Alizarin and tetracycline binding by bone mineral

The nature of alizarin combination with bone is thought to be a chelation with a divalent cation on the surface of the crystal. Reactions of alizarin with bone mineral and apatite, rather than with intact bone, have been demonstrated by several investigators. In vitro titration indicates that a positive divalent or trivalent ion is capable of chelating with alizarin. Aluminum and iron form the strongest chelates, but calcium, magnesium, barium, etc. are capable of reacting. Not only does bone mineral, such as hydroxyapatite, combine with alizarin, but such other calcium‐containing salts as calcium fluoride also bind the dye. The structure of alizarin that reacts with the metal ion appears to require a hydroxyquinone as the ligand. The presence of a second hydroxy group on the ring in the 2 position, or in almost any other position on the ring, facilitates the binding of the ligand to the apatite. This makes the dihydroxy molecules far more effective at low pH, and it is to this family of compounds that alizarin belongs. Alizarin red S is simply the placement of asulfonic acid group on the ring to get additional ionic structure, in order to make it soluble in water. A variety of isomers of alizarin have been studied, and all are capable of reacting with bone mineral on the same molar basis. In fact, removal of the unsubstituted ring of alizarin to form dihydroxy‐naph‐aquinone produces a dye still capable of reacting with apatite. The ratio of the moles of napthaquinone bound to those of anthraquinone may be explained by the relative sizes of the two dye molecules. It has been shown that phosphate competitively replaces alizarin, and that its presence in solution will prevent alizarin attachment to the surface of apatite. This result can be interpreted to mean that both the alizarin and the phosphate tend to occupy the same site when they are being adsorbed onto the surface. Tetracyclines, while more complex in structure, appear to have a binding mechanism to apatite which resembles that of alizarin.