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Isoprene Rule

  • Page ID
    39398
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    Formally, in biosynthesis of terpenes, two or more isoprene molecules are linked to one another. Linking between two isoprene molecules could occur in three ways, given that the head and the tail of the molecule are primarily involved in the linking:

    1-1 Linkage

    The head of one isoprene molecule could link with the head of another isoprene molecule.

    isoprenerule1.png

    Figure 1: This link is called a head-to-head or 1-1 link.

    1-4 Linkage

    The head of one isoprene molecule could link with the tail of another isoprene molecule.

    isoprenerule2.png

    Figure 2: This link is called a head-to-tail or 1-4 link.

    4-4 Linkage

    The tail of one isoprene molecule could link with the tail of another isoprene molecule.

    isoprenerule3a.png

    Figure 3: This link is called a tail-to-tail or 4-4 link.

    Cyclic terpenes also contain links that are neither 1-1, 1-4, nor 4-4, which are called crosslinks.

    Definition: The Isoprene Rule

    The isoprene rule states that, in most naturally occurring terpenes, there are no 1-1 or 4-4 links.

    eg. 1:

    isoprenerule4.png

    Figure 4: Myrcene

    eg. 2:

    isoprenerule5.png

    Figure 4: Limonene

    eg. 3:

    isoprenerule6.png

    Figure 5: Retinol

    A terpene that does not obey the isoprene rule is called an irregular terpene. eg:

    isoprenerule7.png

    Figure 6: β-Carotene


    Isoprene Rule is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Gamini Gunawardena via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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