| Abstract
| - Calix[4]arenes functionalized at the 1,2-, 1,3-, and 1,2,3-positions of the upper rim with [12]ane-N3 ligating units were synthesized, and their bi- and trimetallic zinc(II) and copper(II) complexes wereinvestigated as catalysts in the cleavage of phosphodiesters as RNA models. The results of comparativekinetic studies using monometallic controls indicate that the subunits of all of the zinc(II) complexes and ofthe 1,3-distal bimetallic copper(II) complex 7−Cu2 act as essentially independent monometallic catalysts.The lack of cooperation between metal ions in the above complexes is in marked contrast with the behaviorof the 1,2-vicinal bimetallic copper(II) complex 6−Cu2, which exhibits high catalytic efficiency and highlevels of cooperation between metal ions in the cleavage of HPNP and of diribonucleoside monophosphatesNpN‘. A third ligated metal ion at the upper rim does not enhance the catalytic efficiency, which excludesthe simultaneous cooperation in the catalysis of the three metal ions in 8−Cu3. Rate accelerations relativeto the background brought about by 6−Cu2 and 8−Cu3 (1.0 mM catalyst, water solution, pH 7.0, 50 °C)are on the order of 104-fold, largely independent of the nucleobase structure, with the exception of thecleavage of diribonucleoside monophosphates in which the nucleobase N is uracil, namely UpU and UpG,for which rate enhancements rise to 105-fold. The rationale for the observed selectivity is discussed interms of deprotonation of the uracil moiety under the reaction conditions and complexation of the resultinganion with one of the copper(II) centers.
|
