Ion-exchange chromatography is a laboratory technique that allows scientists to

Question

Ion-exchange chromatography is a laboratory technique that allows scientists to purify or separate polymers present in a solution mixture. The polymers of interest are added to a buffered solution of constant pH, and this solution is poured onto a solid surface that is coated with either anions or cations. The technique relies on the binding of the polymers to the solid surface based on the polymers' net (overall) charges. For instance, cation chromatography relies on the binding of net positively charged polymers to a surface coated with anions.

Consider two short amino acid polymers with the following sequences:

Polymer 1: NH2-Val-Gly-Arg-Asp-Ser-Lys-Asn-Tyr-COOH

Polymer 2: NH2-Gln-Ala-Arg-Asn-Ser-Phe-Glu-Tyr-COOH

Suppose both polymers were dissolved in a buffered solution of pH = 8.0. Which of these two polymers shows a higher likelihood of binding to a surface coated with anions at this pH? The pKa values of any relevant acidic side chains are shown below. The pKa of the alpha amino group is 9.5, and the pKa of the alpha carboxyl group is 2.2.

Polymer 1

Polymer 2

Both polymers would most likely bind equally well at this pH.

Polymer 1

Polymer 2

Both polymers would most likely bind equally well at this pH.

Explanation / Answer

Answer =A polymer 1

Explanation=Affinity to anion depends on net + charge on the polymer,higher the charge more will be the affinity.But the net positive charge in turn depends on the number of protonated species in the polymer.

If pH<pKa , the NH2 groups are protonated.

in polymer 1,

No of basic groups with pka>8 are four in number=Arg,lys,tyr and the alpha NH2. So net charge =+4(-NH3+ is formed)

In polymer 2 No of basic groups with pka>8, are three ,Arg,tyr, alpha NH2 net charge=+3

Higher net charge in polymer 1,so high affinity for anions ,more binding to the surface

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