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On-line version ISSN 1806-9959
Rev. bras. Bot. vol.22 n.1 São Paulo Apr. 1999
(recebido em 12/09/97; aceito em 17/09/98)
ABSTRACT - (Foliar flavonoids of nine species of Bauhinia). Foliar flavonoids of nine species of Bauhinia were isolated and identified. All the compounds correspond to glycosides derived from kaempferol, quercetin, isorhamnetin and myricetin. Derivatives of the latter aglyconhe seem to be rare in Bauhinia. Derivatives of isorhamnetin are commonly found in species of subgenus Bauhinia and were not detected in the two species of subgenus Phanera. Flavonoid patterns of species of the former subgenus are in general more complex than those of the latter.
RESUMO - (Flavonóides foliares de nove espécies de Bauhinia). Os flavonóides foliares de nove espécies de Bauhinia foram isolados e identificados. As substâncias correspondem a derivados glicosilados de campferol, quercetina, isoramnetina e miricetina. Os derivados desta última aglicona parecem ser raros em Bauhinia. Os glicosídios de isoramnetina são comuns em espécies do sugênero Bauhinia e não foram detectados nas duas espécies do subgênero Phanera. Os padrões flavonoídicos das espécies do subgênero Bauhinia são, em geral, mais complexos do que aqueles das espécies de Phanera.
Key words - Bauhinia, Caesalpinioideae, flavonoids, chemotaxonomy
Bauhinia is a very characteristic pantropical genus of Caesalpinioideae, characterized by trees, shrubs and climbers with geminate and sometimes bifoliolate or entire leaves presenting a palmate pattern of vein distribution. It is one of the largest genera of the subfamily, comprising about 300 species. About 250 species native in Brazil have been described. Although a treatment of section Tylotaea (= sect. Caulotretus) from Brazil is available (Vaz 1979), a comprehensive revision of Bauhinia has as yet not been done since Bentham (1870). The genus Bauhinia L. has recently been divided in four subgenera: B. subg. Bauhinia with nearly 140 species; B. subg. Elayuna with ca. six species; B. subg. Barklya with 1 species, and B. subg. Phanera with about 150 species (Wunderlin et al. 1987). Two main phyletic lines were proposed for genus Bauhinia, one giving rise to subgenera Bauhinia, Elayuna and Barklya, and the other to the subgenus Phanera. The first three subgenera are represented by trees and shrubs, never possesing tendrils, while the latter corresponds to lianas and vines, always with tendrils. Zhang (1995) presented a cladistic analysis of the genus Bauhinia, based on 70 equally weighted morphological and leaf venation characters. In his analysis subgenus Bauhinia is monophyletic, formed by sections Bauhinia, Pauletia, Telestria, Gigasiphon and part of the section Afrobauhinia. A survey of 78 species of genus Bauhinia shows that the rpL2 intron in chloroplast genomes is present in subgenera Elayuna, Phanera and the monotypic subgenus Barklya. In contrast, the intron is absent in nearly all species of the subgenus Bauhinia (Lai et al. 1997).
In several countries and continents, there is a widespread and nearly indiscriminate popular use of leaves of any species of Bauhinia as a mean of controlling hyperglycemia, although in Brazil B. forficata Link is considered by many people as more effective medicinally than other species. For this reason, part of the chemical investigation on Bauhinia has been driven by interest in finding compounds with hypoglycemic activity.
The leaf flavonoid chemistry of the genus is poorly known and no studies have so far been carried out with taxonomic aims. It seems that flavonols based on common aglycones predominate in leaves of Bauhinia. Rabaté (1938) obtained quercitrin (quercetin-3-O-rhamnoside) from leaves of B. reticulata DC. Rahman and Begum (1966) found the 3-galactoside and the 3-rhamnoside of kaempferol in leaves of B. variegata L. Abd-El-Wahab et al. (1987 apud Spilková & Húbik 1992) isolated quercetin, rutin, quercitrin, apigenin and apigenin-7-O-glucoside from leaves of B. variegata L. var. variegata and B. purpurea L. Quite different flavonoid profiles can be found in parts of Bauhinia other than in leaves. For example, methoxylated and methylenedioxyflavones were isolated from roots of B. championii Benth. (Chen et al. 1984). Gupta et al. (1980) obtained naringenin-4'-rhamnoglucoside from stems of B. variegata L. Bausplendin (a dimethylenedioxyflavone) was found in the wood of B. splendens H. B. K. (Laux et al. 1985). Seeds of B. purpurea L. were shown to possess butein-4'-arabinosylgalactoside (Bhartiya et al. 1979, Bhartiya & Gupta 1981). Kumar et al. (1990) isolated kaempferol and agatisflavone from pods of B. vahlii Wight & Arn.
This paper describes the leaf flavonoid chemistry of nine species of Bauhinia, belonging to subgenera Bauhinia and Phanera. The liana species here studied - B. alata Ducke and B. radiata Vell. - belong respectively to sections Tylotaea (= Calotretus subsect. Binaria) and Schnella. The arborescent species studied - B. aculeata L., B. cupulata Benth., B. forficata Link, B. longifolia (Bong.) Steud., B. pentandra (Bong.) Vog., B. ungulata L. - belong to section Pauletia, except for B. purpurea which correspond to section Telestria.
