The complex [SnMe2(HTDP)(H2O]Cl.H2O, synthesized by reaction between dimethyltin(IV) dichloride and thiamine diphosphate hydrochloride (H3TDPCl) in water, was characterized by X-ray diffractometry and IR and Raman spectroscopy in the solid state, and by electrospray mass spectrometry (ESMS) and NMR spectroscopy (H-1, C-13, P-31, Sn-119 and inverse-detection H-1,N-15 HMBC) in aqueous solution. In the solid state the HTDP- anion chelates the metal via one oxygen atom of each phosphate group [Sn-O = 2.062(3), 2.292(3) Angstrom], and another oxygen atom belonging to the terminal phosphate links the SnMe22+ cations into chains. The tin atom has distorted octahedral coordination involving the trans methyl groups, the above-mentioned diphosphate oxygen atoms, and the oxygen atom of the coordinated water molecule. The thiamine moiety has F conformation. NMR studies suggest that the interaction between the organometallic cation and the HTDP- ligand persists in D2O solution, which is in keeping with the ESMS spectrum showing a peak corresponding to [SnMe2(HTDP)]. Both in the solid state and in solution, the acidic HTDP- proton in the complex is located on the N(1') atom of the pyrimidine ring. The enzymatic behavior of native pyruvate Decarboxylase (EC 4.1.1.1, PDC), obtained from baker's yeast, was compared in a coupled assay with that shown by the "SnMe2-holoenzyme" created by incubation of apoPDC with [SnMe2(HTDP)(H2O)]-Cl.H2O. The SnMe2-holoenzyme exhibited about 34% of the activity of the native enzyme (with a Michaelis-Menten constant of 2.7 muM, as against 6.4 muM for native PDC), so confirming the very low specificity of PDC regarding the identity of its metal ion cofactor. In view of the observed protonation of N(l '), it is suggested that the role of divalent cations in the mechanism of thiamine-diphosphate-dependent enzymes may be not only to anchor the cofactor in its binding site but also to shift the acidic proton of HTDP- from the diphosphate group to N(1'); protonation of N(1') is widely believed to be important for enzyme function, but the origin of the proton has never been clarified.