Question

2.The titration curve of alanine shows the ionization of two functional groups with pK_a values of 2.34 and 9.69, corresponding to the ionization of the carboxyl and the protonated amino groups, respectively. The titration of di-, tri-, and larger oligopeptides of alanine also shows the ionization of only two functional groups, although the experimental pK_a values are different. The trend in pK_a values is summarized in the table.

Amino acid or peptide	pK1	pK2
Ala	2.34	9.69
Ala-Ala	3.12	8.30
Ala-Ala-Ala	3.39	8.03
Ala-(Ala)n-Ala, n ³ 4	3.42	7.94

Draw the structure of Ala-Ala-Ala. Identify the functional groups associated with pK₁ and pK₂.

Why does the value of pK₁ increase with each additional Ala residue in the oligopeptide?

Why does the value of pK₂ decrease with each additional Ala residue in the oligopeptide

Answer 1

2) On addition of Ala residue to the oligopeptide the electrostatic inteaction between the ammino and carboxylic groups decreases (the non polar chain between the two increases). This decrease in the electrostatic interaction lowers the strength of acidic group (carboxylic group) resulting in the increase in the pK1 value.

3) On addition of Ala residue to the oligopeptide the electrostatic inteaction between the ammino and carboxylic groups decreases (the non polar chain between the two increases). This decrease in the electrostatic interaction lowers the strength of basic group (amino group) resulting in the decrease in the pK2 value [pKb decreases with decrease in basic strength].