Microrheological investigations of biopolymer networks : PhD thesis, research conducted at the Institute of Fundamental Sciences, Massey University of Palmerston North, New Zealand

is a major polysaccharide of the plant cell wall which is known to play a role in many mechanical functionalities, especially when a gel is formed in the presence of calcium. Understanding the gelling abilities of pectin is of great interest to the food industry also, since pectin is a widely used as a gelling agent and thickener. The aim of this study was to apply two complementary microrheological techniques to these systems, multiple particle tracking (MPT) and a light scattering technique called diffusing wave spectroscopy (DWS). While the first one provides fundamental information about the homogeneity of the studied gel, the second gives access to the high frequency behaviour, related to the nature of the basic strands of the network. Firstly, after verifying the validity of the experimental apparati and analysis approaches in a series of careful control experiments on archetypal systems, a regime where pectin gels exhibit the signatures of semi-flexible networks was identified in experiments carried out on gels made of pectin chains pre-engineered by enzymatic deesterification and subsequently assembled with the release of Ca2+. These results were the first showing that polysaccharides networks could be accommodated within the framework of semi-flexible networks, which have become a paradigm for biological gels, such as the well-known F-actin solutions present in the cell cytoskeleton. However, in the plant cell wall, where calcium is already present, the assembly mechanism could be controlled in a different manner, and a more biologically relevant system was studied where the action of the plant enzyme pectinmethylesterase was used to liberate ion-binding groups in the presence of Ca2+. Gels formed according to this alternative methodology were found to behave as punctually cross-linked flexible networks, strikingly different from the first results. This would be explained by the presence of short blocks of charged residues. Finally, experiments on pectins carried out with controlled blocky structures showed that a pectin made of short blocks can exhibit both sorts of network, depending on the polymer and Ca2+ concentrations. This lead naturally to the construction of a state diagram for the regimes of assembly, with proposed control parameters being the polymer concentration and the ratio of the amount of Ca2+ to the quantity of pectic residues which can effectively bind the calcium into cross-links, christened Reff