JOURNAL OF COLLOID AND INTERFACE SCIENCE 178, 681–693 (1996) ARTICLE NO. 0166 Self-Consistent-Field Modeling of Adsorbed b-Casein: Effects of pH and Ionic Strength on Surface Coverage and Density Profile FRANS A. M. LEERMAKERS,* PETER J. ATKINSON,² E RIC DICKINSON,² ,1 AND DAVID S. HORNE‡ * Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands; ² Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, United Kingdom; and ‡ Hannah Research Institute, Ayr K A6 5HL, Scotland, United Kingdom Received May 8, 1995; accepted October 9, 1995 ordered secondary structure ( a-helix or b-sheet ) is attributed The Scheutjens–Fleer self-consistent-field theory has been used (2) to the complete absence of cysteine residues and the to describe the adsorption of a complex linearpolyelectrolyte con- presence of numerous proline residues distributed through- taining a proportion of segments with pH-dependent charges, i.e., out the polypeptide sequence. At neutral pH the isolated a model disordered protein. Each individual segment along the b-casein molecule carries a net negative charge, and the chain is categorized as being apolar, polar (uncharged), or (poten- nonuniform distribution of the charged and hydrophobic res- tially) charged, and the sequence of segments along the chain is idues makes the molecule distinctly amphiphilic. The high taken to mirrorthe known primary structure of the milk protein proportion of nonpolar groups along the 209-residue poly- b-casein. The equilibrium adsorption behavior of this b-casein peptide chain gives bovine b-casein a tendency to form mi- look-alike has been investigated as a function of bulk phase protein celle-type aggregates in aqueous solution and to adsorb concentration, pH, and ionic strength. The total segment density strongly at oil–water or air–water interfaces (3). A note- profile w( z ) and distributions of individual amino-acid residues and electrical charges across the adsorbed layerhave been calcu- worthy feature of the nonuniform hydrophobic residue distri- lated. At not too high adsorbed amounts, the monolayer profile bution (4) is the presence of a predominantly hydrophilic w( z ) falls off in a featureless fashion from a very high value [ w( z ) region of 40–50 segments at the N-terminus which includes Ç 0.95] in a narrow region close to the surface down to values many charged groups (including all of the phosphoserine approaching the polymer bulk concentration [ w( z ) õ 0.01] for z residues). ú 5 nm. Overa wide range of pH, ionic stength, and bulk concen- Over a wide range of bulk concentrations, b-casein ad- tration, there is found to be a very dilute tail region [ w( z ) õ 0.01] sorbs at a variety of hydrophobic surfaces (5–8) to give a extending out from z Å 3–7 nm to z Ç 20 nm. This tail region monolayer coverage of 2–3 mg m 02 . Detailed information mainly corresponds to the hydrophilic sequence of Ç40 segments on the b-casein density profile perpendicular to the planar at the N-terminus of the polypeptide chain. Removal of the phos- oil–water or air–water interface has recently been obtained phate groups from the five phosphoserine residues produces a (9–12) using the technique of specular neutron reflectance substantial increase in the adsorbed amount but a reduction in the (13, 14). The measurements at the air–water interface (pH hydrodynamic layer thickness. The numerical results are in very 7, bulk concentration Å 5 1 10 03 or 5 1 10 04 wt%) are good qualitative agreement with findings from recent neutron re- best fitted (11, 12) by a two-layer model consisting of a flectivity and light scattering studies of adsorbed b-casein at solid and liquid interfaces.  1996 Academic Press, Inc. dense inner layer of volume fraction f inner Ç 0.9 and thick- Key Words: adsorption; casein; protein. ness d inner Ç 1 nm and a diffuse outer layer of volume fraction f outer Ç 0.15 and thickness d outer Ç 4–5 nm. The total layer thickness d total ( d inner / d outer ) of 5–6 nm determined by neutron reflectivity at pH 7.0 increases to d total É 7 nm on INTRODUCTION lowering pH to 6.0 and to d total É 8–9 nm on lowering pH to 5.5. The hydrodynamic layer thickness ( ú10 nm) as One of the most important macromolecular emulsifiers determined by dynamic light scattering (15, 16) is substan- involved in the stabilization of food colloids is the milk tially greater than the thickness d total inferred from neutron protein b-casein (1). Unlike most proteins, which exist in reflectivity or from X-ray scattering ( 17 ) . This is perhaps not their native states as compact globules, b-casein has a disor- unexpected ( 3 ) since the hydrodynamic thickness includes dered structure which can be modeled to a first approxima- dangling segments at the very periphery of the adsorbed tion as flexible linear polyelectrolyte. The relative lack of layer ( i.e., long tails ) where the local protein volume fraction is probably too low to make a contribution to the neutron 1 To whom correspondence should be addressed. reflectivity measurements. 681 0021-9797/96 $18.00 Copyright  1996 by Academic Press, Inc. 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