ZN-2+ INCLUSION COMPLEXES OF ENDODENTATE TRIPODANDS AS CARBONIC ANHYDRASE-INSPIRED ARTIFICIAL ESTERASES

Facile syntheses are reported for three endo-tridentate tris-imidazole podands 1-3, two of which also bear basic amino residues, designed to reproduce the active site of carbonic anhydrase. The architecture was tested for the ability to bind Zn2+ in a ''biomimetic'' fashion and for the esterase activity of the Zn2+ complexes. Complexation of Zn2+ and Co2+ was studied by pH-titrimetry and NMR and UV-vis spectrometry and was found to be moderate. Besides species of LM2+ and +HLM2+ stoichiometries, the formation of LZnOH+ species was facilitated by inclusion, with associated pK(a) values ranging from 7.6 to 8.1. Cooperativity was found in the protonation of 3ZnOH+, however, wherein both the amino vertex and the hydroxozinc moieties are protonated in a single stage with an overall pK(a) 7.95 and no 3Zn2+ (or +H3ZnOH+) was detected. There was some evidence for the formation of 2Zn2+ as its tautomer, +H2ZnOH+. The catalysis of hydrolysis of p-nitrophenyl acetate was studied in buffered H2O between pH 7.35 and 8.15. Even though complexation was incomplete and gave rise to two or three complexes in each case, the residual first-order catalytic rate constants owing to these complexes were obtained after discounting the catalyses owing to other species present in solution. The catalyses were attributed to 1Zn2+, 2Zn2+, and 3ZnOH+ (but not 1ZnOH+ or 2ZnOH+) with second-order rate constants 0.079, 0.098, and 0.139 M-1 s-1, respectively, significantly higher than with several previous artificial esterases.