Question

This graph contains the titration curve of which of these amino acids: leucine, histidine, or lysine? Explain.

14.0r 12.0 10.0 8.0 6.0 4.0 2.0 a 60 Volume of base added

Answer 1

To see it better, let's put the structures of these aminoacids:
Leucine

он

histidine

OH NH2

Lysine

Let's also see the pKa's values for the aminoacids:

Histidine

1.77

9.18

6.10

Lysine

2.18

8.95

10.53

Leucine

2.33

9.74

The titration curve shows only two curves, which means that it has only two pKas values, most of the aminoacids have 3 pkas values, the pKa value for the lost of the COOH, then, the NH2, and finally the side chain. As we can see in the structures, leucine only have the COOH and NH2, and in the side chain it does not contain another molecule that can be hydrolized by the base. So, we can assume that this titration curve belongs to leucine.

Hope this helps