitioned into the interface region is not ionized due to interactions with the negatively charged polar head of the surfactant. The partitioning depends on the water content of the system. In some intermediate [H2O]/[AOT] molar ratio values, two absorption peaks were clearly observed, which can be utilized in the partition coefficient estimation. The partitioning also depends on the buffer used. While partitioning of 4-nitrophenol into the interface is observed in carbonate buffer, the partitioning disappeared in 2-amino-2-methylpropanol buffer presumably due to displacement of 4-nitrophenol from the interface region into the water pool. This displacement is not a salt effect but is due to the amino group of 2-amino-2-methylpropanol, because tert-butylamine, rather than isobutanol, induced the replacement. When the surfactant concentration was increased, while keeping the system water content constant, the absorption peak at 402 nm increased with a concomitant decrease in the A310 peak, which demonstrated the affinity of the non-ionized 4-nitrophenol with the surfactant. Multiple apparent pKa values of 4-nitrophenol were observed in the AOT reverse micellar system. We propose a model of the AOT reverse micelles with a gradient micro-polarity in the water pool that results in a continuous influence on the ionization of 4-nitrophenol in the water pool of the system.

The nucleophilic aromatic substitution (SNAr) between GSH and 2,4-dinitrochlorobenzene was studied in reverse micellar systems composed of limited amounts of water, a surfactant with a polar head and a nonpolar tail, and the organic solvent 2,2,4-trimethylpentane. When the surfactant was positively charged and contained an aromatic ring in the polar head, the second-order rate constant was increased by approximately two orders of magnitude as compared to that in aqueous solution. The rate enhancement could be attributed to the stabilization of the negatively charged Meisenheimer σ-complex by the positively charged polar head and the weak aromatic ring’s electric quadrupole interactions of the surfactant. The reaction rate in reverse micelles composed of neutral polar head groups (Triton X-100) was increased by 3-fold, which may be explained by the interactions of the hydroxy groups of Triton molecules with the π-system of the Meisenheimer complex. An inverse relationship between the molar concentration [H2O]/[surfactant] ratio, which reflects the inclusive volume of the reverse micellar particle, and the rate enhancement was observed for positively charged or hydroxy-containing reverse micelles, but opposite results were obtained with negatively charged reverse micelles. These reverse micellar systems thus mimic the active site of a detoxification enzyme, glutathione transferase, in which stabilization of the Meisenheimer complex by a positively charged arginine residue, on-edge quadrupole interactions of aromatic amino acids, and the hydroxy group of tyrosine or threonine have been proposed in the enzyme-catalysed SNAr conjugation.

Ionisation of 4-nitrophenol in a reverse micellar system prepared by dissolving Aerosol OT (AOT), a surfactant with an anionic polar head, in isooctane, depends on the degree of hydration ([H2O]/[AOT]) of the system. The model system provides a convenient instrumental tool with which to study the leaving group effect of alkaline phosphatase-catalysed hydrolysis of 4-nitrophenyl phosphate. The Bronsted constants, βlg, for kcat and kcat/Km were found to be –0.47 and –1.03, respectively. Assuming that phosphorylation of the enzyme is rate limiting, the strong leaving group effect on catalysis indicates that the apparent pKa-values observed in reverse micelles are true measures of ionisation.