Secondary Deuterium Kinetic Isotope Effects in the Cleavage of Thiamin and N-Methylthiaminium Ion: First Evidence To Identify the Rate-Limiting Step

Article abstract of DOI:10.1021/jo00277a036

Thiamin and 1'-methylthiaminum ion deuterated at the 6'- and N-methylene positions were cleaved by sulfite ion in aqueous phosphate at 25 deg C.Observed secondary kinetic isotope effects (standard deviation of the ratio) were inverse for the 6'-position, 0.95 (0.09) and 0.95 (0.03), respectively, and normal for the N-methylene group., 1.08 (0.09) and 1.10 (0.03), respectively.These results confirm the multistep mechanism first proposed by us (J.Am.Chem.Soc. 1977, 99, 3134).They prove for the first time that sulfite ion adds to the 6'-position and that fragmentation of the CH2-thiazole bond in the resultant adduct contributes to the rate in the multistep mechanism.