Structural elucidation of new ferulic acid-containing phenolic dimers and trimers isolated from maize bran Mirko Bunzel, a, * John Ralph, b,c Carola Funk a and Hans Steinhart a a Institute of Biochemistry and Food Chemistry, Department of Food Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany b US Dairy Forage Research Center, USDA-ARS, 1925 Linden Drive West, Madison, WI 53706-1108, USA c Department of Forestry, University of Wisconsin, Madison, WI 53706, USA Received 20 April 2005; revised 22 June 2005; accepted 27 June 2005 Available online 14 July 2005 Abstract—Four new phenolic dimers and trimers that contain ferulic acid moieties were isolated from the alkaline hydrolyzate of insoluble maize bran fiber and their structures were established by 1D/2D NMR and mass spectrometry. The biological role of one dimer remains unclear whereas the dimeric vanillin-ferulic acid-cross-product probably represents an oxidative degradation product from the corresponding diferulate. Both ferulic acid dehydrotrimers are able to cross-link polysaccharide chains. However, the 5-5/ 8-O-4(H 2 O)-triferulic acid may be a cross-link in its identified structure whereas we assume that the identified 8-O-4/8-5(non-cyclic)- triferulic acid arose from a natural 8-O-4/8-5(cyclic)-triferulate analog during the saponification process. Ó 2005 Elsevier Ltd. All rights reserved. Grasses and cereals contain substantial amounts of cell wall-bound hydroxycinnamate esters linked to polysac- charides. 1 Particularly, ferulates play an important role in cross-linking cell wall polysaccharides. 2 Ferulates dehydrodimerize via a radical, oxidative mechanism to form mainly 8-5-, 8-O-4-, 5-5-, 8-8-, and 4-O-5-coupled dehydrodiferulates (often simply referred to as diferu- lates), thus cross-linking two polysaccharide chains. 3–8 Polysaccharide cross-linking plays a significant role in the plant and plant derived products, for example, by effecting cell wall strength, fiber degradability, and food textural and processing properties. 9–12 Furthermore, ferulates and diferulates act as cross-links between poly- saccharides and lignin. 13,14 Recently, three dehydrotri- ferulic acids have been isolated and identified for the first time, 15–17 indicating that also higher ferulate oligo- mers are involved in cross-linking cell wall polysaccha- rides as proposed by Fry et al. 18 However, to date it is not possible to decide whether triferulates cross-link three polysaccharide chains or whether their cross-link- ing capacity is limited to two polysaccharide chains due to difficulties in the approach of three unwieldy polymer chains in a limited space. In this paper, we describe the isolation and structural identification of four new ferulic acid-containing dimers and trimers from maize bran. These studies aim to char- acterize the range of such products to be found in cereal grains (and grass cell walls in general), and begin to understand their role and impact on grain properties. Compounds 1–4 (Fig. 1) were isolated from maize bran according to a recently published method developed by our group to isolate dehydrodiferulic acids (DFA) on a semipreparative scale. 19 In brief, maize bran was defat- ted, milled, and destarched and partially deproteinated by treatment with a-amylase, protease, and amyloglu- cosidase. Following centrifugation the residue was washed (hot water, ethanol, and acetone) and dried. Alkaline hydrolysis of insoluble maize fiber (40 g, com- pound 1: 30 g) was carried out using 2 M NaOH under nitrogen and protected from light. After acidification (pH <2), liberated phenolic acids were extracted into diethyl ether. The organic layer was purified by liquid– liquid extraction with NaHCO 3 solution and re-extrac- tion into diethyl ether after acidification. The dried resi- due was redissolved in 10 mL MeOH/H 2 O 50/50 (v/v). 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.06.140 Keywords: Zea mays L.; Gramineae; Cell wall cross-linking; Triferulic acid; Triferulate; Ferulic acid; Ferulate; Phenolic dimers; Vanillin; Oxidative degradation; Arabinoxylans; Dietary fiber. * Corresponding author. Tel.: +49 (0)40 42838 4379; fax: +49 (0)40 42838 4342; e-mail: mirko.bunzel@uni-hamburg.de Tetrahedron Letters 46 (2005) 5845–5850 Tetrahedron Letters