The formation constants of ionomycin with divalent cations in 80% methanol/water

The protonation constants and complex formation constants of ionomycin have been determined in 80% methanol/water (w/w) at 25.0 degrees C and mu = 0.050 (tetraethylammonium perchlorate). Potentiometric and spectrometric titration techniques give the following values for the mixed-mode protonation constants of ionomycin: log KH1 = 11.94 +/- 0.02 and log KH2 = 6.80 +/- 0.03. Comparison of these values with those for model compounds indicates that KH1 and KH2 refer to equilibria involving the beta-diketone and carboxylic acid moieties, respectively. Titrations of ionomycin with metal ion at fixed values of pH produced changes in the UV-visual absorbance spectra which were analyzed to give conditional complex formation constants, KMI'. The pH dependence of the values of KMI' indicated that 1:1 divalent metal ion-ionomycin (MI) complexes and protonated MHI+ complexes were formed in the pH range studied. The values of log KMI ranged from 5.30 +/- 0.11 for Sr2+ to 10.25 +/- 0.03 for Ni2+. The selectivity pattern and relative affinities (in parentheses) for the formation of the species MI are as follows: Ni2+ (2000) greater than Zn2+ (600) greater than CO2+ (440) greater than Mn2+ (47) greater than Mg2+ (1.00) greater than Ca2+ (0.21) greater than Sr2+ (0.022). Logarithmic values of KMHI, for the reaction MI + H+ in equilibrium MHI+, ranged from 5.9 (Ni2+) to 8.4 (Sr2+). Calculations using the values of the equilibrium constants determined indicate that an appreciable fraction of the complexed ionophore exists as the protonated complex, MHI+, in the pH range of 6.5-8.5.