**2.1 Chemical-physical properties**

#### **2.1.1 Ulvan composition**

Green algae such as *Ulva sp.* are known to contain high amounts of good-quality protein, carbohydrate, vitamins and minerals (Taboada et al., 2010). Among these, polysaccharides are gaining increasing attention as they possess unique physical and chemical properties representing a versatile material platform for potential biological applications.

Ulvan represents a class of sulphated heteropolysaccharide extracted from the cell wall of green seaweeds belonging to *Ulva sp.* whose composition has been extensively debated (Lahaye & Robic, 2007; Robic et al., 2009; ) and showed to vary according to several factors including the period of collection, the ecophysiological growth conditions, the taxonomic origins and the post-collection treatment of the algal sources (Lahaye & Robic, 2007).

Four types of polysaccharides are reported to be contained in the biomass of *Ulva sp.*, including the water soluble Ulvan and insoluble cellulose as major one and an alkali-soluble linear xyloglucan and glucuronan in minor amounts (Lahaye & Robic, 2007). Ulvan represents the major biopolymeric fraction of the cell wall having the function of maintaining the osmolar stability and protection of the cell (Paradossi et al., 2002). As usually found in polysaccharides present into the cell walls, Ulvan is present in close association with proteins and the conventional methods of extraction and purification resulted not completely effective in the removal of the protein fraction even after a specific deproteinization protocol (Alves et al., 2010).

Extraction is conventionally achieved by using warm water solution (80-90°C) containing ammonium oxalate as divalent cation chelator and the recovery of Ulvan is generally obtained by precipitation in ethanol. The yield of extraction usually ranges from 8% to 29% of the algal dry weight depending on the applied purification procedure (Lahaye & Axelos, 1993; Lahaye et al., 1994).

The sugar composition of Ulvan is extremely variable but rhamnose, xylose, glucuronic and iduronic acid and the presence of sulphate groups have been identified as the main constituents of the polymer (Paradossi et al., 2002; Robic et al., 2009). These monomers are arranged in an essentially linear fashion even though a slight degree of branching has been found (Lahaye & Robic, 2007). The chemical heterogeneity of Ulvan is partially striken by a "structural motif" found within the heteropolymer chain essentially given by the presence of repeating dimeric sequences constituted by aldobiuronic acid disaccharides designated as type A (glucurorhamnose 3-sulphate, A3s) and type B (iduronorhamnose 3-sulphate, B3s) (Figure 3).

Fig. 3 Structure of the main disaccharide repeating units in Ulvan.

The most striking feature that distinguishes the chemical composition of Ulvan from that of the other polysaccharides of marine origin is, therefore, the presence of uncommon sugar such as iduronic and sulphated rhamnose displaying a close similarity with mammalian glycosaminoglycans. To this view Ulvan and related polysaccharides could represent an abundant and cheap feedstock for the substitution of heparinoid substances commonly used in biomedical applications solving the problems related to their isolation and purification (Alban et al., 2002).
