Question

5. Amino acid titration. The graph below shows a titration of an amino acid with NaOH. This experiment reveals several important features of this amino acid. A) What is the identity of the amino acid? [Write its full name.] B) Match the following points in the titration curve. [In the space beside each description (left), write a number (1-6) corresponding to a specific pH (right).] The amino acid is fully protonated. 1) pH=0.0 PH The amino acid is fully deprotonated. 2) pH=3.1 Half of the amino groups are ionized. 3) pH-4.5 The average charge on the carboxyl group is -0.5. 4) pH=6.0 0.5 1.0 3.0 5) pH=8.0 1.5 2.0 2.5 OH" (equivalents) The side chain has its maximum buffering capacity, functioning as an acid catalyst or base catalyst. 6) pH=10.04 The predominant species contains a protonated amino group, a deprotonated carboxyl group, and a protonated side chain (as shown below). "HN-CH-COO R+

Answer 1

A)pKR value of this amino acid is 6. So it is histidine which has pKR near 6.

B) pH 0 the amino acid fully protonated

pH 10   Amino acid is fully deprotonated.

pH 8 Half of amino group are ionised

pH 3.1   the average charge on Carboxylic group is -0.5

pH 6 side chain has its maximum buffer capacity functioning as acid catalyst or base catalyst

pH 4.5   the predominant species contains a protonated amino acid and deprotonated Carboxylic group and protonated side chain.