Mechanistic Imperatives for the Reaction Catalyzed by Isopentenyl Pyrophosphate Isomerase: Free Energy Profile for Stepwise Isomerization in Water through a Tertiary Carbocation Intermediate
✍ Scribed by Maria M. Toteva; John P. Richard
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 198 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0045-2068
No coin nor oath required. For personal study only.
✦ Synopsis
Rate and equilibrium constants are reported for the stepwise allylic 1,3-isomerization of 4-(4-methoxyphenyl)-2-methyl-1-butene (2) to give 4-(4-methoxyphenyl)-2-methyl-2-butene (3) in water through a simple tertiary carbocation intermediate 1 ϩ , and the data are used to construct a free energy profile for the reaction. This profile shows that isopentenyl pyrophosphate isomerase stabilizes the carbocation-like transition state for the stepwise isomerization of simple alkenes by ca. 16 kcal/mol. The barriers for the deprotonation of the tertiary carbocation 1 ϩ by solvent water are significantly smaller than those for the protonation of simple enolates by this solvent. This difference favors a concerted mechanism for the enzyme-catalyzed 1,3-isomerization of alkenes, which avoids the formation of a tertiary carbocation intermediate.