Phytoalexin of infected sweet pepper fruits (Capsicum annuum) Capsidiol is a bicyclic terpene that is biosynthetically derived from the mevalonate pathway via farnesyl pyrophosphate (FPP). The E,E-farnesyl cation is first created by the loss of pyrophosphate. Kinetic studies have indicated that turnover appears to be limited by a chemical step after the initial loss of pyrophosphate. Crystal structures of recombinant tobacco 5-epi-aristolochene synthase (TEAS), alone and also complexed with two FPP analogues have been reported and analyzed to suggest the following mechanism of biosynthesis. The E,E-farnesyl cation undergoes cyclization to form the germacryl cation. The second ring closure gives the bicyclic eudesmyl cation, which is stabilized by various dipole interactions, then H-2 migrates to C-3 producing a tertiary cation at C-2 (farnesyl numbering). Production of 5-epi-aristolochene from FPP by 5-epi-aristolochene 3-hydroxylase, a sesquiterpene cyclase, is considered the critical step in capsidiol biosynthesis. Aristolochene synthase enzymes from Penicillium roqueforti, Nicotiana tabacum have been purified and their crystal structures have been reported suggesting different steriochemistries for aristolochene. Penicillium roqueforti's enzyme appears to synthesizes aristolochene by way of (S)-germacrene A , however, the Nicotiana tabacum enzyme 5-epi-aristolochene synthase produces the diastereoisomeric product by way of (R)-germacrene A.; Capsidiol is a phytoalexin, a natural fungicide present in pepper. (PMID: 10335386); Capsidiol shows bacteriostatic properties in vitro against Helicobacter pylori with a minimum inhibitory concentration (MIC) of 200 microg/mL. (PMID: 17002415); Capsidiol is a bicyclic, dihydroxylated sesquiterpene produced by several solanaceous species in response to a variety of environmental stimuli. It is the primary antimicrobial compound produced by Nicotiana tabacum in response to fungal elicitation, and it is formed via the isoprenoid pathway from 5-epi-aristolochene. (PMID: 11556809); Capsidiol is a terpenoid compound that accumulates in tobacco Nicotiana tabacum and chili pepper Capsicum annuum in response to fungal infection. Capsidiol is categorized under the broad term of phytoalexin, a class of low molecular weight plant secondary metabolites that are produced during infection. Phytoalexins are also characterized as a part of a two pronged response to infection which involves a short term response consisting of production of free radicals near the site of infection and a long term response involving the production of hormones and an increase in enzymes to biosynthesize phyoalexins such as capsidiol.