The binding of trialkyltin(IV) moieties to rat haemoglobin, and the structure of model systems, studied by tin-119 Mössbauer spectroscopy

Abstract

The configuration of the tin environment in binding sites of microcrystalline SnR₃–rat haemoglobin complexes (R = Me, Et, or Buⁿ), as well as in model systems consisting of Sn^IVR₃ moieties in aqueous media and in organic solutions (mimicking hydrophilic and hydrophobic zones of the globin respectively) has been investigated by ¹¹⁹Sn Mössbauer spectroscopy, through the point-charge model rationalization of the experimental nuclear quadrupole splitting, ΔE, measured at 77.3 K, and eventually by Mössbauer–Zeeman spectra taken at 4.2 K in a transverse magnetic field of 6 T. A distorted trigonal-bipyramidal (tbp) structure SnR₃(S_thiol)( N_het) has been inferred for the haemoglobin complexes, where the ligand atoms (axial) would come from a cysteine and histidine side chain respectively, according to previous biochemical work. Moreover, the occurrence of a strong Sn–S bond (with a C–Sn–S angle consistently larger than 90°), as well as of a weak N → Sn co-ordinative interaction, has been proposed. The aqueous model systems [Sn^IVMe₃ and Sn^IVEt₃ derivatives at pH 7.40, eventually in presence of N-(2- hydroxyethyl)piperazine-N′-ethane-2- sulphonic acid (Hepes) buffer, which were reacted with cysteine] have evidenced the formation of quasi-regular ternary tbp complexes SnR₃(S_thiol)(N_am)(the ligand atoms being axially located, coming from cysteine and Hepes respectively) where the N →Sn interaction is consistently lower than Sn–S. Slow-rate reactivities in aqueous solutions of SnR₃ cysteinates are also reported and commented upon. Essentially regular tbp species Sn^IVR₃(SPh)(N_het)(N_het from 1 -methylimidazole; R = Me, Buⁿ, or Ph) have been identified in benzene solution. The distortion of the tbp binding site in the haemoglobin complexes has then been assumed to be peculiar to the SnR₃-globin system. Comments based on crystalline derivatives with Sn–S bonds have been accordingly reported.