Mixed protein-polysaccharide interfacial layers: a self consistent field calculation study

Mixed interfacial films of protein and polysaccharide have been investigated using self consistent field (SCF) calculations. The colloidal interactions mediated by such composite layers between two approaching surfaces have been considered. Two types of systems have been studied: (a) covalently-bonded polysaccharide and protein, and (b) the excess presence of polysaccharide at the interface, occurring through electrostatic interaction with an already existing, oppositely charged, protein layer. Our calculations show that for covalently-bonded complexes, depending on the location of the protein-polysaccharide bond, the attachment of short uncharged chains to the protein can be detrimental to provision of repulsive colloidal forces by such complexes. We have attributed this to an increased tendency of the hybrid polymer to adopt bridging configurations in the gap between two nearby surfaces. For larger grafted chains this bridging effect is eliminated, and the expected enhanced steric stabilization of the protein-polysaccharide conjugate is achieved. For adsorbed films formed through electrostatic interactions between these two biopolymers, stronger repulsive forces between the surfaces are produced, at an intermediate level of charging for the polysaccharides. This has been related to a maximum level of adsorption of polysaccharide, as the number of charged segments of the chain is varied. The peak occurs at higher levels of charging as the salt concentration in the bulk solution is increased. We have also observed the experimentally-reported phenomenon of charge overcompensation, arising from adsorption of the polysaccharide chains onto the primary protein layer. The importance of the non-uniform charge distribution of the polysaccharide molecule, in providing an explanation for this effect, has been demonstrated.