Metal ion catalyzed hydrolysis of ethyl p-nitrophenyl phosphate.
15N isotope effects in the nitro group and 18O 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, 50 degrees C, gave an 15N isotope effect of 0.12% and an 18O one of 2.23%, showing that P-O cleavage is rate limiting and the bond is approximately 50% broken in the transition state. The active catalyst is a dimer and the substrate is presumably coordinated to the open site of one Co(III), and is attacked by hydroxide coordinated to the other Co(III). Co(III)-tacn under the same conditions shows a similar 15N isotope effect (0.13%), but a smaller 18O one (0.8%). Zn(II)-cyclen at pH 8.5, 80 degrees C, gave an 15N isotope effect of 0.05% and an 18O 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 degrees C, shows a very large 15N isotope effect of 0.34% and a 1.6% 18O isotope effect. The large 15N 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.
Rawlings
J
J
The Institute for Enzyme Research, University of Wisconsin, Madison, WI 53705, USA.
Cleland
W W
WW
Hengge
A C
AC
D002384:Catalysis / Q000187:drug effects; D003035:Cobalt / Q000494:pharmacology; D005063:Europium / Q000494:pharmacology; D006868:Hydrolysis / Q000187:drug effects; D007477:Ions / Q000494:pharmacology; D007554:Isotopes / Q000494:pharmacology; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008670:Metals / Q000494:pharmacology*; D015394:Molecular Structure; D009584:Nitrogen / Q000494:pharmacology; D009943:Organophosphorus Compounds / Q000138:chemical synthesis / Q000378:metabolism*; D015032:Zinc / Q000494:pharmacology
D007477:Ions; D007554:Isotopes; D008670:Metals; D009943:Organophosphorus Compounds; C042896:ethyl 4-nitrophenyl methylphosphonate; D003035:Cobalt; D005063:Europium; D015032:Zinc; D009584:Nitrogen