The research in this thesis deals with the influence of charge on the adsorption of milk proteins to surfaces. A variety of charged surfaces were used including negatively charged and zwitterionic liposomes prepared from phosphatidylglycerol and phosphatidylcholine respectively and positively and negatively charged polystyrene latices. Adsorption was determined by measuring the increase in the hydrodynamic radius of the particles by photon correlation spectroscopy and also by solution depletion techniques. In some instances, electrophoretic mobility measurements were also used in order to determine changes in the surface charge of the particle as a result of protein adsorption. The ionic strength and pH of the buffer were found to be important in the adsorption of protein to liposomes. In the absence of NaCl, adsorption did not occur. At low pH values, addition of both k-casein and B-Iactoglobulin to negatively charged liposomes caused very large increases in size presumably as a result of aggregation. At pH6.2, protein layer thicknesses on the negatively charged liposomes were significantly greater than on the zwitterionic ones due to charge repulsion between the negatively charged surface and the negatively charged regions of the proteins. Removal of the negatively charged phosphate groups which form a cluster in the hydrophilic region of B-casein resulted in a reduction in the thickness of the adsorbed protein layer on the negatively charged liposome but did not have any effect on the thickness on the zwitterionic surface. The thickness of adsorbed layers of as1-, k-, and B-casein and B-lactoglobulin on the phosphatidylcholine liposomes were all very similar at around 6nm. Addition of as1-casein to the negatively charged liposomes appeared to cause aggregation as a result of protein molecules bridging between liposomes. Attempts to determine the fraction of added protein which bound to the surface of the liposomes were unsuccessful and therefore, the binding of native, dephosphorylated and methyl-esterified l3-casein to small, monodisperse, positively and negatively charged polystyrene latices was studied. As with the liposomes, the thickness of the adsorbed B-casein layer was greater on the negatively charged surface. Removal of the phosphate groups from the protein decreased the layer thickness by about 4nm on the negatively charged surface but had relatively little effect on the thickness on the positively charged surface, once again showing the effect of charge interactions. As with dephosphorylation, methylation also reduces the net negative charge of the protein, but by a different mechanism. This also resulted in a reduction in the thickness of the adsorbed protein layers but only after a significant proportion of the free carboxyl groups had been esterified. Thus methylation of 35 % of these groups had relatively little effect on the thickness of the layer on the positively charged latex and no effect on the negatively charged, but esterification of a further 9% (equivalent to two residues) caused a substantial decrease in thickness on both surfaces. These changes are believed to result from alterations in both the charge and hydrophilicity of particular regions of the B-casein molecule. Bridging was found to occur when low levels of native or modified B-casein were added to the positively charged latex. Protein loading was found to range from 2.5 to 5.5mg m-2 depending on the nature of the protein and the charge on the surface. The thickness of adsorbed native and dephosphorylated B-casein layers on the negatively charged latex was found to be influenced by the presence of calcium and increasing ionic strength. Increasing levels of either calcium ions or NaCI in the medium resulted in a very pronounced decrease in the thickness of pre-adsorbed phosphorylated B-casein layers. The changes in dephosphorylated protein layers were less pronounced. The results are discussed in terms of the proposed loop-and-train configuration of the B-casein at the surface of the latex. The influence of protein phenotype and the extent of glycosylation on the adsorption of k-casein was also determined. The more highly glycosylated protein molecules, which also had a higher net negative charge, formed thicker layers on the negatively charged surface. Again, layer thicknesses were less on the positively charged surface, but for each x-cn phenotype glycosylation increased the thickness, presumably as a result of the increased hydrophilicity of the protein. k-Casein A, which has one more negative charge than the B phenotype, was found to give a slightly thicker layer on the negatively charged latex. Under certain conditions, adsorbed k-casein could be cleaved by the enzyme chymosin as shown by the reduction in the size of the coated latex.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:301249 |
| Date | January 1993 |
| Creators | Brooksbank, Dawn Victoria |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/8390/ |
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