Discussion
A mixture of four peptides, PKKRK, RGERV, RYRGV and LVVYP, is loaded onto an ion-exchange column at pH = 7.2. If carboxymethyl (CM) cellulose is used as the stationary phase of this column, then which peptide elutes first?
Answer: D
The first peptide to elute is LVVYP.
Carboxymethyl cellulose carries negatively charged carboxylate groups at pH 7.2, so it is a cation exchanger: positively charged peptides bind, and the least positive peptide passes through first.
At pH 7.2 lysine (K) and arginine (R) side chains are positive, glutamate (E) is negative, and the free amino and carboxyl termini cancel. PKKRK carries about +4, RYRGV about +2, and RGERV about +1, so all three are retained. LVVYP has no charged side chains, a net charge of about zero, so it does not bind and elutes first.
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