Metal ion catalyzed hydrolysis of ethyl p-nitrophenyl phosphate

N-15 isotope effects in the. nitro group and O-18 isotope effects in the phenolic oxygen have been measured for the hydrolysis of ethyl p-nitrophenyl phosphate catalyzed by several metal ions. Co(III)-cyclen at pH 7, 50degreesC, gave an N-15 isotope effect of 0.12% and an O-18 one of 2.23%, showing that P-O cleavage is rate limiting and the bond is similar to50% broken in the transition state. The active catalyst is a dimer and the substrate is presumably coordinated to the open site of one Cb(III), and is attacked by hydroxide coordinated to the other Co(III). Co(III)-tacn under the same conditions shows a similar N-15 isotope effect (0.13%), but a smaller O-18 one (0.8%). Zn(II)-cyclen at pH 8.5, 80 degreesC, gave an N-15 isotope effect of 0.05% and an O-18 one of 0.95%, suggesting an earlier transition state. The catalyst in this case is monomeric, and thus the substrate is coordinated to one position and attacked by a cis-coordinated hydroxide. Eu(III) at pH 6.5, 50 degreesC, shows a very large N-15 isotope effect of 0.34% and a 1.6% O-18 isotope effect. The large N-15 isotope effect argues for a late transition state or Eu(III) interaction with the nitro group, and was also seen in Eu(III)-catalyzed hydrolysis of p-nitrophenyl phosphate. [Bioorg. Chem. 28 (2000) 283]. (C) 2002 Elsevier Science Inc. All rights reserved.