Infrared and Raman spectra of 2-(α-hydroxybenzyl)thiamine, 2-(α-hydroxycyclohexylmethyl)thiamine, their protonated forms and their complexes with zinc(II), cadmium(II) and mercury(II)

Silver-Induced Enhancement of Thiochrome-Based Peroxide Measurements

Thiamine is presently one of the most attractive substrates used for sensitive fluorometric measurements of peroxides. Thiochrome (TC), a highly fluorescent product, is formed in enzyme-mediated oxidations. It is assumed that H(2)O(2) is nearly quantitatively converted to TC. The reaction cannot differentiate H(2)O(2) from many other peroxides such as methylhydroperoxide (MHP); to perform differential measurements, H(2)O(2) can first be selectively destroyed by a suitable catalyst such as MnO(2). In substituting Ag(2)O for MnO(2) to accomplish the selective destruction of H(2)O(2), we achieved the stated objective but were puzzled by a 3-fold increase in the MHP response in the presence of Ag(2)O. It was soon discovered that traces of dissolved Ag(+) and Hg(2+) can dramatically increase the yield of TC in this reaction from either H(2)O(2) or MHP; the normal yield in fact is only 20%. We present here a reaction scheme and kinetic model that adequately describes this behavior and should provide a path to substantially increase the sensitivity of this important assay method.

On the mechanism of action of thiamin enzymes in the presence of bivalent metal ions

Results on the interactions between the bivalent metal ions Zn2+, Cd2+, Hg2+, Co2+, Ni2+ and 'active aldehyde' thiamin derivatives are reviewed. The techniques used in these studies include spectroscopic methods, i.e., IR-Raman, UV-Vis, multidimensional and multinuclear NMR in solution and in solid state, and X-ray crystal structure determinations. More recently, potentiometric studies on thiamin pyrophosphate and 2-(alpha-hydroxyethyl)thiamin in combination with NMR and EPR techniques were also undertaken. All these studies lead to useful conclusions on the mechanism of action of thiamin enzymes in the presence of bivalent metal ions.

Complexes of Zn(2+), Cd(2+), and Hg(2+) with 2-(alpha-Hydroxybenzyl)thiamine Monophosphate Chloride

The binding sites of Zn(2+), Cd(2+), and Hg(2+) in complexes with 2-(alpha-hydroxybenzyl)thiamine monophosphate chloride, (LH)(+)Cl(-), have been investigated in the solid state [2-(alpha-hydroxybenzyl)thiamin monophosphate chloride monoprotonated at the phosphate group and protonated at N(1)' is denoted as (LH)(+)Cl(-); therefore, the ligand monoprotonated at the phosphate group and deprotonated at N(1)' is L]. Complexes of formulae MLCl(2), M(LH)Cl(3), and (MCl(4))(2)(-)(LH)(2)(+) (M = Zn(2+), Cd(2+), and Hg(2+)) were isolated in aqueous and methanolic solutions, depending on pH. The crystal structure of the complex of formula HgL(2)Cl(2) was solved, together with that of the free ligand (LH)(+)Cl(-), by X-ray crystallography. HgL(2)Cl(2) crystallizes in C2/c, with a = 32.968(6) Å, b = 7.477(2) Å, c = 21.471(4) Å, beta = 118.19(1) degrees, V = 4665(2) Å(3), and Z = 4. (LH)(+)Cl(-) crystallizes in Cc, with a = 10.951(3) Å, b = 17.579(4) Å, c = 13.373(3) Å, beta = 105.36(2) degrees, V = 2482.4(10) Å(3), and Z = 4. Mercury(II) binds to the N(1') of the pyrimidine ring. Both ligands are in the S conformation [Phi(T) = -98.1(9) degrees and Phi(P) = 176.1(10) degrees for HgL(2)Cl(2) and Phi(T) = 104.1(5) degrees and Phi(P) = 171.9(6) degrees for (LH)(+)Cl(-)]. (31)P and (13)C NMR spectra, together with vibrational spectra (IR/Raman), are used to deduce the binding sites of the metal and the protonation states of the ligand at various pH values. It is found that solid-state (31)P NMR spectroscopy is particularly useful in characterizing these complexes as the (31)P shielding tensors are sensitive to the state of the phosphate group. On the other hand, the (31)P NMR spectra indicate that direct bonding between Zn(2+) and Cd(2+) to the phosphate can occur under certain preparation conditions. Solid-state (13)C NMR and vibrational (IR/Raman) spectroscopic results are also in agreement with the other techniques.