Diferulic acids (also known as dehydrodiferulic acids) are organic compounds that have the general chemical formula C20H18O8, they are formed by dimerisation of ferulic acid. Just as ferulic acid is not the proper IUPAC name, the diferulic acids also tend to have trivial names that are more commonly used than the correct IUPAC name. Diferulic acids are found in plant cell walls, particularly those of grasses.
There are currently nine known structures for diferulic acids. They are usually named after the positions on each molecule that form the bond between them. Included in the group are 8,5'-DiFA (DC) and 8,8'-DiFA (THF), which are not true diferulic acids, but probably have a similar biological function. The 8,5'-DiFA (DC) lost CO2 during its formation, the 8,8'-DiFA (THF) gained H2O during its formation. Ferulic acid can also form trimers and tetramers, known as triferulic and tetraferulic acids respectively.
Diferulic acids are thought to have a structural function in plant cell walls, where they form cross-links between polysaccharide chains. They have been extracted attached to a few sugar molecules at both ends, but so far no definitive proof of them linking separate polysaccharide chains has been found. In suspension-cultured maize cells, dimerisation of ferulic acid esterified to polysaccharides occurs mostly in the protoplasm, but may occur in the cell walls when peroxide levels increase due to pathogenesis. In suspension-cultured wheat cells, only the 8,5'-diferulic acid is formed intraprotoplasmically with the other dimers being formed in the cell wall.
Most diferulic acids are not commercially available and must be synthesised in lab. Synthetic routes have been published, but it is often simpler to extract them from plant material. They can be extracted from plant cell walls (often maize bran) by concentrated solutions of alkali, the resulting solution is then acidified and phase separated into an organic solvent. The resulting solution is evaporated to give a mixture of ferulic acid moieties that can be separated by column chromatography. Identification is often by high performance liquid chromatography with a UV detector or by LC-MS. Alternatively they can be derivatised to make them volatile and therefore suitable for GC-MS. Curcumin can be hydrolyzed (alkaline) to yield two molecules of ferulic acid. Peroxidases can produce dimers of ferulic acid, in the presence of hydrogen peroxide through radical polymerization.
Diferulic acids are more effective inhibitors of lipid peroxidation and better scavengers of free radicals than ferulic acid on a molar basis.
See also 
- Phenolic compounds as cross-links of plant derived polysaccharides, M.Bunzel, J.Ralph and H.Steinhart, Czech J. Food Sci., 22:64-67 (2004).
- Structural identification of dehydrotriferulic and dehydrotetraferulic acids isolated from insoluble maize bran fibre, M.Bunzel, J.Ralph, P.Brüning and H.Steinhart, J. Agr. Food Chem., 54:6409-6418 (2006).
- New discoveries relating to diferulates, J.Ralph, J.H.Grabber, R.D.Hatfield and G.Wende, 1996 USDFRC Research Summary, p70-71 (1996).
- Identification and synthesis of new ferulic acid dehydrodimers present in grass cell walls, J.Ralph, S.Quideau, J.H.Grabber and R.D.Hatfield, J. Chem. Soc. Perk. T. 1, 3485-3498 (1994)
- Ferulic acid dehydrodimers as structural elements in cereal dietary fibre, Eur. Food Res. Technol., A.Renger and H.Steinhart, 211:422-428 (2000)
- European Food Research And Technology, 2002, Volume 214, Number 1, pages 33-42, doi:10.1007/s00217-001-0417-6
- Esterified phenolics of the cell walls of chufa (Cyperus esculentus L.) tubers and their role in texture, M.L.Parker, A.Ng, A.C.Smith and K.W.Waldron, J. Agr. Food Chem., 48:6284-6291 (2000)
- Dehydrodiferulic acids from sugar-beet pulp, V.Micard, J.H.Grabber, J.Ralph, C.M.G.C.Renard and J-F.Thibault, Phytochemistry 44:1365-1368 (1997)
- Cross-linking of arabinoxylans via 8-8-coupled diferulates as demonstrated by isolation and identification of diarabinosyl 8-8(cyclic)-dehydrodiferulate from maize bran, M.Bunzel, E.Allerdings, J.Ralph and H.Steinhart, J. Cereal Sci., 47:29-40 (2008).
- Intraprotoplasmic and wall-localised formation of arabinoxylan-bound diferulates and larger ferulate coupling-products in maize cell-suspension cultures, S.C.Fry, S.C.Willis, A.E.J.Paterson, Planta 211:679-692 (2000)
- Intracellular feruloylation of arabinoxylan in wheat: evidence for feruloyl-glucose as precursor, N.Obel, A.C.Porcina, H.V.Scheller,Planta 216:620-629 (2003)
- Vernetzung von Phenolcarbonsäure-estern von Polysacchariden durch oxydative phenolische Kupplung, T. Geissman and H. Neukom, Helv. Chim. Acta, 54:1108-1112 (1971).
- Ferulic acid dehydrodimers from wheat bran: isolation, purification and antioxidant properties of 8-O-4-diferulic acid, M-T.Garcia-Conesa, G.W.Plumb, K.W.Waldron, J.Ralph and G.Williamson, Redox Rep., 55:2418-2423 (1997)
- Diferulic acid as a component of cell walls of Lolium multiflorum, R.D.Hartley and E.C.Jones, Phytochemistry, 15:1157-1160 (1976).