Question

The tetrapeptide, NRMD, exhibits four pK_a values: pK₁ = 1.88, pK₂ = 3.65,
pK₃ = 8.80, and pK₄ = 12.48.

At what pH is the net charge of this peptide -1.15?

Group of answer choices

pH = 12.5

pH = 11.7

pH = 10.6

pH = 8.8

pH = 7.0

Answer 1

The correct option is when pH= 8.8.

Because as we know that when pKa of a group is greater than pH then the group must be protonated and vice-versa .

From above it is clear that when the pH is 8.8 then one amino acid residue with pK4= 12, will be positively charged or protonated, two amino acid residues with pK1=1.8 and pK2= 3.65 will be negatively charged and one amino acid residue with pK3=8.8 will be neutral are as Zwitter ion.

Thus the net charge at this pH will be

-2 + 1 = -1

The difference of 0.15 is dud to the interaction of the for amino acids with one another which change their pK values