Phosphates cobalt complex catalysis

A series of diaquatetraaza cobalt(III) complexes accelerated the hydrolysis of adenylyl(3 -50adenosine (ApA) (304), an enhancement of 10 -fold being observed with the triethylenetetramine complex (303) at pH 7. The pentacoordinated intermediate (305), which is formed with the complex initially acting as an electrophilic catalyst, then suffers general acid catalysis by the coordination water on the Co(III) ion to yield the complexed 1,2-cyclic phosphate (306), the hydrolysis of which occurs via intracomplex nucleophilic attack by the metal-bound hydroxide ion on the phosphorus atom. Neomycin B (307) has also been shown to accelerate the phosphodiester hydrolysis of ApA (304) more effectively than a simple unstructured diamine. 

Preparations are listed in Table 47. Main interest lies in determining how metal ions catalyze the hydrolysis of ADP and ATP. ATP plays a crucial role in the energy metabolism of all living cells and divalent metal ions (Mg +, Mn " and Ca ) play an important role in these phosphoryl transfer processes. Divalent metal ions such as Mg +, Ca, Zn, Cu and Mn " provide only modest in vitro catalysis "" and stronger, more specific coordination to phosphate units appears to be required by the enzyme. Co " (and Cr" ) complexes of ADP and ATP have been shown to mimic many of the biological functions of the Mg" enzyme, and since the cobalt(III)-phosphatc coordination remains intact, the specificity of alternative coordination sites, and the stereochemical requirements at phosphorus, have been elucidated in some cases. Often the Co "-enzyme species is biologically active and several enzymic functions of ATP have been examined in this manner. 

Recent work has shown that simple nonlabile mono-aqua metal complexes function as general base catalysts in the hydrolysis of neutral phosphate and phosphonate esters in aqueous solution. If general base catalysis operates the reaction of [Co(NH3)50H] with 4-nitrophenyldiethyl phosphate, then (1 = PNPDEP) will give free diethylphosphonic acid in solution, while direct nucleophilic displacement will give the (diethylphosphate)cobalt(III) cation (2). 

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