A study of selective adsorption of Na+ and other alkali-metal ions on isolated proteins: a test of the salt-linkage hypothesis.

According to the salt-linkage hypothesis (a part of the association-induction hypothesis), the binding of alkali-metal ions on isolated proteins may be low as a result of the masking of cation-binding fixed anionic groups by the formation of "salt linkages" between them and fixed cationic groups. This hypothesis has been verified in a quantitative manner by the measurement of free and adsorbed Na+ in solutions of bovine hemoglobin titrated with increasing concentration of NaOH. In addition this communication presents data indicating (1) wide variability in the extent of Na+ adsorption among different isolated proteins and even among different samples of the same protein, (2) auto-cooperativity in the pH titration and Na+ adsorption, and (3) selectivity of the liberated fixed anionic groups for different alkali-metal ions in the rank order Na greater than Li greater than K greater than Rb, Cs.