Peptides can be separated using an ion-exchange column based on their isoelectri

Question

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 pi of 6.0 and the other with a pi of 8.8. 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 pl = 6.0 Peptide B pl = 88 anion-exchange column at pH = 4.0 Peptide A pl = 6.0 Peptide B pl = 8.8 pH = 7.2 Peptide B pl = 8.8 pH = 11.0 cation-exchange column at pH = 4.0 pH = 7.2 Peptide A pl = 6.0 pH = 11.0 Peptide A pl = 6.0 Peptide B pl = 8.8

Explanation / Answer

pI --> isoelectric point, pH in which charges of COO- and NH4+ are the same

if pH is less than pI --> positive charge; binds to cation exchangers

if pH is more than pI --> negtively charge; binds to anions exhcangers

then...

For Protein A; Anion Exchanger:

a) pH = 4.0, Peptide A (pI = 6), Positive charge --> no bind

b) pH = 7.2, Peptide A (pI = 6), Negative Charge -->  binds

c) pH = 11, Peptide A (pI = 6), Negative charge --> bind

For Protein A; Canion Exchanger:

a) pH = 4.0, Peptide A (pI = 6), Positive charge --> binds

b) pH = 7.2, Peptide A (pI = 6), Negative Charge --> will not bind

c) pH = 11, Peptide A (pI = 6), Negative charge --> will not bind

For Protein B; Anion Exchanger:

a) pH = 4.0, Peptide B (pI = 8.8), Positive charge --> no bind

b) pH = 7.2, Peptide B (pI = 8.8), Positive charge --> no bind

c) pH = 11, Peptide B (pI = 8.8), Negative charge --> will bind

For Protein B; Canion Exchanger:

a) pH = 4.0, Peptide B (pI = 8.8), Positive charge --> binds

b) pH = 7.2, Peptide B (pI = 8.8), Positive charge --> binds

c) pH = 11, Peptide B (pI = 8.8), Positive charge --> no bind

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