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 Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Pb(2+), as well as that for the formation of PbL2(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) NaClO(4) at 298.1 +/- 0.1 K. The enthalpy changes for the formation of Cu(2+) 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)(-), P(2)O(7)(4)(-), and [Co(CN)(6)](3)(-) by Land L1 were studied by means of pH-metric titrations in 0.15 mol dm(-)(3) NaClO(4) at 298.1 +/- 0.1 K. The catalytic reactions of ATP dephosphorylation induced by these ligands in solution were followed by (31)P 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.
Bencini
Andrea
A
Department of Inorganic Chemistry, University of Valencia, c/ Dr. Moliner 50, 46100 Burjassot (Valencia), Spain.
Bianchi
Antonio
A
Giorgi
Claudia
C
Paoletti
Piero
P
Valtancoli
Barbara
B
Fusi
Vieri
V
García-España
Enrique
E
Llinares
José M.
JM
Ramírez
José Antonio
JA