JARQ 28, 70-78 (1994) Chemistry and Utilization of Condensed Tannins from Tree Barks Seip .. OHARA. ... Wood Chemistry Division, Forestry and Forest Products Research Institute (Tsukub a, Ibaraki, 305 Japan) Abstract Chemical characteristics, protein-adsorbing capacity, chemical modifications and utilization of condensed tannis from tree barks were reviewed. Various aspects relating to the chemistry of these compounds and prospects for their utilization were discussed. Condensed tannins from Quercusjalcata, Salix sieboldiana and most of the Japanese conifers consist mainly of procyanidins and those from Acacia mearn­ sii of prorobinetinidins. The protein-adsorbing capacity of condensed tannins is closely related to their molecular weights and the positions of phenolic hydroxyl groups in the B-ring. The most effective hydroxyl group is located in the para­ position. In the reactions of catechin, the pyran ring is cleaved by alkaline treatment at 40°C or phenolation in the presence of BF3. The latter reaction can be effective for opening the pyran ring selectively. The tannins can be processed for utilization as cold-setting adhesives, polyurethane foams and adsorbents of heavy metal ions. Prospects for uses of condensed tannins include applications to biologically active compounds and functional materials. Discipline: Forestry and forest products Additional keywords: catechin, proanthocyanidins, protein-adsorbing capacity, pyran ring, tannin-derived polyurethane foams dicotyledoneae. They are present at very high Introduction concentrations in the bark of timber species such as conifers, eucalypts and leguminous Vegetable tannins are widely distributed in hardwoods. Their molecular weights cover a plants and have been used as tanning agents, wider range than those of hydrolysable tannins dyestuffs and drugs since older times. They with the highest one being 20,000. Recently, can be divided into two major groups, con­ many investigations on new forms of utiliza­ densed tannins and hydrolysable tannins. The tion and biological activities have been reported. former group consists of proanthocyanidins and In this paper the classification, chemical the latter of polyesters based on gallic acid or characteristics, protein-adsorbing capacity and hexahydroxydifenic acid and their derivatives. chemical modifications of condensed tannins, Condensed tannins are distributed in ferns and as well as several fields of utilization are allied species, and both monocotyledoneae and reviewed. Furthermore, various aspects relating This work was supported in part by the Integrated Research Program for Bio Renaissance Program from the Ministry of Agriculture, Forestry and Fisheries (BRP93 - V - B-I). Ohara: Chemistry a11d U1iliw1io11 of Tannins from Tree Barks 71 to the chemistry or these compounds and ponding (2R, 3R) isomers are designated by the prospects for their utilization are discussed. addition of the prefix 'cpi' to the beginning of each monomer name, and (2S, 3R) or (2S, Classification 3S) isomers are indicated by the addition of the prefix ent to the beginning of the appropri­ Condensed tannins belong to pro•an­ ate monomer name. thocyanidins as plant secondary metabolites. Proanthocyanidins which are polymers of sever­ Chemical characteristics al kinds or flavan-3-ols are classified as shown in Fig. I, according to the hydroxylation pat­ J) Quercus falcata terns or A- and B-rings of the monomer Q. falcata (southern red oak) is the most im­ units8>. Procyanidins are widely distributed portant species of red oak in the forests of the and prodelphinidins, profisetinidins and southern United States. Related compounds of prorobinetinidins are also found frequently in condensed tannins isolated from the bark of many plants. As each flavan-3-ol has two the southern red oak are listed in Fig. 25>. The asymmetric carbon atoms in the C-ring, there bark of southern red oak is a rich source of are four stereoisomers. Guibourtinidol, quercetin-3-rhamnoside (1) and it contains only fisetinidol, robinetinidol, afzelechin, catechin low concentrations of catechin (2). The three and gallocatechin are the monomer units with major dimeric proanthocyanidins are a (2R, 3S) absolute stereochemistry correspond­ epicatechin-(4.8-8)-catechin (3), catechin-(4a-8)­ ing to progibourtinidin, profisetinidin, catechin (5) and the 3-gallatc ester of prorobinetinidin, propelagonidin, procyanidin epicalechin-(4~·8)-calcchin ( 4). The acetone­ and prodelphinidin, respectively. The corres- water soluble tannins with a higher molecular weight consist of polymeric procyanidins predominantly made up of 2,3-cis chain ex­ R2 OH tender units with a 2,3-trans catcchin terminal. I The polymers contain only small amounts of HO~O2, 0 R3 I A C 3 2,3-trans procyanidin chain extender units and ~ only traces of prodelphinidin units. R, r OH 2) Salix sieboldiana There is only one report on the chemical n