Journal of the American Chemical Society p. 11676 - 11681 (1993)
Update date:2022-08-11
Topics:
Rao, Muralikrishna
Barlow, Paul N.
Pryor, Alton N.
Paneth, Pyotr
O'Leary, Marion H.
Quinn, Daniel M.
Phillip Huskey
The nitrogen-15 isotope effect on V/K for Electrophorus electricus acetylcholinesterase-catalyzed hydrolysis of o-nitroacetanilide has been determined by isotope ratio mass spectrometry. The effect determined in buffered H2O (0.1 M sodium phosphate, 0.1 N NaCl, pH 7.3, 25 °C) is 15V/K = 1.0119 ± 0.0005. A small though palpable decrease of the isotope effect is observed when the reaction is run in equivalently buffered D2O (pD = 7.7), 15V/K = 1.0106 ± 0.0002. The corresponding solvent isotope effect is DV/K = 1.56 ± 0.03. The solvent isotope effect on the nitrogen isotope effect is interpreted in terms of a mechanism in which successive transition states for induced fit and for formation and decomposition of a uninegative tetrahedral intermediate contribute to rate determination of V/K. Numerical modeling allows relatively narrow limits to be placed on the isotope effects for the chemical steps. The solvent and substrate isotope effects for the formation of the tetrahedral intermediate are DK5 = 2.6-3.7 and 15k5 = 1.000-1.009, respectively. The corresponding isotope effects for the decomposition of the intermediate are Dk7 = 1.0-1.5 and 15k7′ = 15k515k7/15k6 = 1.027-1.053. The value of 15k7′ is consistent with a transition state for decomposition of the tetrahedral intermediate in which C-N bond breaking is occurring.
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