Question

What is the net charge of the dipeptide Glu-Ala at pH=4.1, given the following data for the individual amino acids:

Glu: pKa1= 2.1; pKa2= 9.5; pKaR= 4.1
Ala: pKa1 = 2.3; pKa2 = 9.9

Answer 1

At a pH above the pI of the peptide, the peptide carry a net negative charge. pI of the peptide EA is 3.3, if this peptide is at pH 4.1 it the carry a negative charge.

Net charge of the peptide at a given pH is calculated using the following formula:

Q_peptide =Q^- +Q⁺

Q^- = (-1)/(1 + 10 ^{-(pH-pKa (carboxyl group))})

Q⁺ = (+1)/(1 + 10 ^{(pH-pKa (amino group))})

Net charge of the peptide EA at pH 4.1 would be..

Q_peptide(EA)= (-1)/(1 + 10 ^{-(pH-pKa (carboxyl group))}) + (-1)/(1 + 10 ^{-(pH-pKa (carboxyl group of side chain))}) + (+1)/(1 + 10 ^{(pH-pKa (amino group))})

Q_peptide(EA)= (-1)/(1 + 10 ^-(4.1-2.3)) + (-1)/(1 + 10 ^-(4.1-4.1)) + (+1)/(1 + 10 ^(4.1-9.5))

Q_peptide(EA) = (-1)/(1 + 10 ^-1.8)) + (-1)/(1 + 10⁰⁾) + (+1)/(1 + 10 ^(-5.4)) = (-0.99) + (-0.5) + (0.99)= -0.5

Net charge on the peptide EA at pH 4.1 = -0.5 (Peptide would be negatively charged at pH 4.1)

Reference: http://onlinelibrary.wiley.com/doi/10.1016/0307-4412(85)90114-1/pdf