Hello! The answers I have are not correct and I\'m not sure why! Please explain

Question

Hello! The answers I have are not correct and I'm not sure why! Please explain and correct. Thank you!

Peptides can be separated using an ion-exchange column based on their isoelectric (pl) values. At which pH values would two different peptides, one with a pl of 5.4 and the other with a pl of 8.6, bind to a cation- and anion-exchange column? Each peptide may be capable of binding to each column at more than one pH value. Peptide A Peptide B pl 5.4 pl 8.6 anion-exchange column at cation-exchange column at pH 3.6 pH 3.6 Peptide A pl 5.4 pH 7.2 pH 7.2 Peptide A Peptide B pl 5.4 pl 8.6 pH 10.8 pH 10.8

Explanation / Answer

Thumb rules:

1) A peptide at a pH corresponding to its pI exist as a zwitter ion and has no net charge and will hence bind to no charged moeity.

2) At a pH lower than its pI, its -NH2 and -COO- group would have accepted a -H from the highly acidic solution resulting in a positive charge.

3) At a higher pH higher than its pI, its -NH3+and -COOH group would lose their -H to the basic solution resulting in a negative charge.

4) In an anion exchange column, the column is positively charged to be able to bind to an anion and vice versa.

Hence, at pH = 3.6 both peptides will be positively charged and will bind cation exchange column; at pH= 7.2 peptide A will be negatively charged an will bind to anion exchange column while peptide B will be positively charged and wll bind to the cation exchange colum; at pH = 10.8 both the peptides will be negatively charged causing them to bind to anion exchange column.

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