Binding of palladium(II) complexes to guanine, guanosine or guanosine 5′-monophosphate in aqueous solution: potentiometric and NMR studies

The interaction of guanine, guanosine or 5′-GMP (guanosine 5′-monophosphate) with [Pd(en)(H2O)2](NO3)2 and [Pd(dapol)(H2O)2](NO3)2, where en is ethylenediamine and dapol is 2-hydroxy-1,3-propanediamine, were studied by UV–Vis, pH titration and 1H NMR. The pH titration data show that both N1 and N7 can coordinate to [Pd(en)(H2O)2]2+ or [Pd(dapol)(H2O)2]2+. The pKa of N1–H decreased to 3.7 upon coordination in guanosine and 5′-GMP complexes, which is significantly lower than that of ∼9.3 in the free ligand. In strongly acidic solution where N1–H is still protonated, only N7 coordinates to the metal ion, but as the pH increases to pH ∼3, 1H NMR shows that both N7-only and N1-only coordinated species exist. At pH 4–5, both N1-only and N1,N7-bridged coordination to Pd(II) complexes are found for guanosine and 5′-GMP. The latter form cyclic tetrameric complexes, [Pd(diamine)(μ-N1,N7-Guo]44+ and [Pd(diamine)(μ-N1,N7-5′-GMP)]4Hx(4−x)−, (x=2,1, or 0) with either [Pd(en)(H2O)2](NO3)2 or [Pd(dapol)(H2O)2](NO3)2. The pH titration data and 1H NMR data agree well with the exception that the species distribution diagrams show the initial formation of the N1-only and N1,N7-bridged complexes to occur at somewhat higher pH than do the NMR data. This is due to a concentration difference in the two sets of data.