A hydrocarbon biomarkers group whose molecular structure contains the basic unit isoprene, consisting of five carbon atoms with a branch at the second atom. The most common pattern is head to tail. Many petroleum hydrocarbons are diagenetic derivatives of isoprenoid polymers. The most common isoprenoids in petroleum geochemistry are the acyclic isoprenoids that include pristane (iC19) and phytane (iC20).