Forces between macroscopic surfaces in solutions of calcium binding proteins

Forces have been measured between macroscopic sheets of mica exposed to aqueous solutions of the calcium binding protein, calbindin D-9k. The study focuses on the nature of the long-range exponential double layer force and on the adsorption behavior of calbindin on mica. These two features have been monitored as a function of molecular protein composition. We have varied the primary structure of calbindin by genetic substitution of specific amino acids. In particular, we report on the effects of neutralizing individual negative amino acid groups. Advantage is also taken of the protein's ability to bind calcium ions to reduce its net negative charge further. At a fixed protein concentration, in the absence of added salt, the variation in net charge affects the decay length of the exponential force at large separations, in the way expected from classical continuum theory. In fact, after spectroscopic analysis of the solutions we find that the measured decay length agrees with the Debye length. The force at short separations is greatly affected by the presence of calcium bound to the protein. Specifically, when the protein's binding affinity is high, the presence of bound calcium provokes a very strong adhesive force between the protein-adsorbed mica sheets. This we attribute to an ion correlation effect, which in the biological literature would be termed calcium bridging. In this and other cases the conformation of adsorbed caibindin is greatly influenced by calcium.