Question

To the nearest hundredth of a unit, what is the net charge of the peptide GLACIDR at pH 4.5? Use the following set of pKa values:

Group                pKa                                               

N-ter                 8.0

C-ter                 3.1

Asp or Glu         4.0

Lys                  10.5

Arg                  12.4

His                    6.0

Tyr                  10.5

Cys                   8.4

Answer 1

Net charge on Peptide at pH 4.5 GLACIDR

If the pH is below the pKa value amino acid has negative charge and if pH is higher it has positive charge.

NH2- Gly - Leu - Ala - Cys - Ile - Asp - Arg -COOH

By using the values given in table.

N-ter pKa is 8.0 so it will have charge of -1.

Leu has no side chain. Therefore, no charge.

Ala has no side chain. Therefore, no charge.

Cys pKa is 8.4 so it will have charge of -1.

Ile has no side chain. Therefore, no charge.

Asp pKa is 4.0 so it will have charge of +1.

C-ter pKa is 3.1 so it will have charge of +1.

Net charge = -1-1+1+1 = 0.00