Catálogo de publicaciones - libros

Hemicelluloses, comprising the non-cellulose cell-wall polysaccharides of vegetative and storage tissues of annual and perennial plants, represent an immense renewable resource of biopolymers. They occur in a large variety of structural types, divided into four general groups, i.e., xylans, mannans, mixed linkage β-glucans, and xyloglucans. The presented review summarized recent reports on hemicelluloses, including the arabinogalactan from larch wood, focused on new plant sources, isolation methods, and characterization of structural features, physicochemical and various functional properties. Attention was paid to derivatives prepared from these polysaccharides and to application possibilities of hemicelluloses or hemicellulosic materials for food and non-food applications, including the production of composite materials and other biomaterials.

Polysaccharides from plants have been the subject of studies for a very long time, mainly focussed on their physical properties, their chemical and physical modification, and their application. Over the last 20 years there has been increasing interest in the biological activity of the natural polysaccharide polymers. These studies became possible as a result of the scientific development of isolation, purification and characterisation methods concomitant with the development of fairly simple in vitro tests for effects especially on the immune system. The growing acceptance of the knowledge to be gained by people still using so-called traditional medicine in finding sources worthy of study has led to new sources for interesting bioactive plant polysaccharides. This chapter contains only the knowledge on bioactive plant polysaccharides of the pectic type gained over approximately the last ten years and the focus is on those papers where structural characterisation has been performed. For this reason, the reader may not find all of the plants studied during this period within the chapter. Discussions concerning the structural aspects of polysaccharides that may be responsible for activity, are included where relevant.

The impetus for the increased interest in the synthesis of functionalized natural polymers, in particular esters of cellulose, is their easy biodegradability and conformity to the principles of green chemistry. This review is concerned with the preparation of cellulose esters under homogeneous reaction conditions, including products that cannot be obtained by the (industrial) heterogeneous reaction. This scheme, which leads to products of reproducible properties, includes three stages: Cellulose activation (by solvent exchange or heat), dissolution (in derivatizing or non-derivatizing solvent systems), and functionalization of the solubilized polymer. Dissolution in non-derivatizing solvent systems, in particular LiCl/DMAc; (CH)NF ·hydrate/DMSO and ionic liquids (green solvents) is due to the disruption of the H-bonding within the polymer structure. Dissolution in derivatizing solvents, e.g., acid-anhydrides leads to functionalization of cellulose, and may be fruitfully employed in controlling the regioselectivity of polymer substitution. Optimization of each of the above-mentioned reaction stages is a pre-requisite in order to meet the requirements of green chemistry.

Chitin is well characterized in terms of analytical chemistry, is purified from accompanying compounds, and derivatized in a variety of fashions. Its biochemical significance when applied to human tissues for a number of purposes such as immunostimulation, drug delivery, wound healing, and blood coagulation is currently appreciated in the context of biocompatibility and biodegradability. Physical forms (nanoparticles, nanofibrils, microspheres, composite gels, fibers, films) are as important as the chemical structures. Besides being safe to the human body, chitin and chitosan exert many favourable actions, and some chitin based products such as cosmetics, nutraceuticals, bandages, and textiles are presently commercially available. This chapter puts emphasis on the development of new drug carriers and on the interaction of chitosans with living tissues, two major topics of the most recent research activities.

The launch of the XL-I analytical ultracentrifuge by Beckman Instruments (Palo Alto, USA) in 1996 and subsequent development of penetrating software for the analysis of the optical records digitally recorded in this new generation instrument has made some exciting possibilities for the analysis of polysaccharides in a solution environment. We review these developments and investigate the application of the technique to the study of polysaccharide polydispersity, molecular weight analysis, conformation and flexibility analysis, the study of associative interactions, including large complex formation phenomena, and to the measurement of charge and charge-shielding phenomena.