Three-Phase Model of Micellar Reactions. Methylation of Thymidine by (Long-chain-alkyl)dimethylsulfonium Iodides

Article abstract of DOI:10.1021/ja00341a039

Micellar methylation of thymidine to 3-methylthymidine was studied by using mainly n-octadecyldimethylsulfonium iodide at pH 7-12 and 60-75 deg C.The reaction exhibited the presence of an induction period as long as 1 h, a bell-shaped pH dependence of the methylation rate with an optimum pH ca. 10, and competitive inhibitory effects of added salts on the micellar reaction, etc.A three-phase model was proposed to analyze the micellar events, assuming that (i) the anions (dT^-) of thymidine, the active species of the nucleoside, diffuse from the aqueous bulk phase to the Stern layer across the Gouy-Chapman layer, (ii) dT^- ions concentrate in the Stern layer to obey the Langmuir adsorption isotherm theory, and (iii) the dT^- ions in the layer are mediated in a bimolecular fashion with the dimethylsulfonium ions, the head groups of the micelles.The model furnished apparent kinetic constants such as binding constants of counterions (dT^-, I^-, OH^-, etc.) to the micelle, a diffusion rate constant of dT^- and Me-dT from the Stern layer.The effects of temperature on these constants were studied carefully to estimate the respective thermodynamic constants.The three-phase model was compared with other micellar models reported.The micellar methylation was discussed briefly in connection to the enzymatic methylation that utilizes S-adenosylmethionine as a cofactor.