Terpene synthase enzymes build the vast diversity of monoterpenes (C10) from a single linear precursor, geranyl pyrophosphate (GPP), through a carbocation cascade — no external reagents, just controlled ionization and ring closure:
1. Ionization: GPP --(-PPi)--> allylic carbocation
2. Cyclization: allylic cation --(6-endo attack)--> α-terpinyl cation (monocyclic)
3. Branch point (this simulator):
(a) -H+ -> limonene (exocyclic diene)
(b) +H2O -> α-terpineol (tertiary alcohol)
(c) 2nd cyclization -> pinyl cation -> methyl shift, -H+ -> α-pinene (bicyclic)
Every step obeys Markovnikov-type addition: the ring closes so the new positive charge lands on the more substituted (tertiary) carbon, exactly as electrophilic addition predicts. The enzyme's role is purely to pre-organize the flexible chain and electrostatically stabilize the cation — it does not change which products are thermodynamically reachable, only how fast the reaction reaches them.
The rate constant panel applies the Arrhenius equation to the rate-limiting cyclization step:
k = A · e^(-Ea / RT)
A ≈ 1×10¹³ s⁻¹ (typical unimolecular pre-exponential)
Ea ≈ 12 kcal/mol (enzyme-catalyzed; ~30+ kcal/mol uncatalyzed in solution)
R = 1.987×10⁻³ kcal·mol⁻¹K⁻¹
- Pathway buttons — choose which product the terpinyl cation collapses to; the 2D schematic and energy diagram update to match the real mechanism for that branch.
- Progress slider — scrub the reaction coordinate by hand; the bond-breaking, ring-folding and bond-forming steps are geometrically interpolated between the real intermediates.
- Play — auto-advances the coordinate; speed scales with temperature, illustrating that a hotter system explores the coordinate faster (loosely, the k above).
- Temperature slider — recomputes k live via the Arrhenius equation; raising T from 270K to 340K changes k by roughly two orders of magnitude for a 12 kcal/mol barrier.
Real-world relevance: this exact GPP → terpinyl cation branch point is why pine resin, citrus peel, and lavender oil share one biosynthetic ancestor — limonene, α-terpineol and α-pinene are all downstream of the same carbocation, and which terpene synthase an organism expresses decides which scent it makes.