Solution versus solid state conformation of Group 12 metal complexes of active aldehyde derivatives of thiamine

Abstract

Molar conductances in water and dimethylformamide and ¹³C NMR spectra in D₂O and (CD₃)₂SO solutions and solid-state ¹³C cross polarization magic angle spinning NMR spectra have been measured for the chloride salts of 2-(α-hydroxybenzyl)thiamine (hbt), 2-(α-hydroxycyclohexylmethyl)thiamine (hcmt), their protonated forms [hbt]Cl·HCl and [hcmt]Cl·HCl and zwitterionic complexes of the type [M(hbt)Cl₃] and [M(hcmt)Cl₃](M = Zn, Cd or Hg). The data affords good evidence for bonding of the metals to the N(1′) atom of the pyrimidine moieties in the ligands, as well as of the S conformation of the ligands in solution. The small chemical shifts observed for the carbon atoms situated adjacent to the N(1′) site [C(2′), C(6′) and 2′-CH₃] in the solid-state and solution ¹³C NMR spectra of the complexes are attributed to the synergic interaction of metal complexation which causes a downfield shift and the accumulation of negative charge at the same position due to [MCl₃]^– which leads to an opposing upfield shift. The ¹⁹⁹Hg NMR spectra of [Hg(hbt)Cl₃] and [Hg(hcmt)Cl₃] in (CD₃)₂SO further substantiate these conclusions. Overall, the results strongly support earlier suggestions for the role of metal ions in the enzymatic action of thiamine.