We are interested in how polysaccharide fine structure impacts on the molecular and macroscopic functionality, from affecting the molecular force-extension curve, to the way the polymer interacts with other molecules, to the properties of materials assembled from these macromolecules.
Nanomechanical Properties of Single Chains
Single Chain Stretching
Atomic force microscopy (AFM) has emerged as the tool of choice for conducting single molecule force spectroscopy, owing largely to the limited range of forces that can be applied by competing methodologies such as the use of optical tweezers or magnetic beads. Such experimental single-chain stretching data is routinely fitted to statistical mechanical models of the force-extension curve, in particular the freely jointed and wormlike chain models (FJC and WLC models) or their extensible extensions, which involve a high-force Hookean regime that permits the chain to be elongated beyond its contour length. Here the
beta-linked polysaccharides methylcellulose and chitosan are stretched and the data fitted to a extensible wormlike chain model. The work is carried out in collaboration with Richard Haverkamp.
Although examples of single polynucleotides, proteins, polysaccharides and synthetic polymers have all now been stretched using AFM, there remain relatively few studies on polysaccharide molecules. At first sight polysaccharides may appear not to offer the same exciting opportunities as nucleotides, whose duplexes can be mechanically separated, or proteins, that may be mechanically denatured. They do, however, exhibit their own richness of behaviour including the possibility of force-induced conformational transitions of the pyranose rings
under tension. In polysaccharides in which the rings are alpha-linked, the bond can act as a molecular lever, inducing conformational change, as seen here in pectin.
AGAG polysaccharide chains tie togther collagen fibrils in animal extracellular matrices (ECMs).
It has been suggested that such conformational transitions in polysaccharides may have a signaling role in-vivo. However, recent work has demonstrated that force-induced conformational changes occur in the sugar rings of one family of bridging anionic glycosaminoglycans (AGAG), the dermochondans (DS), but not in the closely related chondroitans (CS), as shown here. As tissues requiring significant elastic extensions (e.g. skin) characteristically contain dermochondans rather than chondroitans the work suggests that the nanomechanics of force-induced conformational transitions of pyranose rings may play a role in directly controlling macroscopic tissue properties.
Materials Properties of Biopolymer Matrices
We are interested in monitoring the assembly of biopolymers, in particular when the process results in the production of a soft material. In addition to the effect of fine structure modifications on the resulting materials we also try to understand the relationship between the manifest properties and the assembly mechanism and kinetics.
Here we have used a conventional bulk-rheological measurement in order to monitor gel formation in a pectin solution, induced by releasing calcium from calcium carbonate. By releasing calcium at different rates we alter the kinetics of gel formation. Romaric is currently developing microrheological methods using diffusing wave spectroscopy (DWS) and multiple particle tracking in order to follow the rheological evolution occurring during the gelation process. This will extended the experimental frequency range, and also to allow such tests to be carried out where a minimal amount of material is available.
We are also interested in microstructurally engineering soft materials using mixtures of biopolymers by exploiting thermodynamic incompatibility. A particular interest is the interplay between molecular ordering, phase separation and gelation.
In this gelatin / maltodextrin system gelatin ordering not only leads to gel formation that eventually traps the microstructural evolution, but its initial aggregation is responsible for triggering the phase separation process.
Polysaccharide - Protein Interactions
We are also investigating the molecular interactions between polysaccharides and proteins as well as carrying out work on glycoproteins and proteoglycans.