Thiamine pyrophosphate-metal ion complexation: equilibrium and kinetics

Abstract

Abstract 1. 1. Acid dissociation constants of thiamine pyrophosphate have been determined from pH titration data at 25°C in 0.1 M KNO3. 2. 2. Stability constants of the metal ion-thiamine pyrophosphate complexes MTPP and MHTPP+, where M is Mg2+, Mn2+, Cu2+, Ni2+, Cu2+, or Zn2+, have been determined from pH titration data at 25°C in 0.1 M tetramethylammonium bromide utilizing a Newton-Raphson algorithm to solve simultaneous mass balance equations and a reiterative nonlinear least-squares approach to compute the constants. The stabilities of these complexes do not follow the Irving-Williams order, but instead are typical of the order generally found with phosphate ligands. 3. 3. The kinetics of complexation of Ni2+ and Mg2+ with thiamine pyrophosphate have been studied using the temperature-jump method. The concentration dependence of the relaxation times observed is not consistent with the common assumption that the concentration of the intermediate ion-pair fulfills the steady-state conditions, but is consistent with the more general expression of the two-step mechanism of complex formation. For the reaction, M(H 2 O ) 2+ n + L 2− ( (aq.) ) ⇌ K 12 M(H 2 O) n , L ⇌ K 32 K 23 M(H 2 O) n−x L +x H 2 O , at 25°C and an ionic strength of 0.1 M (tetramethylammonium bromide), the results are: M = Ni, k23 = 4.2·103 (± 6%) s−1, k32 = 250 s−1, K12 = 240 M−1; M = Mg, k23 = 7.5·104 (± 12%)s−1, k32 = 2400 s−1, K12 = 87 M−1; with the probable experimental errors being three times the reported standard deviations. 4. 4. These results are viewed as being in agreement with rate-determining loss of a water molecule from the inner coordination sphere of the metal ion. Nevertheless, for those cases in which the rate of water dissociation from the intermediate ion-pair is known experimentally, this rate invariably is slightly less than the rate of solvent exchange. This effect is examined and its implications are discussed. 5. 5. For systems where calculation of the fraction of ion-pair using experimentally determined values is possible, it can be concluded that the greater the charge at the binding site, the more predominant are the inner sphere complexes.