Material and Methods
Leaves of species of Bauhinia were collected from specimens cultivated in Rio de Janeiro Botanical Garden, in whose Herbarium (RB) voucher specimens were deposited.
Powdered leaves (30-80 g) were extracted four times with 80% MeOH and twice with 50% MeOH under reflux for 60 min. The pooled extracts were concentrated under reduced pressure. The flavonoids were isolated by unidimensional PC with BAW and AcOH 15% and CC of polyvinylpolypyrrolydone with MeOH 80% and Sephadex LH-20 with MeOH. The identification of the compounds followed standard procedures of chromatography, acid and enzymatic hydrolysis, UV/visible spectrometry of glycosides and aglycones and comparison with authentic samples of aglycones (Mabry et al. 1970, Markham 1982). Isorhamnetin was also identified by GC/EI-MS at 70 eV with a HP Ultra-1 capillary column (25 m x 0.25 mm) and He as carrier gas. The temperature of the column was 300°C and that of the injector and detector, 325°C. Characterization of the flavonoids as monoglycosides and di/triglycosides was tentatively achieved by means of TLC (Santos et al. 1995). The position of sugars of 3,7-diglycosides was determined using specific glycosidases (Mabry et al. 1970, Markham 1982).
Results and Discussion
Table 1 lists the flavonoids identified and table 2 shows their distribution among the specimens studied. The interesting feature of this survey is the apparent specialization in production of flavonols by the leaves of species of Bauhinia, a result that reinforces the claim (see Introduction) that this class of phenols predominate in foliar flavonoid profiles of the genus. Derivatives of isorhamnetin and myricetin had not been previously reported in Bauhinia. It seems that derivatives of myricetin are rare in Bauhinia, because only B. longifolia yielded a derivative of the latter aglycone among the nine species investigated.
The distribution of flavonoids presented on table 2 suggests that the subgenus Bauhinia can be distinguished from Phanera by the presence of derivatives of isorhamnetin and myricetin in species of the former. This result is coherent with distinctions between the two subgenera based on cladistic and molecular studies (see Introduction). Two points, however, must be raised: 1. the sampling of the present work, particularly of the subgenus Phanera, is too small, and 2. not all species of the subgenus Bauhinia seem to present derivatives of those aglycones, particularly of myricetin. Hence, care must be taken not to draw definite conclusions before a more comprehensive evaluation is made about the discriminatory value of the foliar flavonoids on distinctions between the two studied subgenera.
Another distinctive feature between the species of the two studied subgenera is the relatively simpler flavonoid composition of species of Phanera and the complexity of the flavonoid patterns of most species of Bauhinia, not only in relation to the number of aglycones, but also to the number of different glycosidic combinations based on the same aglycone (table 2). The only species investigated that presented flavonoids based only on quercetin belongs to Phanera (B. alata, table 2), and only B. radiata (subg. Phanera) presented no glycosides other than monoglucosides (table 2). Thus subgenus Bauhinia seems to be chemically more complex than subgenus Phanera. This observation recalls the relationship between relative chemical complexity and relative evolutionary advancement (Mabry 1973a, b): more advanced taxa tend to present simpler chemical profiles, suggesting that evolution leads to simplification of chemical patterns. This trend has been observed, for example, by Averett et al. (1979, 1990, 1991) in Onagraceae, by Blatt et al. (1994) in Diplusodon (Lythraceae) and Santos et al. (1995) in Cuphea (Lythraceae). The presence of a myricetin derivative in a species of the subgenus Bauhinia could be regarded as a plesiomorphic feature, since the presence of this flavonol has been correlated with primitiviness and woodiness (Harborne 1977), although the establishment of correlations between structural flavonoid characteristics and relative advancement of taxa has been a matter of debate (Crawford 1978, Bohm 1987). All the neotropical arborescent species here studied belong to section Pauletia, which is viewed as primitive (Wunderlin et al. 1987). Studies of species included in more advanced sections of subgenus Bauhinia would be welcome. Neotropical sections of the subgenus Phanera were also regarded as primitive, together with the paleotropical section Lysiphyllum (Wunderlin et al. 1987), the remaining paleotropical sections being viewed as highly derived. Among ten sections of the subgenus Bauhinia, Pauletia is the only one with two species in the paleotropics. This present distribution seems to be relictual and suggests some antiquitity for the group. In additon, most species have 10 fertile stamens in a subgenus with is a tendence to reduce the number of fertile stamens to one. Further flavonoid studies of others sections are desired to test the patterns observed here.
Acknowledgements - This work was carried out with funds provided by CAPES (Fundação de Coordenação e Aperfeiçoamento do Pessoal de Nível Superior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo).
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1. Instituto de Biociências, Universidade de São Paulo, Caixa Postal 11461, 05422-970 São Paulo, SP, Brasil.
2. Instituto de Botânica, Caixa Postal 4005, 01061-970 São Paulo, SP, Brasil.
3. Jardim Botânico do Rio de Janeiro, R. Pacheco Leão 915, 22460-030 Rio de Janeiro, RJ, Brasil.