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Gas-Phase Chemical Dynamics Simulations on the Bifurcating Pathway of the Pimaradienyl Cation Rearrangement: Role of Enzymatic Steering in Abietic Acid Biosynthesis
journal contribution
posted on 2012-04-10, 00:00 authored by Matthew
R. Siebert, Paranjothy Manikandan, Rui Sun, Dean J. Tantillo, William L. HaseThe biosynthesis of abietadiene is the first biosynthetically
relevant
process shown to involve a potential energy surface with a bifurcating
reaction pathway. Herein, we use gas-phase, enzyme-free direct dynamics
simulations to study the behavior of the key reaction (bifurcating)
step, which is conversion of the C20 pimaradienyl cation
to the abietadienyl cation. In a previous study (J. Am. Chem.
Soc. 2011, 133, 8335), a truncated
C10 model was used to investigate this reaction. The current
work finds that the complete C20 pimaradienyl cation gives
reaction dynamics similar to that reported for the truncated C10 model. We find that in the absence of the enzyme, the C20 abietadienyl cation is generated in almost equal quantity
(1.3:1) as an unobserved (in nature) seven-membered ring product.
These simulations allude to a need for abietadiene synthase to steer
the reaction to avoid generation of the seven-membered ring product.
The methodology of post-transition state chemical dynamics simulations
is also considered. The trajectories are initiated at the rate-controlling
transition state (TS) separating the pimaradienyl and abietadienyl
cations. Accurate results are expected for the short-time direct motion
from this TS toward the abietadienyl cation. However, the dynamics
may be less accurate for describing the unimolecular reactions that
occur in moving toward the pimaradienyl cation, due to the unphysical
flow of zero-point energy.