Metal ion-carbonyl oxygen recognition in complexes of acetyl phosphate

Studies on acetyl phosphate (AcP2-), one of the so-called 'energy-rich' mix ed-acid anhydrides, are summarized. Based on stability constants determined by potentiometric pH titrations in aqueous solution, it is shown that the M(AcP) complexes of Ca2+, Mg2+, Mn2+ Cu2+, and Zn2+ are more stable than is expected from the basicity of the phosphate group of AcP2-. This observed stability increase is attributed to an additional interaction of the alread y phosphate-coordinated metal ion (M2+) with the carbonyl oxygen of the anh ydride unit. These conclusions are corroborated by the properties of the co mplexes of the hydrolysis-stable acetonylphosphonate (AnP(2-)). The formati on degrees of the various six-membered chelates occurring in the M(AcP) and M(AnP) systems are presented and evidence is given that these chelates per sist in mixed ligand complexes and that their formation degree is promoted by a low solvent polarity. The biological relevance of these results regard ing carbonyl oxygen-metal ion interactions is briefly indicated. (C) 2000 E lsevier Science Inc. All rights reserved.