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Two 8−8-coupled sinapic acid dehydrodimers and at least three sinapate−ferulate heterodimers have been identified as saponification products from different insoluble and soluble cereal grain dietary fibers. The two 8−8-disinapates were authenticated by comparison of their GC retention times and mass spectra with authentic dehydrodimers synthesized from methyl or ethyl sinapate using two different single-electron metal oxidant systems. The highest amounts (481 μg/g) were found in wild rice insoluble dietary fiber. Model reactions showed that it is unlikely that the dehydrodisinapates detected are artifacts formed from free sinapic acid during the saponification procedure. The dehydrodisinapates presumably derive from radical coupling of sinapate−polymer esters in the cell wall; the radical coupling origin is further confirmed by finding 8−8 and 8−5 (and possibly 8−O−4) sinapate−ferulate cross-products. Sinapates therefore appear to have an analogous role to ferulates in cross-linking polysaccharides in cereal grains and presumably grass cell walls in general. Keywords: Zizania sp.; Gramineae; wild rice; dietary fiber; hydroxycinnamic acid; sinapate; sinapic acid; dehydrodimer; ferulate; ferulic acid; radical coupling; cross-coupling; cell-wall cross-linking; single-electron oxidation
Bunzel et al. (Thu,) studied this question.