Chapter 21
Antioxidant Activity of Flavanols and Flavonoid Glycosides in Oolong Tea 1
2
Shengmin Sang , Nanqun Z hu1, Shoei-Yn Lin-Shiau , Jen-Kun L i n 3, and Chi-Tang H o 1
1Department of Food Science, Rutgers University, 65 Dudley Road, New Brunswick, N J 08901 Institutes of Toxicology and Biochemistry, College of Medicine, National Taiwan University, Taipei, Taiwan 2
3
Fourteen flavanols and flavonoid glycosides were isolated from the extracts of Oolong tea. Their structures were determined by spectral methods (MS and N M R ) and compared with authentic samples. One of them is new to the constituents of tea. D P P H free radical scavenging activity was used to evaluate their antioxidant activity. The most active compounds were found to be compounds 1-6, compound 12 and the mixture of compounds 13 and 14.
The tea plant (Camellia Sinensis) is an evergreen tree belonging to the family of Theaceae. Tea as a beverage has been consumed in many countries for a very long time. Tea can be divided into three types-unfermented, semifermented, and fermented, in terms of the degree of leaf fermentation (Figure 1). Each type of tea has a distinct aroma, color, and flavor, which appeal to our senses of taste, smell, and sight. Green tea, which is unfermented tea, remains the most popular tea in the Asian countries such as China and Japan. About 90% of the world's green tea is produced in China. Black tea is fully fermented and accounts for approximately 70% of world's tea
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© 2002 American Chemical Society
293 consumption. The largest producer of black tea is India. China, Indonesia and Kenya are also major black tea producers. Oolong tea is partially fermented and accounts for less than 3% of world consumption and is produced both in the Mainland China as well as Taiwan.
Fresh Leaf
•
Withering
Rolling —
Firing
Fermentation Firing
Firing
f Oolong Tea
Green Tea
Black Tea
Figure I. Tea manufacturing procedure
According to Chinese legends, the story of tea began in 2737 B C . Emperor Shen Nong discovered Tea. In 800 A D , the returning Buddhist priest Yeisei brought the first tea seeds to Japan. Later tea use was introduced into what is now known as Indonesia and from there through the Dutch colonials into Holland. It was also cultivated in India and subsequently imported to England, where it became popular. In the middle of the 17th century the English played a major role in merchandising and popularizing tea. In the United States of America, tea plays a dramatic role in the establishment of the U S A . Most American knows the Boston tea party which is famous in the history of American Independence. United State of America was also the birthplace of iced tea and teabags.
Health Benefits of Tea A large body of information has been published concerning the effects of tea and its major constituents on human health. A number of chronic diseases,
294 such as cardiovascular disease and cancer, have been associated with excess production of reactive oxygen species and oxidative damage. Antioxidants have been identified as likely candidates responsible for the health benefits of many dietary plant materials. Tea has been proposed as a healthy beverage due to its powerful antioxidant activity. Although the antioxidant activity of catechins in teas has been well-recognized (1,2), the antioxidant activity of other phenolic compounds such as flavonols and flavonoid glycosides has not been extensively studied, especially for oolong tea. In this report, we have isolated flavonols and flavonoid glycoosides from oolong tea and studied their free radical scavenging activity.
Tea Polyphenols Tea contains a wide range of polyphenols, such as catechins, flavonols and glycosides, flavones and glycosides, and phenolic acids and esters. In green tea, catechins are the predominant polyphenols. The major catechins in tea leaves are (-)-epigallocatechin 3-O-gallate (EGCG), (-)-epigallocatechin (EGC), (-)epicatechin 3-O-gallate (ECG), (-)-epicatechin (EC), (+)-gallocatechin (GC) and (+)-catechin (C) (Figure 7). Epicatechins account for up to 15% (w/w) of the dry leaves; E G C G comprises 6-10% of the dry weight (3). These compounds contribute to the bitterness, astringency and sweet aftertaste of tea beverages (4). Moreover, two new catechin derivatives (Epigallocatechin-3-O(3-O-methyl) gallate and Epigallocateehin-3-0-(4-0-methyl) gallate) with potent antiallergic activity were isolated from Taiwanese oolong tea (27). The characteristic color of black tea is generated during its manufacturing process. During this process, the colorless catechins are oxidized both enzymatically and chemically to give two major groups of pigments, theaflavins and thearubigins (5). The theaflavins are formed by oxidative coupling of the dihydroxybenzene and trihydroxybenzene rings of an appropriate pair of flavan-3-ols (e.g. epicatechin and epigallocatechin, respectively), resulting in a benzotropolone ring, which gives a yellow color. It is known that theaflavins make important contributions to the properties of black tea such as color (6), 'mouthfeel' (7) and extent of tea cream formation (8) and therefore studies of the structures of these pigments have long been of interest. Stereoisomers of theaflavin such as iso- and neotheaflavin have been identified (9,10) (Figure 2) and a number of closely related polyphenols compounds, including theaflavic acids (10,11) and theaflagallins (12) have also been isolated from black tea. Recently, three novel minor polyphenol compounds, theaflavate B , isotheaflavin-3 '-O-gallate and neotheaflavin-3-O-gallate, have been characterized in extracts from black tea (13). By contrast, thearubigins are extremely complex, heterogeneous mixture of pigments, and their structures are largely unknown (14). Thearubigins are
295 predominant in black tea leaves (15-20% dry weight) and are believed to make the greatest contribution to taste, depth of color and body of a tea brew and therefore influencing the quality. Recently, a new polyphenols pigment, named theacitrin A , has been isolated from the thearubigin fractions of an Assam black tea (15). Proanthocyanidins are colorless precursors of anthocyanidins (Figure 3). In tea a number of proanthocyanidins have been described by Japanese groups earlier (16-18% especially a series of B,B'-linked bisflavanoids, theasinensins A - G (Figure 3), isolated from the oolong tea (28). More recently, Lakenbrink and Engelhardt (19) reported two new proanthocyanidins in green tea. Contents of proanthocyanidins in green tea are roughly in the same range as the flavonol glycosides (20). In black tea the contents are much lower since there is a decrease during fermentation (21).
