Fucoidan is sulfated polysaccharide found in the cell wall of brown seaweed. The chemical composition of fucoidan includes fucose and some other sugars including galactose, glucose, mannose etc., fucoidan also contains uronic acid and sulfate groups. Depending on the seaweed species, fucoidan may have a backbone built up of α-L-fucopyranose residues with (1 → 3)- or alternating (1 → 3)-, (1 → 4)- glycosidic linkages and sulfated at positions C-2 and/or C-3 and/or C-4 [1,2,3]. Fucoidan has valuable biological activities such as anti–tumor, antioxidant, antibacterial, antifungal and anticoagulant activities [4,5,6], immunomodulating activity [3] and anti–HIV activity [7]. It was known to have other biological effects such as lowering cholesterol [8], and lowering blood fat [9].

Recently, many studies aimed to elucidate the relationship between structure and biological activity of fucoidan [10,11,12], however, the overall relationship remains unclear, although it is believed that biological activity of fucoidan is related to the content and position of sulfate groups, monosaccharide composition, molecular weight, and structure of the backbone and branches [13,14,15]. Previous researches indicated that low molecular weight fucoidan had the ability to reduce blood lipids of mice [8,9,16] and anticancer activity increased when reducing molecular weight [17]. Low molecular weight fucoidan had higher cytotoxicity in some human cancer cell lines than native fucoidan [18] and it showed the ability to protect endothelial function, ameliorating hypertension and cardiovascular complication in diabetic rats [19]. Ustyuzhanina et al. [20] reported that immunological activity of fucoidans was greatest when the molecular weight was between 13,500 Da and 40,000 Da. A fucoidan fraction from Sargassum fusiforme with a small sulfate content was unable to inhibit the angiogenesis of HMEC–1 cell, however, other fraction with higher sulfate content, showed an inhibitory effect on the angiogenesis of HMEC–1 cell [21]. In a previous work, we proposed that branched structure is required for having a significant cytotoxic activity of fucoidan, but the relationship of branch length and the level of bioactivity is not detected [22]. The above works studied the separate effects of each structural factor on biological activity of fucoidan. In this study, we aim to study the simultaneous effect of structural factors on anticancer activity of fucoidan.

Nowadays, colon cancer is the most common form of cancer worldwide, accounting for approximately 10 % of all new cancer cases and mortality, the prevalence rates of colon cancer are increasing among nations [23]. Colon cancer can be successfully treated by surgery and combination chemotherapy. However, chemotherapeutic agents provide only minimal survival benefits as a result of several factors: drug resistance, side effects and toxicity [24,25]. Therefore, it is worthwhile to find new compounds derived from natural sources for colon cancer treatments. Fucoidan was suggested being a potential therapeutic reagent against the growth of human colon cancer cells [26,27].

Turbinaria decurrens is a species of brown algae (Phaeophyta) that has a narrow distribution and wide bioactivity potential. Meenakshi et al. [28] reported that T. decurrens fucoidan has the neuroprotective effect against dopaminergic neurodegeneration for the treatment of Parkinson's disease. T. decurrens fucoidan showed antioxidant activity and antiproliferative activity [29]. However, a very limited number of studies on structural characteristics has been done in fucoidan from T. decurrens. Each new fucoidan of seaweed origin has unique structural characteristics, which may display varied bioactivity and would be a potentially new drug.

In this context, the aims of this study are to determine structure of fucoidan from Turbinaria decurrens collected at Vietnam's Sea and to investigate its anticancer effect on HT29 human colon cancer cell line. Besides, in order to contribute a better understanding on structure–activity relationship of fucoidan, in this work, we prepared a series of fucoidans with different molecular weights by hydrolyzing native fucoidan by HCl 0.1 N at various times. Content and position of sulfate groups attached to the main chain of native and hydrolyzed fucoidans were determined, their anticancer activity against HT29 human cancer cell line was evaluated, cross–sectional radius of gyration (Rgc) of all fucoidans was estimated based on SAXS (small angle X–ray scattering) measurements by assuming that fucoidans have a partial rod–like structure.