Heavy-atom tunneling occurs when an atom larger than hydrogen, such as carbon, does not have sufficient energy to go over a barrier, so it tunnels through it instead. This phenomenon is reported to be prevalent in certain reactions in which atoms have to move only short distances. Whereas numerous hydrogen transfer processes in biochemistry are dominated by hydrogen tunneling, there are not yet any reported observations of heavy-atom tunneling in biochemical reactions. However, heavy-atom tunneling was recently predicted to make a modest contribution to the rates of pericyclic reactions in the biosynthesis of tetrahydrocannabinol. Motivated by the common occurrence of pericyclic reactions in biochemistry, we used computational chemistry to study various model reactions relevant to biosynthetic pathways, such as the electrocyclization of hexatriene. This work focuses on the 6-electron electrocyclization step in the biosynthesis of occidentalol. M06-2X/6-31G* results of multidimensional tunneling calculations suggest that tunneling by carbon is predicted to dominate the rate (&gt;50%) at temperatures of 220K or lower. The prediction of significant tunneling in such processes potentially broadens the scope of heavy-atom tunneling into biochemistry and allows us to identify structural features that enhance such tunneling in biochemical reactions.

Heavy-atom tunneling in biosynthetic reactions: A computational study on occidentalol

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