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Secondary 18 O isotope effects as a tool for studying reactions of phosphate mono‐, di‐, and triesters 1
Author(s) -
Cleland W. W.
Publication year - 1990
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.4.11.2199287
Subject(s) - chemistry , sn2 reaction , kinetic isotope effect , leaving group , isotope , bond cleavage , bond order , transition state , stereochemistry , medicinal chemistry , bond length , organic chemistry , molecule , deuterium , catalysis , physics , quantum mechanics
Secondary 18 O isotope effects have been developed as a tool for determining transition state structures in enzymatic and nonenzymatic phosphoryl transfer reactions. 18 O substitution in the nonbridge oxygens of a phosphoryl group makes the reaction go faster when the bond order is higher to these oxygens in the transition state than in the reactant, whereas the reaction goes slower if the bond order is less. The isotope effects are measured by the remote label method, using an isotope ratio mass spectrometer for analysis. The bond order to p ‐nitrophenolate ion when it is the leaving group is indicated by the secondary 15 N isotope effect in the nitro group, with a value of 1.0028 representing nearly complete bond cleavage. It appears that the transition states for phosphoryl transfer have no more than one negative charge on the nonbridge oxygens, so that reactions of monoesters are dissociative, reactions of triesters are associative, and reactions of diesters are S N 2 with half bond order to entering and leaving groups.— C leland , W. W. Secondary 18 O isotope effects as a tool for studying reactions of phosphate homo‐, di‐, and triesters. FASEB J. 4: 2899‐2905; 1990.