PHOSPHORYL-TRANSFER REACTIONS OF PHOSPHODIESTERS - CHARACTERIZATION OF TRANSITION-STATES BY HEAVY-ATOM ISOTOPE EFFECTS

The secondary O-18 and N-15 isotope effects have been measured for phosphoryl-transfer reactions of the phosphodiester 3,3-dimethyl p-nitrophenyl phosphate under conditions of acid and alkaline hydrolysis, cleavage by beta-cyclodextrin, and cleavage by snake venom phosphodiesterase. Isotopic labeling and other experiments show that these reactions proceed by an S(N)2(P) mechanism. The secondary O-18 isotope effect measures changes in transition state bonding to the nonbridge oxygen atoms in the central phosphoryl group in these S(N)2 reactions. The N-15 isotope effects measure transition-state bond cleavage to the leaving group p-nitrophenol. The isotope effect data indicate that weaker nucleophiles lead to a more associative transition state with respect to the central phosphoryl group (decreased bonding to the nonbridge oxygens) while the extent of bond cleavage to the leaving group is affected little. Comparison of isotope effect data from the attack of hydroxide on a phosphodiester monoanion with that of a neutral phosphotriester suggests the oxyanion in the diester may assist in expulsion of the leaving group.