Question

The tetrapeptide, NRMD, exhibits four pKa 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

(provide the calculation please)

Answer 1

The correct option is when pH=8.8.

will . 25

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 or as a Zwitter ion.

Thus the net charge at this pH will be

-2+1 = -1

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