Correct option is B
1. The peptide forms a dimer through disulfide bonds:
- Incorrect. The peptide MFTGPYCPRW includes cysteine (C), which has a thiol group (-SH) that can form disulfide bonds with another cysteine residue. However, the presence of 5 mM DTT (dithiothreitol) in the buffer plays a key role here. DTT is a reducing agent that breaks disulfide bonds by reducing the thiol groups, which prevents cysteine from forming disulfide linkages. Therefore, the peptide will not form a dimer through disulfide bonds in the presence of DTT.
2. The peptide has a net positive charge:
- Correct. The peptide contains arginine (R), an amino acid with a positively charged guanidinium group at pH 7.0. The other amino acids in the peptide (methionine, phenylalanine, threonine, glycine, proline, tyrosine, cysteine, tryptophan) are neutral at this pH. Therefore, the overall net charge of the peptide will be positive because of the positively charged arginine residue.
3. The peptide has a net negative charge:
- Incorrect. There are no negatively charged residues (such as aspartate or glutamate) in this peptide at pH 7.0. Arginine has a positive charge, and all other amino acids are neutral. Therefore, the peptide will not have a negative charge.
4. The peptide is neutral:
- Incorrect. Given that arginine is positively charged at pH 7.0, the peptide will not be neutral. It will have a net positive charge due to the guanidinium group of arginine.
Option 2 is Correct:
Peptide Sequence: MFTGPYCPRW consists of various amino acids, most of which are neutral at pH 7.0. However, arginine (R) is the only amino acid in this sequence that carries a positive charge at neutral pH. The guanidinium group in arginine remains positively charged at physiological pH (pH 7.0).
DTT (Dithiothreitol): This is a reducing agent used to break disulfide bonds. Since it is present in the buffer, it prevents the formation of dimers that might arise from cysteine residues connecting via disulfide bonds. Therefore, the peptide remains as individual monomers.
Net Charge: Since only arginine contributes a positive charge, the peptide has a net positive charge. This is the key reason why Option 2 is correct.
