Question

A mixture of the following dipeptides was applied to a cation exchange column. 1. Ala-Cys 2. His-Asp 3. Tyr-Glu 4. Leu-Arg 5. Val-Lys. Which dipeptide(s) would bind at pH 4.5?

a) 1 and 3
b) 2 and 4
c) 3 and 5
d) 1 and 4

Answer 1

At pH 4.5, the dipeptides His-Asp and Leu-Arg would bind to a cation exchange column due to the presence of basic amino acids histidine and arginine, which are positively charged at this pH level.

Step-by-step explanation:

The question asks which dipeptides from the list would bind to a cation exchange column at pH 4.5. To answer this question, we need to know the pKa values of the amino acids present in each dipeptide and how they behave at pH 4.5. Cation exchange columns typically bind positively charged molecules. Amino acids can be positively charged if their side chains or terminal amino groups have pKa values above the pH of the environment and are not yet deprotonated.

At pH 4.5, basic amino acids like arginine (Arg), lysine (Lys), and histidine (His) will be positively charged because their side chain pKa values are higher than 4.5. Hence they will bind to the cation exchange column.

Looking at the dipeptides given:

• Ala-Cys will not bind firmly because neither have a side chain with a pKa above 4.5
• His-Asp will bind due to the positively charged histidine at this pH.
• Tyr-Glu will not bind strongly as both are neutral or negatively charged at this pH.
• Leu-Arg will bind because of the positively charged arginine.
• Val-Lys will not bind as it is also positively charged.

Therefore, the dipeptides that would bind at pH 4.5 are His-Asp and Leu-Arg, which correspond to option b) 2 and 4.