- An Isotopic Study (2H and 18O) of the Enzymatic Conversion of Linoleic Acid into Colneleic Acid with Carbon Chain Fracture: the Origin of Shorter Chain Aldehydes
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Contrary to earlier reports, the divinyl 9-ether oxygen of colneleic acid is shown by experiment with 18O2 to originate from oxygen, not water.Using -9(S)-hydroperoxyoctadeca-10(E),12(Z)-dienoic acid, made enzymatically from synthetic linoleic acid, it is found that the distribution of deuterium as determined by NMR and mass spectrometry in the fractured carbon chain of colneic acid formed by potato enzyme, is consistent with the intervention of an epoxy carbonium ion intermediate.Though divinyl acids such as colneleic and colnelenic acid give the expected shorter chain aldehydes on treatment with aqueous acid, it is likely that the latter are formed in most plants by trapping of a monovinyl oxonium ion rather than by rehydration of colneleic and colnelenic acid.
- Crombie, Leslie,Morgan, David O.,Smith, Elisabeth H.
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p. 567 - 575
(2007/10/02)
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- The Biosynthesis of Calendic Acid, Octadeca-(8E,10E,12Z)-trienoic Acid, by Developing Marigold Seeds: Origins of (E,E,Z) and (Z,E,Z) conjugated Triene Acids in Higher Plants
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Using an homogenate of marigold seeds, gathered 15 days after flower-drop, linoleic acid and oleic acid were incorporated into calendic acid with little randomisation of the label. Linolenic acid was not incorporated.Despite requiring 12,13-dehydrogenation to form linoleic acid, the putative precursor, oleic acid was better incorporated than administered linoleic acid.Stearic acid, requiring both 12,13- and 9,10-dehydrogenation, was a poor precursor.The results of a series of doublelabelling experiments support and supplement these conclusions. Octadec-9-enoic acid was synthesised and employed in a mass-spectral experiment to show that conversion into calendic acid involves loss of two deuterium and two hydrogen atoms (deuterium at C-16 and C-17 was introduced for loading purposes only, in order to increase the sensitivity of the experiment).Taken with -labelling work, the experiment indicates that during conversion of linoleic acid into calendic acid, there is no loss of the labelled hydrogens at C-9, -10, -12, or -13, but loss of hydrogen from each of C-8 and C-11. (9S)-Hydroxyoctadeca-(10E,12Z)-dienoic acid (α-dimorphecolic acid) was isolated and converted into (R/S)-hydroxy- and hydroperoxy-octadeca-(10E,12Z)-dienoic acids.Neither labelled specimen was converted into calendic acid by marigold seed homogenate.Abstraction of a hydrogen atom from C-11 of linoleic acid is viewed as giving an (E)-allylic radical which, as in lipoxygenase reactions, can be trapped by oxygen at C-9, thus providing a source of α-dimorphecolic acid, a minor component of marigold seed oil.However, this hydroxyacid is apparently a terminus rather than an intermediate for calendic acid.Formation of the latter seems best accounted for by formal loss of a hydrogen atom from C-8 of the (E)-allylic redical.The general position relating to the formation of (E,E,Z) and (Z,E,Z)-trienes is summarised.
- Crombie, Leslie,Holloway, Stephen J.
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p. 2425 - 2434
(2007/10/02)
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