Elucidation of the partial structure of polymeric thearubigins from black tea by chemical degradation

Butanol-soluble neutral and acidic thearubigins were prepared by a combination of solvent extraction, fractional precipitation, and Toyopearl column chromatography. These pigments showed similar properties to those of Roberts' thearubigins on the paper chromatogram, and a convex broad band with several peaks on reversed-phase HPLC, The thearubigins were methylated, degallated, and chemically degraded with KMnO4 under alkaline conditions, From the degradation products, methyl esters of 4-methoxy benzoic acid, 3,4-dimethoxy benzoic acid, 3,4,5-trimethoxy benzoic acid, 3,4-dimethoxy-1,2-benzenedicarboxylic acid, 3,4-dimethoxy-1,5-benzenedicarboxylic acid, 3,6-dimethoxy-1,6-benzenedicarboxylic acid, 3,4,5-trimethoxy-1,2-benzenedicarboxylic acid, 4,5,6-trimethoxy-1,2,3-benzenetricarboxy acid, 4,5,4',5'-tetramethoxy-2,2'-diphenic acid, and 3,4,5,3',4',5'-hexamethoxy-2,2'-diphenic acid were detected by using GC-MS and GC-SIN. The butanol-soluble neutral thearubigins, which had been purified by Toyopearl column chromatography, mere hydrolyzed by treating with HCl-BuOH. Cyanidin and delphinidin were identified in the reaction mixture, These results suggest that thearubigins are heterogeneous polymers of flavan-3-ols and flavan-3-ol gallates which have a bond at C4, C6, C8, C2', C5', and C6' in the flavan-3-ol units, In addition to the C4-C8 or C6 interflavanoid linkage, a C6'-C6' linkage was also found, and a possible partial structure for thearubigin is proposed.