| Abstract: | Magnetic resonance techniques have been applied to study the stability of the complexes formed between Mn(II) ions and NADP in aqueous solutions at a pH of 7.5 and 20 degrees C. The electron paramagnetic resonance (epr) data indicate that at low Mn(II) ion concentrations ([Mn(II)] less than 1 mM; [NADP] approximately 5 mM), a 1:1 complex is formed with an apparent stability constant K1 = 370 +/- 50 M-1 at an ionic strength of 0.22 in the presence of 0.20 M Cl-. At high Mn(II) ion concentrations, a Mn(II)2-NADP species, with an apparent stability constant K2 = 54 +/- 17 M-1, is present in significant amounts. When the epr data are corrected for the presence of the MnCl+ ion, the analysis of the new Scatchard plot yields stability constants for the two sites of K1 = 640 +/- 90 M-1 and K2 = 88 +/- 13 M-1, respectively. The presence of two metal ion binding sites on the NADP molecule has not been observed previously, and previous workers have always analyzed their data in terms of the 1:1 Mn(II)-NADP complex. An epr temperature study of K1 yields a value of delta H equal to 1.3 +/- 0.2 kcal/mol (1 cal = 4.187 J). |
