Phenolic acid dimers in the cell walls of barley

Using gas chromatography/mass spectrometry we have observed six peaks in the single ion chromatograph at m/z 674 of the alkaline extract of barley straw cell walls. The breakdown pattern of these ions suggests that they could all be isomers of 5,5′-bis-dehydroferulic acid. Since only the 5,5′ linkage is reported to be present in plants this tentatively suggests that these compounds exhibit stable orientational isomerism about this bond. Plausible breakdown mechanisms are presented which allow for the complete assignment of structure to all the isomers. These mechanisms suggest that the two pairs of breakdown ions at m/z 556 and 467, and 265 and 193 are representative of trans and cis double bonds, respectively.