Question

The amino acid leucine has two titratable groups, the α-amino, pKa 9.6 and the α-carboxyl, pKa 2.4. Calculate the ratio of acidic and basic forms and express them as [A]/[HA] ratio or [B]/[BH] of the titrating group at the following pH values: α-carboxyl @ 1.4, 1.9, 2.4, 2.9, 3.4; α-amino @ 8.1, 8.6, 9.1, 9.6, 10.1. Plot pH vs. [B]/[BH] for each group, on the same graph. Compare similarities or disparities in the titration of each group.

Answer 1

For the given amino acids,

Using Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

For alpha-carboxyl group

pH          [A]/[HA]

1.4             0.1

1.9           0.316

2.4               1

2.9            3.162

3.4               10

Plot

For alpha-amino group

pH          [B]/[BH]

8.1          0.0316

8.6             0.1

9.1          0.3162

9.6              1

10.1        3.162

Plot

Both are straight line plots with increase concentration of base B or conjugate base A in the solution as the pH goes up.