Final answer:
Covalent bonds between R groups in amino acids occur through disulfide linkages found in cysteine. The correct answer is option 2: Cysteine and Methionine, but note that only cysteine can form disulfide bonds.
Step-by-step explanation:
The question is asking which pairs of amino acids can form covalent bonds between their R groups. The only amino acids mentioned that form covalent bonds with each other through their R groups are cysteine molecules. This is due to the disulfide linkages that form when the sulfur atoms of the sulfhydryl (SH) groups in cysteine side chains are brought together as the protein folds, and they oxidize and link together. This results in the formation of cystine, a dimer of two cysteine amino acids joined by a disulfide bond.
Therefore, the correct answer to which amino acids can form covalent bonds between R groups is option 2: Cysteine and Methionine. However, it is important to clarify that while cysteine can indeed form disulfide bonds, methionine cannot. Methionine's sulfur is involved in nonpolar, hydrophobic interactions but does not form disulfide bonds. Thus, the accurate pairing for this question should be cysteine and cysteine, since only cysteine residues can form these covalent disulfide bonds with one another.