Question

Consider the amino acid lysine (H3Lys2+ in the fully protonated form) which has pKa1 = 2.04, pKa2 = 9.08, and pKa3 = 10.69.

a. What is the charge on the principal species at pH 7.4 (blood)?

Circle one of +2 +1 0 -1 -2

b. What is the charge on the principal species at pH 10.0?

Circle one of +2 +1 0 -1 -2

c. Calculate the fraction of the total lysine which exists in the neutral form (HLys0) at pH 7.40.

Answer 1

Solution.

The structures of lysine at different pH are given below:

a. The pH=7.4 is between pKa1 and pKa2, it corresponds to the second form with a charge of (+1);

b.  The pH=10.0 is between pKa2 and pKa3, it corresponds to the third form with a charge of (0);

c) At pH =7.4 the predominant forms of lysine are H₂L and HL, which are related by K_a2:

Let's assume that solving a system of equation we can get the fraction of the total lysine which exists in the neutral form

or 2.06 %.