Abstract
The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Ni2+, Zn2+, or Cd2+ and AMPS2-, i.e., of the M(AMPS) complexes, were determined by potentiometric pH titrations (25 °C; I = 0.1 M, NaNO3). For the Mn2+/AMPS, Co2+/AMPS, Ni2+/AMPS, and Cd2+/AMPS systems also the protonated species M(H;AMPS)+ were quantified, and for the Zn2+/AMPS system, the stability of the hydroxo species Zn(AMPS)(OH)-, which results from the Zn2+−thio coordination, could be determined. On the basis of previously established log versus p straight-line plots (R-MP2- = simple monophosphate ester ligands without further coordinating groups; Sigel, H.; et al. Helv. Chim. Acta 1992, 75, 2634), it is concluded that the alkaline earth ions in the M(AMPS) complexes are coordinated to the thiophosphate group with the same intensity as to a normal phosphate group. For the M(AMPS) complexes of Mn2+, Co2+, Ni2+, Zn2+, and Cd2+, it is shown by comparison with the corresponding M(AMP) complexes and by employing the mentioned straight-line plots that the stability increase is larger than may be expected due to macrochelate formation, which means that the metal ions also bind to the sulfur atom of the thiophosphate group. The stability increases amount for Mn(AMPS), Zn(AMPS), and Cd(AMPS) to about 0.2, 0.7, and 2.4 log units, respectively, and the estimated approximate percentages of the sulfur-coordinated species are about 30, 80, and 100%, respectively. Furthermore, comparisons between these stability increases and the solubility products for the corresponding metal ion sulfides, MIIS, as well as with the stability increases due to the M2+−thioether interaction observed for the complexes of tetrahydrothiophene-2-carboxylate, which also result in straight-line plots, further support the conclusions about metal ion−sulfur binding in the mentioned M(AMPS) complexes. The indicated correlations allow also an estimate for the extent of the M2+−sulfur interaction in Pb(AMPS) and Cu(AMPS). The various isomers of the M(H;AMPS)+ species are analyzed in a microconstant scheme, and estimations about their formation degrees are presented; for example, for the Cd2+ system, (H·AMPS·Cd)+ is the dominating isomer, which has the proton at N1 and Cd2+ at the thiophosphate group. It is evident that for metal ions like (Mn2+), Zn2+, or Cd2+ the metal ion binding properties of the parent compound AMP2- and its thio analogue AMPS2- differ considerably, and therefore, great care should be exercised in enzymatic studies where AMPS2- is employed as a probe for AMP2- in the presence of metal ions. Regarding studies of ribozymes, it is of interest that plots are presented (pseudo-first-order rate constants versus complex stabilities) which suggest that on top of a sulfur−metal ion interaction during the transition state of the rate-determining step of the hydrolytic cleavage of an oligonucleotide containing a bridged internucleotide 5‘-phosphorothioate RNA linkage also an oxygen−metal ion interaction occurs and that the two effects are “additive”.
Sigel, Roland K O