Ternary Complexes in Solution: Intramolecular Stack Formation in Mixed Ligand Complexes Containing CuII, 2,2' - Bipyridyl, and the 5'-Monophosphate of Inosine (IMP), Guanosine (GMP) o

Abstract

Stability constants of mixed ligand Cu(bpy)(NMP) complexes (bpy = 2,2'-bipyridyl; NMp2e = 5'_IMP28, 5'-GMp2e, or 5'_AMP28) were determined by potentiometric pH titrations in aqueous solution at 1=O.lM (NaNO) and 25 C. These ternary Cu(bpy)(NMP) complexes are more stable than the corresponding Cu(bpy)(R-MP) complexes, where R_Mp28 represents a phosphate monoester with a group R that is unable to participate in any kind of interaction within the complexes as, e.g., D-ribose 5'-monophosphate. This increased stability is attributed, in agreement with previous results, to intramolecular stack formation within the Cu(bpy)(NMP) complexes between the purine residue of the NMPs and the aromatic rings of bpy. The formation degree of the Cu(bpy)(NMP) species with the intramolecular stacks increases in the series, IMP2? (72 ± 5%) < AMP2? (81 ± 4%) ? GMP2? (85 ± 3%). This order is different from that previously observed for the self-stacking tendency of nucleic base residues.