Equilibrium studies on the protonation and Cu(II) complexation by an hexaaza macrocycle containing p -xylyl spacers. The crystal structure of the hexaprotonated ligand and the kinetics of decomposition of the Cu(II) complexes
Abstract The protonation constants of the ligand 3,6,9,16,19,22-hexaazatricyclo[22.2.2.211,14]triaconta-1(26),11(12),13,24,27, 29-hexaene (L, BPXD) have been determined in water at 25°C by potentiometric procedures and indicate a lower basicity than that corresponding to the analogue ligand with m-xylyl spacers between the diethylenetriamine subunits (BMXD). The crystal structure of a salt of H6L6+ containing Br− and NO3 − as anions has been solved by X-ray diffraction procedures and reveals that the macrocycle adopts an almost planar configuration. The protonated amine groups are involved in a complex network of hydrogen bonds with the anions and water molecules, with two anions being placed close to the cavity of the macrocycle. Despite the lower basicity of BPXD, the stability of the mononuclear CuL2+ and HCuL3+ complexes is several log units higher than that corresponding to the analogous BMXD complexes, which suggests the possibility of a different coordination mode for both closely related ligands. Actually, the whole set of stability constants for the mono and binuclear Cu(II)–L complexes can be rationalised by considering that the macrocycle acts as tetradentate in the mononuclear species. In contrast, the kinetic parameters for decomposition of the mono and binuclear Cu(II)–L complexes in acid solutions are only slightly different from those previously determined for the BMXD complexes.
Stability and kinetics of decomposition of binuclear Cu(II) complexes with a symmetrical hexaaza macrocycle: the effect of SCN− as ancillary ligand
Abstract The effect of added KBr and KSCN on the stability constants of the mono and binuclear Cu(II) complexes with a symmetrical hexaazamacrocycle L has been examined in 0.1 M KNO 3 . The presence of these salts does not cause any change in the ligand protonation constants, which indicates that, in the presence of 0.1 M KNO 3 , there is not preferential interaction of the Br − and SCN − anions with the highly protonated forms of the ligand. No ternary CuLBr complexes are detected in the potentiometric study of the equilibrium, but several mono and binuclear CuLSCN complexes are formed at significant amounts and their stabilities are reported. The kinetics of decomposition of the binuclear CuL and CuLSCN complexes upon addition of an excess of acid has been also measured. The results obtained for the CuL complexes agree well with those previously reported in 1.0 M KNO 3 , and they indicate that the release of both Cu(II) ions is statistically controlled. The existence of some differences between the kinetic data corresponding to decomposition of solutions at different starting pH is interpreted in terms of parallel decomposition of the binuclear Cu 2 L 4+ , Cu 2 L(OH) 3+ and Cu 2 L(OH) 2 2+ complexes, the kinetic parameters for the three complexes being slightly different. This interpretation is also supported by the kinetics of decomposition of the CuLSCN − complexes that also reveals differences between the several complexes in solution. If the present data are interpreted in terms of the classical mechanism for decomposition of Cu(II)-polyamine complexes, they suggest that the nature of the ancillary ligands does not cause large changes in the lability of the CuN bonds but it largely affects to the relative rates of attack by H + and water.
New method for analysis of enzymatic reactions: Thermolysin-catalyzed synthesis of aspartame precursor
Aspartame is an artificial sweetener which is 200 times sweeter than sucrose. A precursor of aspartame, N-(benzyloxycarbonyl)-l-aspartyl-l-phenylalanine methyl ester (Z-APM) was synthesized and hydrolyzed with thermolysin in an aqueous monophasic system. Graphical plotting methods and/or nonlinear regression have been used to analyze enzymatic reactions because of the nonlinearity of enzymatic rate equations. There are, however, problems: the former require accurate measurement of the initial reaction rate and the latter gives no analytical information. The authors proposed a method to solve the nonlinear enzymatic rate equations with respect to the concentration of a product with the Taylor expansion, especially for the reversible Theorell-Chance mechanism (Murakami, Y., Hirata, M., and Hirata, A., J. Ferment. Bioeng., 82, 246–252, 1996). This solution, however, is complex for practical use. In this paper, a simpler solution to explain the thermolysin-catalyzed synthesis of Z-APM is obtained. New methods to determine the kinetic parameters, especially for the reversible Theorell-Chance mechanism, based on the approximated solution are proposed. The results calculated using the obtained kinetic parameters can satisfactorily explain the experimental results, which demonstrates the effectiveness of the proposed analysis. We must know the relation between the concentrations of substrates and a product, and accurately evaluate of the result obtained using Taylor expansion when we determine the kinetic parameters using our newly developed methods. The methods to determine the kinetic parameters of the reversible Theorell-Chance mechanism described in this paper is, however, expected to be applied to general enzymatic reactions.