Polissacarídeos das sementes de guaraná (Paullinia cupana Kunth) : análises estruturais, reológicas e atividade antioxidante

Abstract

Resumo: O guarana (Paullinia cupana) e um fruto nativo da regiao Amazonica, rico em amido, metilxantinas, compostos fenolicos e saponinas. Devido ao seu elevado potencial estimulante, seu extrato e utilizado principalmente no preparo de bebidas carbonatadas. No presente trabalho foram obtidas fracoes extraidas das sementes do guarana, utilizando-se diferentes solventes. Inicialmente as sementes foram deslipidificadas com tolueno-etanol, tratado com metanol-agua, obtendo-se a fracao GMW Posteriormente, o amido das sementes foi extraido com dimetilsulfoxido, obtendo-se as fracoes GD-I e GD-II, que foram avaliadas quanto ao comportamento reologico em DMSO 90%. A fracao GD-II apresentou viscosidade aparente (Pa.s) e modulo de armazenamento (G f) superiores aos encontrados para a fracao GD-I, caracterizando um comportamento de gel fraco nas concentracoes superiores a 15%. Alem o amido, outras fracoes polissacaridicas foram obtidas, utilizando-se agua e solucoes aquosas de NaOH. Uma fracao pectica (GHW-IIETF) foi isolada com agua quente, purificada com enzimas ƒ¿-amilase e amiloglucosidase, e ultrafiltracao. Atraves de analises quimicas e RMN-13C foi possivel identificar uma homogalacturonana com algumas insercoes e amnogalacturonanas, contendo cadeias laterais compostas por arabinose e galactose. O grupo de fracoes hemicelulosicas extraidas com solucao alcalina foi separado em GHA e GHB por metodos diferentes de recipitacao dos polissacarideos. O grupo GHA era composto majoritariamente pelos monossacarideos Xyl e Glc e o grupo GHB por Glc, Xyl e Ara. Todas as hemiceluloses apresentaram elevados percentuais de proteina. Uma xilana contendo acido glucuronico foi purificada a partir da fracao extraida com NaOH 2M, empregando-se cromatografia de troca ionica para remocao das proteinas, ratamento enzimatico para remocao do amido, e dialise. Testes de atividade antioxidante foram realizados com duas fracoes de interesse, um extrato metanolico (GMW) e uma fracao pectica (GHW-IIET). O extrato GMW mostrou elevado potencial antioxidante, tanto no ensaio de sequestro de radical DPPH quanto de radical hidroxila (OH.). A fracao GHW-IIET apresentou maior atividade no ensaio de sequestro de radical hidroxila (OH.) e somente quando testada na maior concentracao (10 mg/mL) apresentou consideravel atividade sequestrante de radical DPPH. Os resultados obtidos demonstram que, as sementes de guarana contem grande quantidade de amido (39%), alem de polissacarideos pecticos e hemiceluloses, que podem ser obtidos apos remocao de componentes de menor massa molar. Amidos com diferentes percentuais de amilose foram obtidos exibindo diferentes caracteristicas reologicas. A fracao contendo componentes de menor massa de uma fracao pectica exibiram atividade antioxidante.

Abstract: Guarana (Paullinia cupana) is a native fruit from the Amazon region, rich in starch, methylxanthines, phenolic compounds and saponins. Because of its high stimulant potential, its extract is used primarily in the preparation of carbonated beverages. In the present study, fractions extracted from the seeds of guarana, using different solvents were obtained. The lipids were removed from the seed with toluene-ethanol and then treated with methanol-water, yielding the fraction GMW. Subsequently, the starch of the seeds was extracted with dimethyl sulfoxide, resulting in fractions GD-I and GD-II, which were evaluated for the rheological behavior in DMSO 90%. Fraction GD-II showed storage modulus (G') and apparent viscosity (Pa.s) higher than those found for the GD-I fraction, showing a weak gel-like behavior at concentrations higher than 15%. In addition to starch, other fractions of polysaccharide were obtained, using water and aqueous NaOH. A pectic fraction (GHW-IIETF) was isolated with hot water and purified by enzymatic treatment with á-amylase and amyloglucosidase, and ultrafiltration. Using chemical analysis and 13C-NMR was possible to identify a homogalacturonans with some insertions of rhamnogalacturonans, with side chains containing galactose and arabinose. The fractions of hemicelluloses, extracted with alkaline solutions, were fractionated in GHA and GHB by different methods of precipitation. The GHA group was composed mostly by monosaccharides Xyl and Glcand GHB group by Glc, Xyl and Ara. All the hemicellulosic fractions showed high percentages of protein. A xylan containing glucuronic acid was purified from fraction obtained by 2M NaOH extraction, using ion-exchange chromatography for removal of the proteins, enzymatic treatment for removal the starch, and dialysis. Antioxidant activities tests were performed with two fractions, a methanolic extract (GMW) and a pectic fraction (GHW-IIET). GMW fraction showed high antioxidant activity in vitro by the DPPH radical and hydroxyl (OH•) scavenging experiments. The GHW-IIET fraction showed higher hydroxyl radical scavenging activity, and only when tested at highest concentration (10 mg/mL) showed scavenging activity of DPPH radical. According to the results, a part from starch (39%), guarana seeds contain pectic polysaccharides and hemicelluloses, which can be obtained after removal of lower molecular weight compounds. Starches with different percentages of amylose were obtained showing different rheological properties. The fraction containing lower molar mass compounds and a pectic fraction exhibited antioxidant activity.