Polysaccharides of the red seaweed Grateloupia turuturu

TOMÁS BENECH ARNOLD; DIEGO A. NAVARRO; CARLOS A. STORTZ

Lisboa

Simposio; 29th International Carbohydrate Symposium; 2018

Grateloupia turuturu is a red seaweed native from the Asian Pacific Ocean shores that belongs to the order of Halymeniales. It is an invasive species with a high rate of reproduction and growth; thus, it is common to find it in different parts of the world, including the Argentine seashore. A previous report from 1975 indicates that, as it is usual in the Rhodophyta, this seaweed biosynthesizes large amounts of sulfated galactans.[1]The polysaccharides produced by this seaweed were extracted from specimens collected from the coast of Miramar (Province of Buenos Aires, Argentina). We worked on two different stages of the plant: cystocarpic (F1) and juvenile (F2). They were cleaned and ground to be subjected to a sequential extractive process with water at different temperatures: 20 h at room temperature (FnA), 8 h at 90 °C (FnA90 I) and a second step of 8 h at 90 °C (FnA90 II). The first extraction with hot water was the most effective: F1A90 I and F2A90 I were obtained with yields around 50% and 30% respectively, whereas the other fractions were obtained with yields lower than 20% (F1A and F2A 9% , F1A90 II 19% and F2A90 II 17%). The analysis of the composition of the extracts showed that they were constituted mainly by carbohydrates (50-60%), with a decreasing tendency in the proportion of sulfate as the extraction sequence progressed (for FnA, 27%, for FnA90 I, 25%, decreasing to 16% for FnA90 II). In addition, the extracts obtained contained proteins and uronic acids, in proportions close to 7% each. As expected, galactose was the major neutral sugar in all products (83-90%). Small amounts of xylose, glucose, 3-O-methylgalactose and traces of rhamnose and 3,6-anhydrogalactose were also found. The absolute configuration of the galactose units present in the polysaccharides was determined by reductive amination with chiral amines and subsequent analysis by gas chromatography[2]. A D / L ratio greater than one was determined in all cases (close to 2.5 in FnA and FnA90 II, and reaching 3.5 in FnA90 I). This result could indicate the presence of carrageenans and agarans as separate entities, or the presence of the so-called D / L hybrids, whose existence was never proved yet. GPC analysis showed that all fractions carried high molecular weights, and also large dispersions. The average molecular weights of these fractions ranged between 1,800 and 2,000 KDa.The F2A extract was fractionated by column chromatography using a mixture of DEAE Sephadex A-50 / Sephadex G-100 (ratio 1:3).[3] Eluted was effected with NaCl solutions of increasing concentrations (0.1 to 4M). Two main fractions were obtained (P1 and P2), eluted with NaCl concentrations of 1 M and 1.5 M respectively, and similar yields (26%). The composition analysis of P1 and P2 showed about 57% of carbohydrates, 21.4% of sulfate for P1 and 24.5% for P2, proportions of uronic acids close to 8% for both, and less than 4% proteins. The uronic acid present was identified as D-glucuronic acid. The sugar components of both fractions showed no appreciable differences with respect to the F2A extract, disregarding enantiomeric issues. However, the D / L ratio of the galactose units present in each of the fractions varied with respect to the initial extract, yielding values of 1.68 for F2A-P1 and 3.09 for F2A-P2. Further studies in progress will throw light on the fine structure of the system of galactans from Grateloupia turuturu.References [1] A. I. Usov, L. I. Miroshnikova & V. V. Barbakadze. Zh. Obshch. Khim. 1975, 45, 1618-1624.[2] M. R. Cases, A. S. Cerezo & C. A. Stortz. Carbohydr. Res. 1995, 269, 333-341.[3] C. A. Stortz, M. R. Cases & A. S. Cerezo. Carbohydr. Polym. 1997, 34, 61-65.