Zn2+ inclusion complexes of endodentate tripodands as carbonic anhydrase-inspired artificial esterases. Part 2. Micellar systems

Facile syntheses of two lipophilic, endo-tridentate tris-imidazole podands (3 and 4) are reported. These were designed for micellar media, where pre-o rganization for metal binding was anticipated to better reproduce the activ e site of carbonic anhydrase (CA). pH-Metric titrations and p-nitrophenyl a cetate (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 see n in the enzyme. The second-order rate constant for pNPOAc hydrolysis by [Z n(3)OH](+) was estimated at 0.19(1) M-1 s(-1), the second-highest value mea sured for a biomimetic CA model.