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

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.