Phlobaphenes

Phlobaphenes are phenolic polymers that can be present in floral organs of maize (Zea mays L.), including the pericarp (the hard, outermost layer of the kernel, derived from the ovary wall), the cob, the husks (the leaves covering the ear), the tassel glumes, the cob pith, and the tassel pith. Accumulation of phlobaphenes results in a red pigmentation (Styles and Ceska, 1989). Certain lines of sorghum (Sorghum bicolor L. (Moench)) also produce phlobaphenes (Boddu et al., 2005).

The structure of phlobaphenes is poorly understood. These compounds are believed to be polymers of flavan-4-ols, notably apiferol (1.100) and luteoferol (1.101) (Shirley-Winkel, 2001). Both of these monomers are derived from naringenin (1.35).

The polymerization is thought to be under chemical, rather than enzymatic control, and give rise to a polymer (1.102) in which the monomers are linked via a 4-8' linkage. The C-C bonds between the flavan-4-ol monomers would be difficult to break, which could help explain the difficulties with the structural elucidation of phlobaphenes. The reaction mechanism whereby the hydroxyl group on C-4 is eliminated and a C-C

bond between two monomers is formed, is however, not easily imagined, and it is possible that the structure of phlobaphenes is different than is currently postulated.

OH OH

OH OH

OH OH

OH OH

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