Transesterification of RNA by Cu(II) terpyridine

Kinetics and Mechanism of Hydrolysis of a Model Phosphate Diester by [Cu(Me3tacn)(OH2)2]2+ (Me3tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane)

The kinetics of hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) by [Cu(Me3tacn)(OH2)2]2+ has been studied by spectrophotometrical monitoring of the release of the p-nitrophenylate ion from BNPP. The reaction was followed for up to 8000 min at constant BNPP concentration (15 microM) and ionic strength (0.15 M) and variable concentration of complex (1.0-7.5 mM) and temperature (42.5-65.0 degrees C). Biphasic kinetic traces were observed, indicating that the complex promotes the cleavage of BNPP to NPP [(p-nitrophenyl)phosphate] and then cleavage of the latter to phosphate, the two processes differing in rate by 50-100-fold. Analysis of the more amenable cleavage of BNPP revealed that the rate of BNPP cleavage is among the highest measured for mononuclear copper(II) complexes and is slightly higher than that reported for the close analogue [Cu(iPr3tacn)(OH2)2]2+. Detailed analysis required the determination of the pKa for [Cu(Me3tacn)(OH2)2]2+ and the constant for the dimerization of the conjugate base to [(Me3tacn)Cu(OH)2Cu(Me3tacn)]2+ (Kdim). Thermodynamic parameters derived from spectrophotometric pH titration and the analysis of the kinetic data were in reasonable agreement. Second-order rate constants for cleavage of BNPP by [Cu(Me3tacn)(OH2)(OH)]+ and associated activation parameters were obtained from initial rate analysis (k = 0.065 M(-1) s(-1) at 50.0 degrees C, deltaH = 56+/-6 kJ mol(-1), deltaS = -95+/-18 J K(-1) mol(-1)) and biphasic kinetic analysis (k = 0.14 M(-1) s(-1) at 50.0 degrees C, deltaH = 55+/-6 kJ mol(-1), deltaS = -92+/-20 J K(-1) mol(-1)). The negative entropy of activation is consistent with a concerted mechanism with considerable associative character. The complex was found to catalyze the cleavage of BNPP with turnover rates of up to 1 per day. Although these turnover rates can be considered low from an application point of view, the ability of the complexes to catalyze phosphate ester cleavage is clearly demonstrated.

Ester hydrolysis with 2,6-di(pyrazol-3-yl)pyridines and their CoII complexes in homogeneous and micellar media

The 1:1 complex of Co(ClO4)2 with the H2O-insoluble tridentate 2,6-di(1H-4,5,6,7-tetrahydroindazol-3-yl)pyridine (H21) was found to be an excellent catalyst for the hydrolysis of para-nitrophenyl acetate in aqueous buffers over the pH 7.05-7.90 range, with an estimated second-order rate constant of 0.50 M(-1) s(-1). The Co2+ complexes of the N,N'-di-1-dodecyl analogue in micellar media and the N,N'-di-(4-carboxyphenyl) analogue in aqueous media were much poorer catalysts, poorer than the free ligands. In all cases, the pH-rate profiles indicated that free base, deprotonated or hydroxo forms were the active species. The greater success with Co(H(2)1)2+ indicated a catalytic role for N-H deprotonation.

Hydrolysis of phosphates, esters and related substrates by models of biological catalysts

Why study hydrolases, and why model them? First, hydrolases themselves are of fundamental importance and utility. Examples of their utility in organic synthesis include kinetic resolutions of optical isomers. Restriction endonucleases (DNA hydrolases) are key tools for biotechnology and are vital biological catalysts. Peptidases are necessary for protein digestion and can be harnessed to perform the reverse reaction (peptide synthesis). Thus, for these and many other reasons, hydrolases receive the attention of fundamental and applied research. Models of hydrolases can contribute to our understanding of reaction mechanisms and may also supplant the enzymes as useful catalysts under some conditions. Altering or even increasing the specificity of natural catalysts are also goals of these model studies.