Apparent stability constants of H+ and Mg2+ complexes of 5-phosphoribosyl α-1-pyrophosphate

Apparent Mg²⁺ and H⁺ stability constants of 5-phosphoribosyl α-1-pyrophosphate (ligand, L) complexes were determined from pH titration data at 25°C with an average of 0.17 M NaCl or KCl and 0.20 M ionic strength. The logarithms of calculated macroscopic overall stability constants are: 3.2 (MgL^3-), 4.8 (Mg₂L^-), 6.5 (HL^4-), 12.4 (H₂L^3-), 9.4 (MgHL^2-), and 11.0 (MgH₂L). Comparison of the stepwise Mg²⁺ stability constants (log k = 3.2 and 1.6) with those of MgADP^- and MgAMP or Mg-hexose-1-P suggests that the first and second Mg²⁺ bind to the 1-PP and 5-P groups of the ligand, respectively. Reasonable assumptions about relative microscopic constants indicate that several of the microscopic isomers do not achieve significant concentrations over a large range of conditions. Judging from other data on organophosphate complexes, it is likely that the constants of this study may be extrapolated with little error to other conditions of ionic strength 0.1–0.2 M) and temperature (e.g., 15–35°C), and widely different monovalent ion concentrations.