Theaflavin: Theaflavin-3-gallate: Theaflavin-3*-gallate: Theaflavin-3,3 -gallate: ,
R^R^H R ^ H ; R =galloyl R^galloyl; R =H Ri=R =galloyl 2
2
2
Figure 2. Structures of the major theaflavins in tea.
The flavonols and flavones in tea are present as aglycones (traces) and to a much higher extent as their glycosides. In tea leaves all the flavonol glycosides derive from the aglycones myricetin, quercetin and kaempferol, and the flavone glycosides derive from the aglycone apigenin (Figure 4). For the flavonol glycosides, the carbohydrate moieties, in most instances located at position 3 of the aglycone, consist of various combinations of glucose, galactose, and rhamnose and, in a single case, fructose. Mono-, di- and triglycosides have been observed. In contrast to flavonol glycosides, flavone glycosides in tea appear nearly exclusively as C-glycosides (22).
296
A . C - L i n k e d proanthocyanidin
Theasinensin A: R i=OH; R2=R3=G Theasinensin B: Ri=OH; R2=G; R3=H Theasinensin C: Ri=OH; R2=R3=H Theasinensin F: R l=H; R2=R3=G
Theasinensin D: R l=OH; R2=R3=G Theasinensin E: Ri=OH; R2=R3=H Theasinensin G:Ri=H; R2=R3=G
Figure 3. Some structures of proanthocyanidins (G = gallate).
297
Kaempferol: R,=OH, R =R =H Quercetin : R ^ R ^ O H , R =H Myricetin : R =R =R =OH Apigenin : R =R =R =OH 2
3
3
1
2
2
1
2
3
Figure 4. Structures offlavonolandflavone aglycones in tea.
Figure 5. Structures of gallic acid and theogallin.
298
COOH
COOH
HOOC OH Derivatives of Quinic acid
Caffeic acid
OH p-coumaric acid
3- CQA : R2=R3=H; R i=Caffeic acid 4- C Q A : R i=R3=H; R2=Caffeic acid 5- C Q A : R i=R2=H; R3=Caffeic acid 3- CouQA: R2=R3=H; Ri=p-coumaric acid 4- CouQA: R i=R3=H; R2=p-coumaric acid 5- CouQA: R i=R2=H; R3=p-coumaric acid
Figure 6. Structures of caffeoyl- andp-coumaroylquinic acids.
The major phenolic acids and esters of tea constituents are gallic acid and its tea-specific ester with quinic acid (theogallin) (Figure 5) and hydroxycinnamoyl acid/quinic acid esters (Figure 6). Gallic acid is the most important phenolic acid in tea. The amount of gallic acid increases during the fermentation owing to its liberation from catechin gallates. Theogallin is a particularly interesting compound as it specifically appears in tea (23). Derivatives of hydroxycinnamic acids are widely distributed in the plant kingdom. Caffeoyl and p-coumarylquinic acids (CQAs and CouQAs) have been described in tea (24).
Material and Methods Chemicals Silica gel (130-270 mesh), Sephadex LH-20 (Sigma chemical Co., St. Louis, M O ) and Lichroprep RP-18 column were used for column chromatography. A l l solvents used for chromatographic isolation were analytical grade and purchased from Fisher Scientific (Springfield, NJ).
