Experimental and theoretical study of the mechanism of hydrolysis of substituted phenyl hexanoates catalysed by globin in the presence of surfactant

The bimolecular rate constants for the globin-and alkali-catalysed hydrolysis of substituted phenyl hexanoates in the absence and presence of cetyltrimethylammonium bromide (CTAB) obey Bronsted equations with beta(1g)=-0.53 (globin-catalysed), -0.68 (globin-catalysed in CTAB), -0.34 (in water) and -0.74 (in CTAB), respectively. The slopes indicate that the microsolvation environments associated with the transition states of the catalysed reactions are different from those that occur in aqueous medium. The slope (-0.74) for the reaction in CTAB implies that it proceeds in a less polar medium. The larger beta(1g) value (-0.53) obtained for the globin-catalysed reaction compared to that for the uncatalysed one may be attributed to either the less polar microenvironments of the transition states or the involvement of one of the imidazole groups as a nucleophile. The results from a study of the effect of pH on the reactivity provide evidence for the latter assumption. All of the ligands were docked into the hydrophobic pocket of the protein, and the resulting docking scores ranged from -30.76 to -23.61 kcal mol(-1). Molecular dynamic simulations and MM-PBSA/GBSA calculations performed for the complexes gave insight into the binding modes of globin to the esters, which are consistent with experimental results. The calculations yielded comparable free energies of binding to the experimental ones for 4-nitrophenyl and 4-chloro-2-nitrophenyl hexanoates. In conclusion, information obtained from the linear free-energy relationship is still very useful for elucidating the mechanisms of organic reactions, including enzyme-catalysed reactions. This approach is further supported by the utilization of computational tools.