A mixture of the following peptides is subjected to electrophoresis: peptide A:

Question

A mixture of the following peptides is subjected to electrophoresis:

peptide A: lys-trp-arg-asn-met peptide B: asp-tyr-ala-his-leu peptide C: ile-glu-gln-cys-val

a) Calculate the approximate (nearest integer) net charge on each peptide at pH 7. Predict the pattern that would be obtained by subjecting a mixture of the three peptides to electrophoresis at pH 7. (Note: Each peptide migrates as a single molecule giving a single spot, not as a group of amino acids giving five spots!) (Answer: Peptide A has an approximate charge of +2 at pH 7.)

b) Predict the order of elution of the peptides from a cation exchange resin at pH 7 as the salt concentration is increased.

Explanation / Answer

Answer.

Positive Charged:
3 amino acids (arginine, lysine) have hydrogen donor atoms in their side chains.

Negative Charged:
2 amino acids (aspartic acid, glutamic acid) have hydrogen acceptor atoms in their side chain.
(Histidine) will have mostly (dominating) Negative charge

Ans. a

Net Charges :
A: lys-trp-arg-asn-met; A = +2
(Lys:+ve; Arg:+ve)

B: asp-tyr-ala-his-leu B= - 2
(Asp: -ve; His: -ve)

C: ile-glu-gln-cys-val C = - 1
(Glu: -ve)

Electrophoresis is such placed that
Negatively Charged will migrate to Top
and Positively Charged will migrate to Bottom
Therefore,
Peptide A will migrate to Bottom
and Peptide B will migrate to Top Faster than C

On Gel,
Pep A will be appear on Bottom, and above it pep C and on Top of all Pep B.

Ans. b

On Increase of salt concentration the Ion Interactions will weaker
Thereby,
On a Negative Field
Order of Elution in a Negative Field will be,
pep A > Pep C > Pep B (pep A will be eluded First)

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