Effect of nitrogen methylation on cation and anion coordination by hexa- and heptaazamacrocycles. Catalytic properties of these ligands in ATP dephosphorylation

The stability constants of the complexes formed by 1,10-dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane (L) and 1,4,7-trimethyl-1,4,7,10,13,16,19-heptaazacyclohenicosane (L1) with Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+, as well as that for the formation of PbL(2+)?+ (L2 = 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane), were determined by means of potentiometric (pH-metric) titrations in 0.15 mol dm(-3) NaClO4 at 298.1 +/- 0.1 K. The enthalpy changes for the formation of Cu2+ complexes with L and L1 were measured by means of microcalorimetry. These thermodynamic data were compared with those previously reported for L2, 1,4,7,10,13,16-hexaazacyclooctadecane (L3), and 1,4,7,10,13,16,19-heptaazacyclohenicosane (L4) evidencing that nitrogen methylation can produce lower or higher complex stability depending on the metal ion and the number of methylated nitrogens. The equilibria of complexation of ATP(4-), ADP(3-), AMP(2-), P2O74-, and [Co(CN)(6)](3-) by L and L1 were studied by means of pH-metric titrations in 0.15 mol dm(-3) NaClO4 at 298.1 +/- 0.1 K. The catalytic reactions of ATP dephosphorylation induced by these ligands in solution were followed by P-31 NMR spectroscopy at different temperature and pH values. L is the most appropriate receptor, among L-L4, in the recognition of the nucleotide. The catalytic efficiency of hexa- and heptaazaligands increases in the order L < L3 < L2 and L1 < L4, respectively, L4 being the most efficient. Namely, di- and tetramethylation of L3 produces opposite effects on its catalytic properties.