Methylation and ethylation of uridylic acid and thymidylic acid. Reactivity of the ring and phosphate as a function of pH and alkyl group.

At pH 6.8 in aqueous solution (4 hr, 22 degrees), all methylating agents tested, i.e., dimethyl sulfate, methyl methanesulfonate, and methylnitrosourea, react with both the N-3 of the ring and the phosphate of UMP and dTMP. Although the extent of reaction varies from 17 to 76%, the ratio of phosphate/ring methylation is approximately 4. Both the 3-methyl nucleotides and methyl ester of 3-methyl nucleotides are identified, as well as the methyl esters of unmodified UMP and dTMP. At pH 8.2 the extent of total methylation is similar but reactivity of the N-3 is increased and that of the phosphate decreased so that the phosphate/ring ratio is approximately 1. At pH 6 almost all reaction is with the phosphate group. Uridine, under the same conditions, is methylated at pH 6.8 to form 15% 3-methyluridine and, at pH 8.2, the N-3 of uridine and thymidine is methylated to about 50%. Neither uridine nor UMP forms detectable ribose methyl products at any of these pH's. The comparable ethylating agents (diethyl sulfate, ethyl methanesulfonate, and ethylnitrosourea) are less reactive and the total ethylation of UMP or dTMP is about 1/5 that of methylation. There is little ethylation of the N-3 but the phosphate is alkylated to a relatively high extent so that the phosphate/base ratio at pH 6.8 is 10-23, and at pH 8.2 the ratio is 5-8. The fact that ethylating agents have a greater affinity than methylating agents for alkylating phosphates is proposed as the basis for the previously reported analytical data in which ethylating agents, acting on DNA or RNA at neutrality, form more phosphotriesters than the analogous methylating agents.