Quantum mechanics could help explain a long-held mystery surrounding triterpene compounds.

Triterpenes are a class of cyclic biological molecule important for many biological processes of plants and animals, made in nature by enzymes in a sequence of reactions. Although these enzymes catalyse the construction of specific triterpene molecules, understanding how different reaction mechanisms give the different products has not been looked at in detail, until now.

Seiichi Matsuda and colleagues at Rice University, US, used quantum mechanical calculations to model the energy level profiles of triterpene synthesis. Such computational methods help with understanding how the differences between enzymes' reaction sites could lead to the formation of a range of triterpenes. 

Matsuda calculated the energy profiles of a series of positively-charged (cationic) intermediate structures involved in triterpene synthesis. He showed there to be 'tremendous error' in many of the previously calculated profiles; for example, one ring-forming step was found to be an exothermic process having originally been predicted to be endothermic.

The group also showed how the different alignment of certain bonds within the cationic intermediates could drive the reaction forward. The size of groups close to the enzyme's reactive site also affected the ability of the intermediates to line up, exerting a subtle but powerful electronic control over the reaction that dictates which product is formed.

Many goals remain for such studies in the future, said Matsuda. A more detailed modelling of enzymes' active sites together with molecular dynamics studies will continue to improve overall calculation accuracy. When enlarged molecular models are involved this could be a major challenge, Matsuda said.

David J Parker