Ternary complexes in solution. Part 51. Intramolecular hydrophobic and stacking interactions in mixed ligand complexes containing Cu(II), 2,2′-bipyridyl or 1,10-phenanthroline, and a simple phosphate monoester, D-ribose 5′-monophosphate or a nucleoside 5′-monophosphate (CMP, UMP, TMP, TuMP) with a non-coordinating base residue

Abstract

Stability constants of mixed ligand Cu(Arm)(R-MP) complexes (where Arm = 1,10-phenanthroline (Phen) or 2,2′-bipyridyl (Bpy) and R-MP 2− = phosphate monoester) have been determined by potentiometric pH titrations in aqueous solution at I = 0.1 M (NaNO 3) and 25 °C. The phosphate monoesters employed were 4-nitrophenyl phosphate (NPhP 2−), phenyl phosphate (PhP 2−), n-butyl phosphate (BuP 2−), D-ribose 5′-monophosphate (RibMP 2−) and the nucleotides: cytidine 5′-monophosphate (CMP 2−), uridine 5′-monophosphate (UMP 2−), thymidine 5′-monophosphate (TMP 2−) and tubercidin 5′-monophosphate (TuMP 2− = 7-deazaadenosine 5′-monophosphate). The ternary Cu(Arm)(R-MP) complexes containing a phosphate monoester with an aliphatic or aromatic residue are significantly more stable than the corresponding Cu(Arm)(RibMP) complexes. This increased stability is attributed to intramolecular hydrophobic or stacking interactions between part of the residues R of R-MP 2− and the aromatic rings of Bpy or Phen. The stability of the Cu(Arm)(RibMP) complexes is used as a basis for a quantitative evaluation of the situation in the other Cu(Arm)(R-MP) complexes. The formation degree of the species with the intramolecular ligand-ligand adduct increases for the Cu(Arm)(R-MP) complexes in the series: BuP 2− < PhP 2− ⋍ NPhP 2−; the formation degree for R-MP 2− ligands with a six-membered aromatic-ring system, i.e. PhP 2−, NPhP 2−, CMP 2−, UMP 2− and TMP 2−, is rather similar. The tendency of the nucleic base residues to form intramolecular stacks in the Cu(Arm)(R-MP) complexes follows the order: uracil ≲ cytosine ≲ thymine ⪡ 7-deazaadenine; this series reflects approximately the hydrophobic properties of the base residues and the size of the aromatic-ring systems. The relevance of the results with regard to bio-systems is shortly indicated.