Structure and biosynthesis of β-diketones in barley spike epicuticular wax

The structure and composition of the β-diketones in barley spike and carnation leaf epicuticular waxes have been determined by gas liquid chromatography-mass spectrometry. Barley wax contained, in addition to the well known hentriacontan-14,16-dione, small amounts of tritriacontan-16,18-dione, nonacosan-12,14-dione and nonacosan-14,16-dione. Five β-diketones were identified in carnation wax: nonacosan-10,12-dione, nonacosan-12,14-dione, hentriacontan-12,14-dione, tritriacontan-12,14-dione and tritriacontan-14,16-dione. The biosynthesis of the β-diketones was studied by incorporating various precursors into tissue slices of barley spikes with awns remowed. Incorporation of [1-14C]-pentadecanoic acid labelled a novel C30 β-diketone which upon cleavage contained the 14C in the Csub(16-30) end. This confirms that the carbon chain of the C31 β-diketone is formed in vivo by an elongation mechanism which proceeds from the C31 toward the C1 end of the molecule. A comparison of the position of label within the β-diketone chains synthesized from C12, C14, C16 and C18 labelled precursors revealed that the β-keto acyl elongase which is responsible for β-diketone carbon chain synthesis has the following specificities: C14 or C16 chains serve equally well as primers; C12 chains are first elongated to C14 or C16 before they can serve as primers; C18 chains can not serve as primers. After protection of the β-diketo groups by the β-keto acyl elongase either 7 or 8 additional C2 units are added before the presumed decarboxylation to yield the complete β-diketone molecule. (author)