Discussion
From a carboxymethyl-cellulose column at pH 6.0, arginine, valine and glutamic acid will elute in the order
Answer: D
The order of elution is glutamic acid, valine, arginine.
Carboxymethyl-cellulose carries -CH2COO- groups, so it is a cation exchanger: it holds positively charged molecules and lets negative ones through. What matters is each amino acid's net charge at pH 6.0, which follows from its isoelectric point:
- glutamic acid, pI about 3.2: net negative, repelled by the column, elutes first;
- valine, pI about 6.0: near zero net charge, weakly held, elutes next;
- arginine, pI about 10.8: net positive, bound strongly, elutes last.
The orders starting with arginine would be right for an anion exchanger such as DEAE-cellulose, which is the usual confusion here.
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