299 General Procedures , 3
Ή N M R and C N M R spectra were obtained on a VXR-200 instrument (Varian Inc., Palo Alto, C A ) , operating at 200 and 50 M H z . Compounds were analyzed in C D O D with tetramethylsilane (TMS) as an internal standard. A P C I M S (atmospheric pressure chemical ionization mass spectrometry) was obtained on a Fisons/VG Platform II mass spectrometer. Thin-layer chromatography was performed on T L C plates (250 um thickness, 2-25 um particle size, Sigma-Aldrich), with compounds visualized by spraying with 5% (v/v) H S 0 in ethanol. 3
2
4
Extraction and Isolation Procedures The dried leaves of oolong tea (1.5 kg) were extracted with hot water 3 times. After concentration, the extract was passed through a Diaion HP-20 column eluted with the water-ethanol solvent system. The adsorbed material was eluted with ethanol/water (60%) to give a brown residue (150 g). A part of this residue (30 g) was chromatographed on silica gel column, Sephadex LH-20 column and preparative T L C plate to afford 14 compounds. The concentrations of these compounds are shown in Table I.
Table I. Concentration of the Flavanols and Flavonoid Glycosides of Oolong Tea. Compound
1
2 3 4 5 6 7 8 9 10 11 12 13+14
%Dry weight (mg/lOOg) 2.7 10 120 93 47 270 3.3 8.0 3.7 1.3 100 73 20
300
Results and Discussion
Structure determination of Isolated Compounds
13
To identify compounds isolated, we used Ή-ΝΜΕ, C - N M R , CI-MS, and authentic samples. To make sure the structures were accurately assigned, the data of these compounds were compared with those in the literature (22,25). The structures of these compounds are shown in Figures 7 and 8. Among them, compound 9 is a novel compound isolated for the first time from tea leaves.
OH
OH (1) E C
:Ri=R2=H
(2) E G C
:Ri=OH,R =H
(3) E C G
:Ri=H,
2
(5) C : R=H (6) G C : R=OH
R =Gallate 2
(4) E G C G : R i = O H , R2=Gallate
Figure 7. Structures of compounds 1-6.
Determination of the Scavenging effect of Oolong Tea Flavanols and Flavonoid Glycosides on DPPH Radicals This method was adapted from Chen and Ho (26). D P P H radicals were prepared in ethanol (l.OxlO" M ) . This D P P H solution was mixed with different compounds (final concentration was 20 μΜ) and kept in a dark area for 0.5 h. The absorbance of the samples was measured on a spectrophotometer (Milton Roy, model 301) at 517 nm against a blank consisting of ethanol without DPPH. A l l tests were run in triplicate and mean values reported. 4
301 The scavenging effect of compounds 1-14 is shown in Table II. A l l compounds at a concentration of 20 μΜ exhibited scavenging activity compared to the control sample. Compounds 1-6, compound 12 and the mixture of compound 13 and 14 showed strong D P P H radical scavenging activity.
OH
OH
Ο
Ο
(10): R l=R2=H, R3=Rha i ^ è Glc (7) :Ri=Rha (8) :Ri=Ara (9) :Ri=H
Glc
R2=H R2=Glc R2=Rha
(ll):Rl=R2=H,R3=
Rha lc G
Glc 1-^2 Rha
(12) :Ri=OH,R =H,R = 2
3
G l c
1*6 ^
G
l
c
(13) :Ri=R2=OH, R3=Glc (14) :Ri=R2=OH, R3=Gal
Figure 8. Structures of compounds 7-14.
The D P P H radical-scavenging ability of catechins, as given in Table II, is in the order of EGCG>ECG>EGC>GC>C>EC, and the ability of the four aglycones is myricetin>quercetin>kaempferol>apigenin; and the ability of the two types glycoside is flavonol glycosides>flavone glycosides. By comparing the ability of the aglycones and the ability of the glycosides, it is clear that sugars of these glycosides do not play an important role in their activity According to our study, the flavonol and flavone glycosides also play an important role in the antioxidant activity of tea. The structural requirements of D P P H radical inhibitors are: 1) requirement of the 3 ,4'-dihydroxy group in the Β ring configuration; 2) enhancement in activity with the additional hydroxy group at 3 position or 5* of Β ring. We also noticed that the fermentation procedure in oolong tea did not affect the integrity of flavonoid glycosides. f
302 Table Π. Scavenging Effects of Compounds1-14 and Four Aglycons on DPPH Radical Compound 1 2 3 4 5 6 7 8 9 10 11 12 Mixtures of 13 and 14 Quercetin Apigenin Kaempferol Myricetin DPPH
Absorbance at 517 nm 0.181 0.119 0.109 0.082 0.154 0.152 0.593 0.746 0.784 0.718 0.868 0.257 0.147 0.207 0.796 0.329 0.174 0.960
Inhibition (%) 81.1 87.6 88.6 91.4 84.2 84.0 38.2 22.3 18.3 25.2 9.6 73.2 84.7 78.4 17.1 65.7 81.2 —
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