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 H-1 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 pK(a) of N1-H decreased to 3.7 upon coordination in guanosine and 5'-GMP complexes, which is significantly lower than that of similar to9.3 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 similar to3, H-1 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)(mu-N1,N7-Guo](4)(4+) and [Pd(diamine)(mu-N1,N7-5'-GMP)](4)H-x((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 H-1 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. (C) 2003 Elsevier B.V. All rights reserved.