Question

. In what order will the following amino acids elute from a Dowex-50 cation exchange column at pH 3.2: alanine (pI = 6.02); arginine (pI = 10.76); glutamate (pI = 3.22); serine (pI = 5.68); and tryptophan (pI = 5.88)? (Ignore the polarity characteristics of the side-chains). 


Wouldnt the basic ones bind and the acidic one (glu) elute off first??

Answer 1

Dowex-50 cation exchange is a cation exchane resin. It is negatively charged and it will attract positively charged species and It will let negatively charged species go due to repulsion.

First we will look over positively and negatively charged amino acids.

If pH > pI , Negative charged amino acid

If pH < pI , Positive charged amino acid

Arginine (pI = 10.76) is basic amino acid and it is positively charged at pH = 3.2 .It will attarcted by Dowex-50 cation exchange column so it will elute in last . glutamate (pI = 3.22) is acidic amino acid.It will be negatively charged so Dowex-50 cation exchange column will repelled by glutamate and it will elute first.

alanine (pI = 6.02) and tryptophan (pI = 5.88) are non polar amino acid whereas serine (pI = 5.68) is polar amino acid .If pI value of amino acid is high then it will elute late.

So elution order :

Glutamate > serine >tryptophan >alanine > Arginine