analysis of proanthocyanidins in the Salix spe­ cies grown in Japan. Proanthocyanidins and their derivatives from the bark of S. sieboldi­ ana, a shrub common in Japan, arc shown in Fig. 3 2>. This species contains a homologous series of acylated procyanidins (7), (9) and R1 (I/), together with the widespread procyanidin 8-1(6), B-3(8) and trimer (10). Their struc­ R,:H, R2:H, R3=H -·· Proglbourtlnldln R1:H, R2:0H, R3:H --· Prollsetinidin tures are assigned to the 3-0-(1-hydroxy-6-oxo- R :H, R :0H, R :0H -··· Proroblnetlnidln 1 2 3 2-cyclohexcne carboxylic acid esters) of pro­ R1: 0H, R2:H, R3=H --- Propelargonldln R1:0H, R2:0H, R3:H •··· Procyanldln cyanidins B -1, 8-3 and trimer. As 1-hydroxy- R :0H, R :0H, R : 0H ····Prodelphinldin 1 2 3 6-oxo-2-cyclohexene carboxylic acid is structur­ Fig. I. Proanthocyanidin classification ally related to saligenin, it is considered to be 72 JARQ 28(1) 1994 OH 0 HO Q(X'-:::, 6 h OH H 0 (1) (2) OH 0 HOC(X'6 OH I OH 'OR OOH h OH OOH OH I o , ::::,... I HO Gu'-:::O , ::::,... OH h OH H (3) A: H (5) (4) A= Galloyl Fig. 2. Compounds isolated from the bark of southern red oak a biosynthetic precursor of salicin which is a conifers amount to 5- 18% and 2-7%; common constituent in the Salicaceae. respectively9>. The composition of the poly­ meric proanthocyanidins from 9 species of 3) Acacia mearnsii conifers is shown in Table I 10>. Although poly­ The bark of A. mearnsii is a good source meric proanthocyanidins from the bark are of tannins (phenol content 380Jo, tannin con­ composed generally of polymeric procyanidins tent 24% 1>). it contains a large amount of low in many coniferous species, Abies sachalinen­ molecular weight proanthocyanidins. About sis and Picea jezoensis contain a certain amount 30% of 70% acetone extracts from the bark of polymeric prodelphinidins. The level of are dimers or trimers. Major dimeric proan­ stereochemistry of the flavan-3-ol monomer thocyanidins are robinetinidol-(4a-8)-catechin units of the polymers varies considerably with ( 12) and robinetinidol-(4a-8) -gallocatechin ( 13) the species. As for the extender units, the poly­ (Fig. 3). The acetone-water soluble tannins with mers from Cupressaceae and Cryptomeria a higher molecular weight are proanthocyanidin japonica show nearly the same or higher ratios polymers with a 2,3-trans heterocyclic ring, the of 2,3-trans units to 2,3-cis. On the other hand, extender units of which consist mainly of the polymers from Pinaceae show extremely prorobinetinidin units. high ratios of 2,3-cis units to 2,3-trans. In the terminal units, 2,3-trans units predominate and 4) Conifers almost all the units consist of 2,3-trans in P. Many Japanese conifers contain a large jezoensis and Pinus densiflora. amount of phenolic extractives in the bark, and proanthocyanidins are the main components of Protein-adsorbing capacity these phenolic extractives. Plavanol and tan­ nin contents in the bark of the Japanese main Condensed tannins are able to precipitate Ohara: Chemistry and Utilization of Tannins from Tree Barks 73 OH OH OH OH I O HOco~0 O OH OH OH h OH O OH I OH I O HOco~ OR OH OH OR (6) A = H (8) A= H 0-x,H 0-x,OH (7)R= V (9)R= V OH OH HOC(XOO OH I OH h OH OOHI HO ~ R h COOH OH (12)R:H (13)R:OH -CO OH (11)R= OD Fig. 3. Proanthocyanidins and their derivatives from the bark of Salix sieboldiana a.nd A. meamsii Table 1. Composition of polymeric proanthocyanidins from coniferous bark Ratios of stereochemistry Pcy:Pdp•J (2,3 -trans: 2,3-cis) Extender units Termination units Cupressaceae Chamaecyparis obtusa 100:0 60:40 59:41 Chamaecyparis pisijera 100:0 54:46 64:36 Taxodiaceac Cryptomeria japonica 100:0 78:22 69:31 Pinaceae Ables sachalinensis 71 :29 73:27 84 : 16 Larix leptolepis 100:0 11 :89 80:20 Picea jezoensis 95:5 11: 89 100 :0 Pinus densijlora 100:0 7 :93 97:3 Tsuga sieboldii 100:0 11 :89 44:56 Pseudotsuga japonica 100:0 16:84 23:77 a): Pcy; Procyanidins, Pdp; Prodelphinidins. 74 JARQ 28(1) 1994 proteins and it is assumed that they are involved adsorbing capacity increases with the increase in the chemical defense of some plants against of the molecular weights. 3) Tetramers show predators such as insects and herbivores. In a capacity nearly equal to that of polymers. the tannin-protein interaction, phenolic hydroxyl groups in the A- and B-rings play / / important roles. Results of BSA-precipitating 3.0 / I 00<';!, 1>rcci11i1a1ion-, / tests of synthesized condensed tannin oligomers / with mono-, di-, and tri-hydroxylated B-rings / 6 3 / --6 < 2.0 are shown in Fig. 4 >. BSA-precipitating ca­ v> / ---- 8 pacity of oligomers D and G accounts for 83 a, ,,.---- a= "<.> /~~ and 87% of that of oligomer F, respectively. s These results indicate that the condensed tan­ l 1.0 / / J: / nins with only a 4 ' -hydroxylated B- ring show / almost a similar capacity to that of the tannins / with 3 ', 4 '-dihydroxylated or 3 ' , 4 ', 5 '-tri­ hydroxylated B-rings.