A mixture of the amino acids leucine, glutamate, and arginine is added to a cation exchange column at neutral pH. In what order will they elute from the column?

The elution order of amino acids in a cation exchange chromatography column depends on their respective pKa values and the pH of the eluent. In this case, the amino acids leucine, glutamate, and arginine have different acidic functional groups (carboxyl groups) that can be protonated or deprotonated based on the pH of the solution.

At neutral pH:

1. Glutamate (pKa around 4.3): Glutamate has a lower pKa compared to the other two amino acids. At neutral pH, its carboxyl group (-COOH) will mostly be deprotonated (-COO⁻), carrying a negative charge. In a cation exchange column, negatively charged molecules are repelled, so glutamate will elute first.
2. Leucine (pKa around 2.3): Leucine's carboxyl group will also be deprotonated at neutral pH, but it has a higher pKa than glutamate. It means that it's less acidic than glutamate. Leucine will elute after glutamate because it is less negatively charged and therefore less repelled by the cation exchange column compared to glutamate.
3. Arginine (pKa values around 2.2 for the α-amino group, 9.0 for the guanidinium group, and 12.5 for the carboxyl group): Arginine contains multiple ionizable groups. At neutral pH, its α-amino group and guanidinium group will be positively charged (+NH₃⁺ and +NH=C(NH₂)NH₂⁺, respectively), while its carboxyl group will be deprotonated (-COO⁻). In a cation exchange column, positively charged molecules are attracted. Thus, arginine will elute last due to its strong positive charge compared to leucine and glutamate.

Therefore, the elution order from the cation exchange column at neutral pH would be: Glutamate, Leucine, Arginine.