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Chapter 27: Problem 36

Do the following compounds migrate to the cathode or to the anode on electrophoresis at the specified pH? (a) Histidine at \(\mathrm{pH} 6.8\) (b) Lysine at \(\mathrm{pH} 6.8\) (c) Glutamic acid at \(\mathrm{pH} 4.0\) (d) Glutamine at pH \(4.0\) (e) Glu-Ile-Val at pH \(6.0\) (f) Lys-Gln-Tyr at \(p \mathrm{H} 6.0\)

Short Answer

Expert verified
(a) Histidine at pH 6.8, (b) Lysine at pH 6.8, (c) Glutamic acid at pH 4.0, (d) Glutamine at pH 4.0, (e) Glu-Ile-Val at pH 6.0, and (f) Lys-Gln-Tyr at pH 6.0. Answer: (a) Histidine at pH 6.8 migrates towards the cathode. (b) Lysine at pH 6.8 migrates towards the cathode. (c) Glutamic acid at pH 4.0 migrates towards the cathode. (d) Glutamine at pH 4.0 migrates towards the cathode. (e) Glu-Ile-Val at pH 6.0 migrates towards the anode. (f) Lys-Gln-Tyr at pH 6.0 migrates towards the cathode.

Step by step solution

01

Determine net charge of the compound at given pH

First, we need to compare the pH value to the pKa values of the functional groups present in the compound. If the pH is less than the pKa value, the functional group will be protonated and have a positive charge. If the pH is greater than the pKa value, the functional group will be deprotonated and possess a negative charge.
02

Identify migration of the compound based on net charge

Based on the net charge of the compound, we determine the direction of migration during electrophoresis. Positively charged compounds move towards the negative electrode (cathode), and negatively charged compounds move towards the positive electrode (anode). (a) Histidine at \(\mathrm{pH} 6.8\)
03

Net Charge

Histidine has two pKa values: imidazole side chain (\(\mathrm{pK_a} = 6.0\)) and the \(\mathrm {NH_3^+}\) group (\(\mathrm{pK_a} = 9.17\)). At pH 6.8, the imidazole side chain will be uncharged, whereas the \(\mathrm{NH_3^+}\) group will be positively charged.
04

Migration

Histidine has a net positive charge and hence will migrate towards the cathode. (b) Lysine at \(\mathrm{pH} 6.8\)
05

Net Charge

Lysine has two pKa values: amino group (\(\mathrm{pK_a} = 10.5\)) and the \(\mathrm {NH_3^+}\) group (\(\mathrm{pK_a} = 9.0\)). At pH 6.8, both groups will be positively charged.
06

Migration

Lysine has a net positive charge and hence will migrate towards the cathode. (c) Glutamic acid at \(\mathrm{pH} 4.0\)
07

Net Charge

Glutamic acid has two pKa values: \(\mathrm{COOH}\) side chain (\(\mathrm{pK_a} = 4.07\)) and the \(\mathrm {COOH}\) group (\(\mathrm{pK_a} = 2.19\)). At pH 4.0, the side chain will be uncharged, while the \(\mathrm{COOH}\) group will be protonated and positively charged.
08

Migration

Glutamic acid has a net positive charge and hence will migrate towards the cathode. (d) Glutamine at pH \(4.0\)
09

Net Charge

Glutamine does not have any ionizable side chains. Only the \(\mathrm{NH_3^+}\) group has a \(\mathrm{pK_a}\) value of 9.0. At pH 4.0, the \(\mathrm{NH_3^+}\) group will be positively charged.
10

Migration

Glutamine has a net positive charge and hence will migrate towards the cathode. (e) Glu-Ile-Val at pH \(6.0\)
11

Net Charge

The net charge is determined by the sum of charges on each amino acid. Glutamic acid (\(\mathrm{COOH}\) side chain) has a pKa value of 4.07 and will be deprotonated and negatively charged at pH 6.0. Isoleucine and valine do not have any ionizable side chains, so their contribution to the net charge will be zero.
12

Migration

The net negative charge on the peptide will cause it to migrate towards the anode. (f) Lys-Gln-Tyr at \(p \mathrm{H} 6.0\)
13

Net Charge

The net charge is determined by the sum of charges on each amino acid. Lysine (amino group \(\mathrm{NH_3^+}\)) has a pKa value of 10.5 and will be positively charged at pH 6.0. Glutamine and tyrosine do not have any ionizable side chains, so their contribution to the net charge will be zero.
14

Migration

The net positive charge on the peptide will cause it to migrate towards the cathode.

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