GaIII and ZrIV Complexes as Phosphatase Analogues
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In purple acid phosphatase (PAP)-catalysed phosphate monoester hydrolysis, it is accepted that the ferric ion of the active site can mediate the reaction through Lewis acid activation of the substrate and through nucleophilic attack on phosphorus by coordinated hydroxide. These modes are antagonistic, OH bound to a strongly acidic FeIII center will not be an efficient nucleophile, whereas OH coordinated to a less acidic FeIII centre will be a better nucleophile. Ligand electron donating strength influences the acidity of the metal ion. Employing GaIII as a model for the FeIII site, a series of GaIII complexes of tripodal ligands of various electron-donating capacity were synthesised and their activities towards the hydrolysis of the phosphate diester bis(2,4-dinitrophenyl) phospate (BDNPP), compared. It was observed that complexes with stronger ligand electron-donating groups provided greater rate accelerations. It is proposed that in the PAP models, the efficiency of the coordinated hydroxide nucleophile prevails over Lewis acid activation in deciding the intrinsic reactivity in PAP-mediated phosphate ester hydrolysis. In RNase and Dnase activity, divalent Mg is frequently required as metal cofactor. This is explained by the high natural abundance and favorable chemical properties of Mg. However, the activity of Mg in model systems is greatly reduced relative to enzymatic systems. The low dielectric constants of enzyme active sites is considered to be better-mimicked by organic solvents like alcohols or DMSO than by aqueous solvent. Dinuclear MgII and ZnII complexes were synthesised and their activities towards the RNA model 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP) in DMSO were tested. However, no activity was observed due to poor association between the negatively charged complexes and anionic phosphate ester. A dinuclear ZrIV complex of the ligand 5-methyl-2-hydroxy-1,3-xylene-¿,¿-diamine-N,N,N¿,N¿-tetraacetic acid (HXTA) has been generated and its activity towards phosphate mono- and diester hydrolysis tested between pH 5.5 and 8.5. At neutral pH, monoester hydrolysis was observed to be ~60-fold faster than for an analogous diester. A possible explanation for this observation is that monoanionic phosphate diesters are weak ligands and so, binding to the catalyst is poor relative to phosphate monoesters. Metal ion cooperativity, through synthesis of an analogous mononuclear complex was studied. Ligand electron donating effects were investigated.
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