3. The amino acid methionine is characterized by __________side chain

Question

BlackTom AI · Accepted Answer

Methionine has a nonpolar, hydrophobic side chain because its R group is a thioether (–CH2–CH2–S–CH3) composed mainly of carbon–hydrogen bonds and a sulfur atom that does not ionize or form strong hydrogen bonds.

1) Relevant knowledge points and concepts
- Amino acids are classified by the chemical properties of their side chains (R groups): nonpolar (hydrophobic), polar uncharged, polar acidic (negatively charged at physiological pH), or polar basic (positively charged at physiological pH).
- Nonpolar side chains are rich in C–H bonds, lack ionizable groups, and do not form strong hydrogen bonds with water.
- Methionine’s side chain is a thioether (a sulfur bound to carbons), not a charged or strongly polar functional group.

2) Step-by-step reasoning
- Inspect the R group: methionine’s R group is –CH2–CH2–S–CH3.
- Assess polarity: the R group contains mainly C–H bonds and a sulfur that is relatively nonpolar and not strongly electronegative enough to make the group polar or to ionize.
- Assess ionizability: there is no carboxylate (–COOH/–COO–) or amino group in the side chain that could carry a charge at physiological pH.
- Conclude classification: an uncharged, hydrocarbon-like chain with a thioether behaves hydrophobically and is classified nonpolar.

3) Calculations
- No numeric calculations are required; structural/functional analysis suffices.

4) Why the correct answer is correct
- Because methionine’s side chain lacks ionizable or strongly polar groups and is dominated by nonpolar C–H bonds and a thioether linkage, it does not interact favorably with water and is therefore nonpolar (hydrophobic).

5) Why the other options are incorrect
- B (polar acidic): would require a side-chain carboxyl group (e.g., aspartate, glutamate) — methionine lacks this.
- C (polar uncharged): would require polar groups like –OH, –CONH2, or –SH that can hydrogen bond (e.g., serine, asparagine) — methionine’s thioether is not strongly hydrogen-bonding.
- D (polar basic): would require an amino group that is protonated at physiological pH (e.g., lysine, arginine) — methionine lacks this.

6) Conclusion
- The correct classification is A. nonpolar (hydrophobic), which matches the provided answer.

3. The amino acid methionine is characterized by __________side chain单项选择题

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