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Pectin is an integral component of non-graminaceous plant cell walls. It is believed to form an interconnected network structure independent of the cellulose-xyloglucan network structure. Pectin gels are often used as a model for the pectin network structure within the plant cell wall. Atomic force microscopy studies of calcium-induced gel precursors, and fragments released from gels, suggest that association leads to a branched fibrous structure within the gels. Enzymatic de-esterification of high-methoxyl pectin in the presence of calcium ions can induce gelation of the pectin. Thus pectin gel networks may provide a model for a self-assembled network structure within the middle lamella region of the plant cell wall. The pectin network in plant cell walls is a source of soluble and insoluble fibre. In addition to the health benefits associated with the dietary fibre aspects of pectin new health claims are emerging. Recently published in vitro and in vivo animal studies, and human studies, suggest that oral consumption of a modified form of pectin may have anti-cancer properties. These studies suggest that the modified pectin may act on a range of cancers at several stages of progression of the cancer. It has been hypothesised that this generic action is due to the modification allowing release of bioactive fragment(s) which are claimed to bind specifically to and inhibit the action of the mammalian lectin galectin 3 (Gal3). Gal3 is a key regulator of cellular homeostasis and plays important roles in several stages of cancer metastasis. Studies using force spectroscopy, flow cytometry and fluorescence microscopy suggest that the bioactive fragments of pectin may be pectin-derived galactans.

Unlike pectins from other origins, sugar beet pectin (SBP) acts as an emulsifier, a property which has been correlated to its more hydrophobic character and high protein content. In this work, we have investigated the structure of SBP at interfaces by atomic force microscopy (AFM). Three situations were studied: the mica/water, graphite/water, and air/water interface. For the latter, the interfacial film was transferred onto mica using the Langmuir-Blodgett method. While the adsorption of individual pectin chains on mica requires the addition of divalent cations, on graphite a thin layer containing amorphous areas and rodlike chains forms spontaneously. We suggest that the layer contains proteins and pectin chains which are bound to the graphite via CH-pi interactions. SBP adsorbed at the air/water interface forms an elastic layer, as evidenced by pendant drop and surface shear rheology measurements. AFM Images reveal the layer is crippled with holes and contains rodlike chains, suggesting that the pectin chains prevent the formation of a densely packed protein layer. Nevertheless, we show that the interfacial pectin film is more resistant to displacement by surfactants than a pure protein film, possibly because of the formation of linkages between the pectin chains. In contrast, alkali treatment of the pectin appears to remove the pectin chains from the air/water interface and leaves a film that behaves similarly to pure protein. This work gives a new insight into the nanoscale organization of polysaccharides and polysaccharide-protein mixtures at macroscopic surfaces. The results gathered from the different interfaces studied permit a better understanding of the likely structure of SBP at the interface of emulsion droplets. Such knowledge might be used to modify rationally the pectin in order to improve its emulsifying properties, leading to broader commercial applications.

We have used atomic force microscopy to study the morphology of hydrophobic polyelectrolytes adsorbed on surfaces. The polyelectrolytes consisted of polystyrene sulfonate (PSS) chains made with three charge densities: 32%, 67%, and 92%. They were adsorbed on two types of surfaces: mica, and phospholipid bilayers made of mixed neutral and cationic lipids. We show that the chains with a low charge density (32%) are collapsed in spherical globules while highly charged chains (67% and 92%) are fully extended. End-to-end distances and contour lengths of the extended chains were measured. Statistical analysis shows that the persistence length of these chains depends on the surface where they adsorb. On lipid bilayers, highly ordered monolayers are formed upon increase of the proportion of cationic phospholipids. These results show that highly charged PSS chains behave in a similar manner than the stiffer, hydrophilic DNA when adsorbed on surfaces. It could lead to the design of new types of nanostructured surfaces using polyelectrolyte molecules synthesized with specific properties.

Recently AFM has been used to measure the interactions in aqueous media between a deformable oil drop attached onto an AFM cantilever and another drop on a substrate. In this study this method has been applied to oil droplets immersed in a solution of a polysaccharide extract having emulsifying properties, sugar beet pectin (SBP). In parallel with the force spectroscopy studies, images of the SBP interfacial films were obtained, in order to probe the effects of interfacial structures on the interactions between the drops. The influence of the SBP concentration in the bulk phase was studied: at a low bulk SBP concentration, where the adsorbed SBP layer is relatively flat, non-adsorbed SBP in the bulk solution gives rise to a depletion interaction between the drops. Upon increasing the SBP concentration, a hysteresis appears in the interactions between the drops. It is hypothesised that this effect is induced by the strong liquid structural correlations occurring within the liquid film separating the droplets. This conjecture is supported by a theoretical description which incorporates depletion interactions into a model for the interactions between deformable droplets. At high SBP concentration a strong repulsive force, attributed to steric repulsion between the thick SBP layers adsorbed onto the drops, is observed. Under different bulk conditions polymer bridges were observed to be formed between the SBP-coated droplets. The results demonstrate how AFM can be used to probe different types of interactions between deformable oil drops in aqueous media at the molecular level. When combined with theoretical models, this approach will advance the understanding of molecular mechanisms that govern the stability of emulsions.