Search

Temperature-dependent formation of gelatin−poly(galacturonic acid) (PGA) multilayer films based on the layer-by-layer approach and their stability to changes in temperature, pH, and NaCl concentration were investigated for the first time using quartz crystal microbalance with dissipation monitoring (QCMD). Changes in the viscoelastic properties of the gelatin−PGA films with temperature were also observed using atomic force microscopy (AFM). Four different assembly temperatures ranging from 20 to 37 °C were chosen, and the findings revealed that the assembly temperature was crucial to gelatin−PGA multilayer film formation. The highest amount adsorbed and consequently the thickest gelatin−PGA assemblies were formed at 20 °C, whereas limited growth was observed for gelatin−PGA films assembled at 37 °C. Although the experimental conditions favored electrostatic interactions to be the driving force during the gelatin−PGA assembly, the inhibited multilayer growth observed for layers formed at the temperature of 30 and 37 °C clearly indicated very weak electrostatic interactions between gelatin and PGA and the continuous multilayer growth at lower temperatures was mainly due to hydrogen bonding. The stability of assembled gelatin−PGA multilayer films to various environmental stresses was found to be significantly influenced by the changes in temperature. At low temperatures the behavior of gelatin was dominated by helix formation and association through hydrogen bonding. Raising the temperature promoted melting of the helix, disruption of the multilayer network, and disassembly of the films, which was also indicated by AFM study. In terms of the responsiveness to pH and salt, this study showed that gelatin−PGA multilayer films fabricated at 20 °C underwent almost completely reversible alternate (de)swelling changes. In contrast, gelatin−PGA multilayer films formed at 25 °C showed a partial decomposition when exposed to various pH and salt concentrations. [ABSTRACT FROM AUTHOR]

It is increasingly recognized that changes in the composition of the oil−water interface can markedly affect pancreatic lipase adsorption and function. To understand interfacial mechanisms determining lipase activity, we investigated the adsorption behavior of bile salts and pancreatic colipase and lipase onto digalactosyldiacylglycerol (DGDG) and dipalmitoylphosphatidylcholine (DPPC) monolayers at the air−water interface. The results from Langmuir trough and pendant drop experiments showed that a DGDG interface was more resistant to the adsorption of bile salts, colipase, and lipase compared to that of DPPC. Atomic force microscopy (AFM) images showed that the adsorption of bile salts into a DPPC monolayer decreased the size of the liquid condensed (LC) domains while there was no visible topographical change for DGDG systems. The results also showed that colipase and lipase adsorbed exclusively onto the mixed DPPC−bile salt regions and not the DPPC condensed phase. When the colipase and lipase were in excess, they fully covered the mixed DPPC−bile salt regions. However, the colipase and lipase coverage on the mixed DGDG−bile salt monolayer was incomplete and discontinuous. It was postulated that bile salts adsorbed into the DPPC monolayers filling the gaps between the lipid headgroups and spacing out the lipid molecules, making the lipid hydrocarbon tails more exposed to the surface. This created hydrophobic patches suitable for the binding of colipase and lipase. In contrast, bile salts adsorbed less easily into the DGDG monolayer because DGDG has a larger headgroup, which has strong intermolecular interactions and the ability to adopt different orientations at the interface. Thus, there are fewer hydrophobic patches that are of sufficient size to accommodate the colipase on the mixed DGDG−bile salt monolayer compared to the mixed DPPC−bile salt regions. The results from this work have reinforced the hypothesis that the interfacial molecular packing of lipids at the oil−water interface influences the adsorption of bile salts, colipase, and lipase, which in turn impacts the rate of lipolysis. [ABSTRACT FROM AUTHOR]

The competitive displacement of a model protein (β-lactoglobulin) by bile salts from air−water and oil−water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of β-lactoglobulin adsorbed layers at the air−water and the olive oil−water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl 2, 37 °C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil−water interface compared to the air−water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed β-lactoglobulin film at both the air−water and olive oil−water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial β-lactoglobulin networks. AFM images of the displacement of spread β-lactoglobulin at the air−water and the olive oil−water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]