chapter  19
16 Pages

Walnut Polyphenols: Structures and Functions

WithToshiyuki Fukuda

Crushed walnuts were extracted with 70% aqueous ethanol at room temperature for 24 h, and the concentrated ltrate was partitioned into n-hexane-, ethyl acetate-, and n-butanol-soluble portions. Each extract was separated by successive column chromatography over DIAION HP-20 (Mitsubishi Kasei Co., Tokyo, Japan), TOYOPEARL HW-40F (Tosoh Co., Tokyo, Japan), and MCI GEL CHP20P (Mitsubishi Kasei Co., Tokyo, Japan) columns and preparative high-performance liquid chromatography (HPLC) to give 37 compounds (Figure 19.1). The compounds identi ed were as follows: casuarictin (1) [12], casuariin (2) [12], casuarinin (3) [12], 1,2-di-O-galloyl-4,6-O-(S)-HHDP-βd-glucopyranose (4) [13], euprostin A (5) [14], gemin D (6) [15], 2,3-O-(S)-HHDP-d-glucopyranose (10) [16], isostrictinin (11) [17], pedunculagin (12) [12], 1,2,3,4,6-penta-O-galloyl-β-dglucopyranose (13) [18], platycaryanin A (14) [19], praecoxin A (15) [20], pterocarinin A (16) [21], rugosin C (17) [22], rugosin C methyl ester (18), stachyuranin B (19) [23], stenophyllanin A (20) [24], strictinin (21) [12], tellimagrandin I (22) [25], tellimagrandin II (23) [25], 1,2,3,6-tetraO-galloyl-β-d-glucopyranose (24) [18], 1,2,4,6-tetra-O-galloyl-β-d-glucopyranose (25) [18],

TABLE 19.1 SOD-Like Activity of Various Nut Extracts

1-desgalloylrugosin F (26) [26], heterophylliin D (28) [27], rugosin F (29) [28], ellagic acid (30), ellagic acid 4-O-xylopyranoside (31) [29], (+)-catechin (32), adenine (35), adenosine (36), and l-tryptophan (37) by spectral analyses including nuclear magnetic resonance (NMR), mass spectrometry (MS), and circular dichroism (CD) spectra, and by degradation reactions as well as direct comparisons with authentic samples (Figures 19.2 through 19.5). Structures of new compounds,

glansrin A (7) [30], glansrin B (8) [30], glansrin C (9) [30], glansrin D (27) [31], glansreginin A (33) [31], and glansreginin B (34) [31] were elucidated based on spectral and chemical methods as shown in Figure 19.6

The antioxidant activity of various components isolated from walnuts was evaluated using the relevant EC50 values from measurements of SOD-like activity and 1,1-diphenyl-2-picrylhydrazyl (DPPH)

radical-scavenging activity (Table 19.2) [30,31]. All polyphenols exhibited antioxidant activity. Gallotannins, which have several galloyl groups in the molecule, such as 13 and 24, and ellagitannins, which have both a galloyl group and a hexahydroxyldiphenoyl (HHDP) group, such as 1, 3, 7, 8, 17, and 22, had the strongest activity. A complex tannin, 20, also showed strong antioxidant activity. Ellagitannins with no galloyl group, such as 10 and 12, had weaker effects than tannins which have galloyl groups on both SOD-like activity and DPPH radical-scavenging activity. The nonpolyphenols, such as nucleic acids and carboxylic acid derivatives, showed no antioxidant activity.