Interaction of vitamin B12a with 8-azaguanine and 6-mercaptopurine: kinetic and thermodynamic characterizations

Abstract

Rate constants for the formation, k₁^app., and decomposition, k_–1^app. of 6-mercaptopurine and 8-azaguanine adducts of vitamin B_12a, B₁₂–mpur and B₁₂–agua, and hence their stability constants K^app.=k₁^app./k_–1^app., have been determined in aqueous buffered solutions as a function of pH at 25.0 °C. The pH–rate profiles for the formation of B₁₂–mpur and B₁₂–agua are bell shaped with maxima at pH 7.5 and 7.0, respectively. Rate constants for the decomposition of B₁₂–mpur decrease curvilinearly with increasing pH, having a short plateau in the pH 7–9 region. k_–1^app. Values for the decomposition of B₁₂–agua do not change between pH 6 and 8, but they increase exponentially with increasing hydrogen-ion concentration at pH <6. Kinetic treatment of the data in terms of dissociation constants for vitamin B_12a, the ligand, and the vitamin B₁₂ complexes, and in terms of the reactivities of these species, affords pH-independent rate constants for the formation, k₁, and for the decomposition, k_–1, of these vitamin B₁₂ complexes. k₁ and k_–1 values for B₁₂–mpur are 800 dm³ mol^–1 s^–1, and those for B₁₂–agua are 220 dm³ mol^–1 s^–1 and 2.0 × 10^–2 s^–1, respectively. The mechanism of these reactions and their pharmaceutical potential are discussed.