Step 1: Understanding the Concept:
The question is about methotrexate, which is built from folic acid through an N-methylation at the nitrogen linking the pteridine and para-aminobenzoic acid parts, and asks which group is the isostere involved in that step.
Step 2: Key Formula or Approach:
Bioisosterism means substituting one atom or group with another of similar size or electronic character to change a drug's properties while keeping its basic shape and activity. The classical isosteric replacements include exchanging a hydrogen atom for a small group like methyl.
Step 3: Detailed Explanation:
Folic acid has a free N-H at the nitrogen bridging its pteridine ring and the para-aminobenzoic acid unit.
In methotrexate, this hydrogen is replaced with a methyl group, giving the N-methyl substitution that distinguishes methotrexate from folic acid.
Since methyl is used here as a classical isostere of hydrogen, close enough in size not to disturb the molecule's overall fit into the dihydrofolate reductase enzyme, this substitution both blocks metabolic breakdown at that nitrogen and strengthens the drug's binding compared to folic acid.
Groups like SH, NH2, and CF3 are isosteres used in other well known replacements elsewhere in medicinal chemistry, but none of them describes this specific N-methylation step.
Step 4: Final Answer:
The isosteric group involved is CH3.