Zn2+ inclusion complexes of endodentate tripodands as carbonic anhydrase-inspired artificial esterases. Part 2.1 Micellar systems

Abstract

Facile syntheses of two lipophilic, endo-tridentate tris-imidazole podands (3 and 4) are reported. These were designed for micellar media, where pre-organization for metal binding was anticipated to better reproduce the active site of carbonic anhydrase (CA). pH-Metric titrations and p-nitrophenyl acetate (pNPOAc) hydrolyses were carried out in the presence of Zn²⁺ and the results compared with those obtained with hydrophilic tripodand varieties. The results confirm stronger metal binding with the micelle-bound ligands and show reasonably low pK_a values near 8 for the deprotonation of metal-bound H₂O, as well as evidence for a proton shuttling analogous to that seen in the enzyme. The second-order rate constant for pNPOAc hydrolysis by [Zn(3)OH]⁺ was estimated at 0.19(1) M^–1 s^–1, the second-highest value measured for a biomimetic CA model.