Binding sites and stabilities of transition metal ions with nucleosides and related ligands

Abstract

Stability constant logarithms for Ni 2+, Cu 2+, and Zn 2+ binding at pyridine or purine N1 type nitrogens and imidazole or purine N7 type nitrogens display a linear relationship with pK a for each metal ion and nitrogen type. The slopes of all lines vary only from 0.3 to 0.5. For all three aqueous metal ions and dienPd 2+, at the same pK a, the stability constant for N7 binding is 0.8 to 1.2 log units stronger than for N1 binding. For neutral adenosine the N1 site is intrinsically 320 times more basic than the N7 site. However, for the above three aqueous metal ions the ratio of N1 to N7 bound adenosine complexes is projected to be 3, 2.5, and 1, respectively. Thus solutions of neutral adenosine and these aqueous metal ions contain comparable amounts of N1 metalated and N7 metalated complexes. Intrinsic protonation and metal ion stability constants are compared for adenosine, guanosine and inosine. N7 coordination in purine bases predominates at low pH and gives way to favored N1 coordination for dienPd 2+, Cu 2+, and Zn 2+ between pH 1.5 to 2.7 for adenosine, pH 6.1 to 6.7 for inosine, and pH 6.9 to 7.5 for guanosine. Absolute chemical shifts for both tetramethylammonium ion and acetonitrile are found to be dependent on ionic strength in opposite senses. There seems to be no reliable internal chemical shift reference when ionic strength is varied.