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The increasing number of roles discovered for Al3+ in physiological processes demands an understanding of how Al3+ interacts with compounds in biological systems. Al3+ is expected to complex with oxygen donor ligands, especially phosphates, and it does so in soils, in the gastrointestinal tract, and in cells. The stability of Al3+ complexes has generally been misjudged because of lack of recognition that free, aqueous Al3+ is not the dominant form in neutral solutions and that the solubility of Al(OH)3 limits the free Al3+ at the plasma pH 7.4 to less than 10(-11) mol/L. In the presence of inorganic phosphate, the permitted free Al3+ is decreased further, through formation of insoluble aluminum phosphate. This precipitate facilitates the elimination of Al3+ from the body. In contrast, citrate solubilizes Al3+, and an appreciable fraction occurs as a neutral complex that may pass through membranes and provide a vehicle for Al3+ absorption into the body. In the blood plasma the most likely small-molecule complex is that with citrate, while the only competitive protein complex is that with transferrin, a protein built to transport Fe3+ but whose sites are only 30% occupied.

Calcium ion plays an essential role in many biological processes. The environment about Ca2+ may be probed by substitution of tripositive lanthanide ions, Ln3+. Ca2+ proteins fall into two broad classes: those that are inhibited by Ln3+ substitution and those that are not. It is suggested that although Ca2+ undertakes a primarily structural role in the Ln3+ non-inhibited proteins, Ca2+ may be near the active site or participate in the mechanism of action of Ln3+ inhibited proteins. Ca2+ and Ln3+ radii are similar; most Ln3+ are slightly larger than Ca2+ in complexes of the same coordination number, and substitution of Ln3+ for Ca2+ is accommodated by a slight decrease in bond distance or by an increase in coordination number. Luminescence from Tb3+ has been demonstrated to be a sensitive environmental probe of Ca2+ binding sites in proteins.

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070ChemInform Abstract The stability constants for the divalent metals Mg, Ca, Mn, Co, Ni, Cu, Zn, and Cd are determined by potentiometric pH titrations at 25 rc C. The results show that the stability of most of the M(ATP)2-complexes is significantly larger than that of the corresponding complexes formed with the pyrimidine-nucleoside 5'-triphosphates (PNTP). This increased stability is attributed to the formationof outer-sphere macrochelates. Inner-and outer-sphere forms of M(ATP)2-occur for Mn, Co, Ni, Zn, and Cd in comparable amounts, Cu forms no outer-sphere species to any significant extent and Ca(ATP)2-exists only in the open, phosphate-coordinated form. Of all the M(PNTP)2-complexes, only Cu(CTP)2-forms a base-back-bound species.