Self-consistent-field modeling of adsorbed beta-casein: Effects of pH and ionic strength on surface coverage and density profile

The Scheutjens-Fleer self-consistent-held theory has been used to describe the adsorption of a complex linear polyelectrolyte containing a proportion of segments with pH-dependent charges, i.e., a model disordered protein. Each individual segment along the chain is categorized as being apolar, polar(uncharged), or(potentially) charged, and the sequence of segments along the chain is taken to mirror the known primary structure of the milk protein beta-casein, The equilibrium adsorption behavior of this beta-casein look-alike has been investigated as a function of bulk phase protein concentration, pH, and ionic strength. The total segment density profile phi(z) and distributions of individual amino-acid residues and electrical charges across the adsorbed layer have been calculated. At not too high adsorbed amounts, the monolayer profile phi(z) falls off in a featureless fashion from a very high value [phi(z) similar to 0.95] in a narrow region close to the surface down to values approaching the polymer bulk concentration [phi(z) < 0.01] for z > 5 nm. Over a wide range of pH, ionic stength, and bulk concentration, there is found to be a very dilute tail region [phi(z) < 0.01] extending out from z = 3-7 nm to z similar to 20 nm. This tail region mainly corresponds to the hydrophilic sequence of similar to 40 segments at the N-terminus of the polypeptide chain. Removal of the phosphate groups from the five phosphoserine residues produces a substantial increase in the adsorbed amount but a reduction in the hydrodynamic layer thickness. The numerical results are in very good qualitative agreement with findings from recent neutron reflectivity and light scattering studies of adsorbed beta-casein at solid and liquid interfaces. (C) 1996 Academic Press, Inc.