Protein-polysaccharide interactions effects

Figure 7.18 Protein-polysaccharide interactions in emulsions subjected to high pressure treatment (HPT). Influence of pH on average effective particle diameter d43 determined by static light scattering (Malvern Mastersizer) in emulsions (20 vol% soybean oil, 0.5 wt% p-lactoglobulin) prepared with untreated protein (open symbols) and high-pressure-treated (800 MPa for 30 min filled symbols) protein in the absence (O, ) and presence (A, ) of 0.5 wt% pectin. Reproduced from Dickinson and James (2000) with permission.

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

Polysaccharides can regulate weak interactions between protein molecules. A recent example is the effect of low molecular weight heparin molecules on the weak dimerisation of the plasminogen growth factor NKl, or at least a mutant thereof [135]. 

Let us turn now to consider systems with thermodynamically favourable interaction (A24 < 0) (i.e., mutual attraction) between protein and polysaccharide. Here there is little measurable effect on the protein loading (see Table 3.1) (Semenova et al., 1999). However, an important con-... 

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