Cation bindint to alpha-sl-casein B. A comparison of electrostatic models

System characteristics which determine calcium binding to and subsequent proton release from alpha-sl-casein B are reported at pH 6.6 and [Na-plus] equal to 0.04, 0.0, and 0.16M. Values of protein solvation, G, site bound calcium, Ca,S, and net monomer charge, Z, permitted distributed charge models to be constructed. The models examined proved inadequate in that it was impossible to keep the dielectric constant, D, within acceptable limits and/or predict measured proton release. Three discrete charge models were constructed. At D equals 4, all three gave good agreement between predicted and experimental data as Ca, S increased. The known amino acid sequence was used to make rodlet models for the whole molecule andfor just the phosphate-containing acidic peptide portion. A comparison of these shows the electrostatic dominance of the acidic peptide and suggests that the electrostatic environment for the remainder of the binding sites is essentially constant as Ca, S increases during addition of calcium ion. The third discrete charge model bends the acidic peptide rodlet into a torus. In this case, data were matched with less assumed bond strain under conditions of high molecular charge than with the other two models. This indicates that conformation and association may be important factors to consider when constructing discrete charge models to calculate electrostatic free energy.