Gas-Phase Chemical Dynamics Simulations on the Bifurcating Pathway of the Pimaradienyl Cation Rearrangement: Role of Enzymatic Steering in Abietic Acid Biosynthesis

The 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 C<sub>20</sub> pimaradienyl cation to the abietadienyl cation. In a previous study (<i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 8335), a truncated C<sub>10</sub> model was used to investigate this reaction. The current work finds that the complete C<sub>20</sub> pimaradienyl cation gives reaction dynamics similar to that reported for the truncated C<sub>10</sub> model. We find that in the absence of the enzyme, the C<sub>20</sub> 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.