Transition State Analysis of Model and Enzymatic Prenylation Reactions

Abstract

To obtain a transition state (TS) structure for an enzyme-catalyzed prenylation reaction, S N 1 and S N 2 model substitution reactions with dimethylallyl chloride were first studied. 13 C Kinetic isotope effects (KIEs) for the model reactions were measured by a natural abundance NMR method and used to validate the computational methods that would be used in the subsequent determination of the enzymatic TS structure. Using a primary 13 C KIE and a secondary 2 H KIE measured via mass spectrometry, a TS structure for the enzyme-catalyzed reaction was computed; a density functional level of electronic structure theory using the m PW1N functional in combination with the 6-31+G(d) basis set was employed for those calculations. That structure has a C−O bond length of 1.69 Å and a C−S bond length of 3.70 Å. While the former bond length is similar to that for a nonenzymatic S N 2 reaction, the latter is considerably (0.90 Å) longer, indicating that the enzyme effects catalysis via an “exploded” TS structure.