Effect of Organic Ions on Solubility of Enamel and Dentin in Acid Buffers

¹ Eastman Dental Dispensary, Rochester, New York

The effect of the addition of nineteen different acids to decalcifying buffers on the solubility rates of enamel and dentin over the pH range 2.6-6.0 was determined. The experimental approach used permitted an evaluation of the specific effects of the characteristic ions of the acids. The ions evaluated were derived from naturally occurring and biologically important organic acids and certain synthetic substances, such as ethylenediaminetetraacetic and tricarballylic acid.

It was found that the capacity of an acidic environment to decalcify enamel and dentin could be significantly increased or decreased by the addition of certain acid ions. In this respect enamel appears to be considerably more sensitive to the composition of the decalcification medium than dentin, whose solubility appears to be influenced only by those ions which show the strongest effects with enamel. Moreover, depending on the pH of the environment, the same ion can demonstrate opposite effects. As the pH increases from 2.6 to 6.0, the ions of citric acid and EDTA have no effect, then decrease, and finally increase the solubility rates of enamel in acid buffers. The increases in solubility observed with EDTA and citric acid are attributed to the ability of their ions at certain levels to form soluble calcium complexes, thus reducing the common ion effect which tends to suppress dissolution.

With respect to the solubility reductions observed, the following have been noted:

The monobasic acids produce little or no effect at any of the pH levels tested.

The dibasic acids demonstrate moderate effects.

The tribasic acids and EDTA show strong effects.

The presence of additional polar groups, such as amino or hydroxy, in these acids considerably enhance their capacity to reduce solubility.

From the contrasting findings with the homologues, aspartic and glutamic acids, and the sterioisomers, maleic and fumaric acids, there is an indication that the spatial arrangement of functional groups is important in determining the capacity for reduction.

The finding that the multicharged basic amino acid ions of lysine and ornithine, which have a net unit positive charge, produce reductions as strong as the ions of the acidic amino acid aspartic, which has a net unit negative charge, suggests that the nature of the charge is not so important as the existence of multiple charges on the ions.

It has been postulated that the ions which demonstrate reductions concentrate at or near the surfaces of the enamel and dentin crystallites, forming an ionic shell or surface-bound complexes that retard decalcification by interfering with the diffusion processes necessary for dissolution.

The possibility is suggested that specific ion effects of the sort presented may be of importance in biologic processes involving deposition and resorption of calcified tissues and the destruction of the teeth as occurs in caries.