Question

At pH 8, how many charged groups are present in the pentapeptide ADHRSK?

a) 0
b) 1
c) 2
d) 3

Answer 1

At pH 8, the pentapeptide ADHRSK has d) 3 charged groups: one negative charge from Aspartic acid (D) and two positive charges from Arginine (R) and Lysine (K).

Step-by-step explanation:

At pH 8, the pentapeptide ADHRSK will have a total of three charged groups. The amino acids Aspartic acid (D) and Glutamic acid (E) have side chains that can lose protons and hence will be negatively charged at this pH level. However, there is no E in this peptide. Histidine (H) has a side chain that can gain protons, but its pKa is close to 6; therefore, it is likely to be uncharged at pH 8. Arginine (R) and Lysine (K), on the other hand, have side chains that will be positively charged due to their higher pKa values. Therefore, with one negative charge from Aspartic acid (D) and two positive charges from Arginine (R) and Lysine (K), the overall count is three charged groups.