Isoionic exchange

Mention has already been made of substitution within the crystal lattice. A foreign ion, provided that it is similar in size to the ion which it replaces, may exchange for a normal hydroxyapatite constituent. This process is called heteroionic exchange, whereas the exchange of like ion for like is called isoionic exchange. There are, thus, two ways in which ions other than calcium, phosphate or hydroxyl may become part of the structure of biological apatite. Firstly,... 

Pak, C. Y. C. Bartter, F. C. 1967. Ionic interaction with bone material, I. Evidence for isoionic calcium exchange with hydroxyapatite. Biochimica Biophysica Acta, 141, 401-409. 

An isoionic solution is defined as a solution which contains only protein ions of average charge Z, ions, and OH ions. No other ions may be present. An isoionic solution can be prepared by means of a mixed-bed ion-exchange column (Dintzis, 1952), or by electrodialysis, and only for proteins which are soluble in salt-free water. (Refer to Section 111, B). 

The pH at which a protein or particle has an equivalent number of total positive and negative charges as determined by proton exchange is the isoionic point. The pH at which a protein or a particle does not migrate in an electric field is called the isoelectric point. The isoionic point is a whole fiber property of hair and is reflected in the equilibrium acid-base properties of the total fiber the isoelectric point is related to the acid-base properties of the fiber surface. 

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