NOVELTY - Method (M1) to form fuel constituent from bioisoprene composition by chemically transforming substantial portion to non-isoprene compound(s) involves: (a) contacting with catalyst for olefin metathesis to form olefin(s) and catalytically hydrogenating to alkane(s) fuel constituent; (b) partially hydrogenating to isoamylene and alkylating with isoparaffin to high octane alkylate fuel constituent; or (c) partially hydrogenating to form isoamylene(s), contacting with catalyst for olefin metathesis to form olefin(s), and catalytically hydrogenating to alkane(s) fuel constituent.    USE - For forming fuel constituent from bioisoprene composition by chemically transforming to non-isoprene compound(s) (claimed). The fuel constituents can be used for making various fuel compositions and other products from isoprene.    ADVANTAGE - The method and composition produce fuel constituent from bioisoprene composition, by using unique combination of starting materials and chemical reactions to convert isoprene to fuel constituents. The starting material includes commercially beneficial amount of highly pure isoprene from bioisoprene composition. Fuel constituents that can be produced by method include hydrocarbons derived from isoprene metathesis, isoprene oligomers and alkylates of isoamylenes derived from isoprene. As compared to isoprene from bioisoprene composition, isoprene derived from petrochemical sources is impure 5C hydrocarbon fraction and requires extensive purification before polymerization or other chemical transformations. Several impurities are particularly problematic due to their structural similarity to isoprene and they can act as polymerization catalyst poisons. The biologically produced isoprene is substantially free of any contaminating unsaturated 5C hydrocarbons without undergoing extensive purification. The biological system producing the isoprene yields a composition that contains isoprene, in addition to other impurities that are specific for a biological system i.e. bioisoprene composition. Some bioisoprene compositions contain ethanol, acetone, and 5C prenyl alcohols. These components are more readily removed from isoprene stream than isomeric 5C hydrocarbon fractions that are present in isoprene compositions derived from petrochemical sources. Further, these impurities can be managed in bioprocess e.g. by genetic modification of producing strain, carbon feedstock, alternative fermentation conditions, recovery process modifications and additional or alternative purification methods. Purification of petroleum-derived takes time and additional resources and some of isoprene in crude mixture can be lost during process, leading to lower overall yields of desired fuel constituents. In addition, isoprene is a highly reactive compound, and tends to form gums and polymers under many reaction conditions. In method (M1), at least 95% of isoprene in bioisoprene composition is converted to non-isoprene compounds. The fuel composition (C1) produced by the method (M1) is substantially free of isoprene. Fuels derived from isoprene from bioisoprene composition can be distinguished from fuels derived from petrochemical carbon on basis of dual carbon-isotopic fingerprinting. For isoprene derived from extractive distillation of 5C streams from petroleum refineries, delta 13C is -22 to -24%, which is typical for light, unsaturated hydrocarbons derived from petroleum, and products derived from petroleum-based isoprene comprise isoprenic units with same delta 13C. Isoprene from a bioisoprene composition produced by fermentation of corn-derived glucose ( delta 13C = -10.73%) with minimal amounts of other carbon-containing nutrients (e.g. yeast extract) produces isoprene which can be polymerized into polyisoprene with delta 13C of -14.66 to -14.85%. Products produced from bioisoprene composition are expected to have delta 13C values that are less negative than those derived from petroleum-based isoprene. For isoprene derived from reaction of isobutylene with formaldehyde, delta 13C can be about -34.4% because formaldehyde is often derived from feedstocks with much more negative delta 13C values. The fuel constituent (C') and the fuel composition (C1) have delta 13C value of greater than -22% or -32 to -24%. Thus derivatives of isoprene from bioisoprene compositions and associated fuels, intermediates, and mixtures may be completely distinguished from their petrochemical derived counterparts on basis of dual carbon-isotopic fingerprinting, indicating new compositions. Further, the fuel constituent produced has energy density higher than that of ethanol; boosts cetane number of fuel e.g. petroleum-based fuel; reduces emission of petroleum based fuels; and has octane number of 80-120 or 30-130.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a system for producing a fuel constituent from a bioisoprene composition, comprising: at least one catalyst for catalyzing metathesis of isoprene in the bioisoprene composition to form an olefin product, and a catalyst capable of hydrogenating the olefin product to form an alkane fuel constituent; or a chemical capable of partially hydrogenating isoprene in the bioisoprene composition to produce an isoamylene, and isoparaffin for alkylating the isoamylene derived from the bioisoprene composition to produce a fuel constituent; or a chemical capable of partially hydrogenating isoprene in the bioisoprene composition to produce an isoamylene, at least one catalyst for catalyzing metathesis of the isoamylene produced from partial hydrogenation of the bioisoprene composition to form an olefin product, and a catalyst capable of hydrogenating the olefin product to form an alkane fuel constituent; where a substantial portion of the isoprene in the bioisoprene composition is chemically converted to at least one non-isoprene compound; and optionally a fermentation system comprising bioisoprene composition;    (2) a fuel constituent (C') produced by the method (M1); and    (3) a fuel composition (C1) comprising: the fuel constituent (C').