Imagine a pentapeptide molecule having a total of one - SH , - CONH ₂ , - NH ₂ group each and two - COOH groups. If the pH is maintained at 13.2, what would be the total number of negative charges on the pentapeptide?

Step-by-step solution

At pH 13.2, groups with pK_a Two -COOH groups ( pK_a 2-4 ): both deprotonated 2 (-1) = -2 charges One -SH group ( pK_a 8.3 ): deprotonated to -S^- -1 charge One -NH₂ group ( pK_a of conjugate acid 9-10 ): at pH 13.2, exists as neutral -NH₂ 0 charge One -CONH₂ group: amide, non-ionizable 0 charge Total negative charges = 2 + 1 = 3