The formidable-looking molecule 5-methyltetrahydrofolic acid (abbreviated 5-methyl-FH4) is the product of sequences of biological reactions that convert carbon atoms from a variety of simple molecules, such as formic acid and the amino acid histidine, into methyl groups.

The simplest synthesis of 5-methyltetrahydrofolic acid is from tetrahydrofolic acid (FH4) and trimethylsulfonium ion, a reaction carried out by microorganisms in the soil.

(a) Can this reaction be reasonably assumed to proceed through a nucleophilic substitution mechanism? Write the mechanism, using the "electron-pushing" arrow notation.
(b) Identify the nucleophile, the nucleophilic and electrophilic atoms participating in the reaction, and the leaving group.
(c) On the basis of the concepts presented in Sections 6-7, 6-8, 9-2, and 9-9, are all the groups that you identified in (b) behaving in a reasonable way in this reaction? Does it help to know that species such as H3S+ are very strong acids (e.g., pKα of CH3SH2+ is -7)?

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OH r steps Formic acid NH2 CH CHCOH HN NCH NH CNHCHCH CH2COH NH Seven steps H2 5-Methyltetrahydrofolic acid (5-Methyl-FH) Histidine CH CH3-S-CH+H CH3 5-Methyl-FH FH4 Trimethylsulfonium ion