The pH at which a molecule carries no net electric charge is known as its isoele

Question

The pH at which a molecule carries no net electric charge is known as its isoelectric point, pI. For the delta-amino acids, pI = 1/2 (pK_i+pK_j) Where K_i and K_j are the dissociation constants of the two ionizations involving the neutral species. (p. 82 of your text) Given the values of pk_1, pK_2, and pK_R for the amino acids in the table below, calculate the values of pI for the amino acids Ile, Leu, Pro, Asp, Glu, Arg, Lys, His, Cys, and Tyr. When the pH of a solution is below the pI value, the amino acid will have a net positive electric charge; when the pH of a solution is above the pI value, the amino acid will have a net negative electric charge. Your answers to this question will be useful in answering question 9 below.

Explanation / Answer

Calculate PI for the amino acids using the table above.

Ile : 1/2(2.32 + 9.76) = 6.04

Leu : 1/2(2.33 + 9.74) = 6.03

Pro : 1/2(1.95 + 10.64) = 6.29

Asp : 1/2(1.99 + 3.90) = 2.94

Glu : 1/2(2.10 + 4.07) = 3.08

Arg : 1/2(8.99 + 12.48) = 10.73

Lys : 1/2(9.06 + 10.54) = 9.81

His : 1/2(9.33 + 6.04) = 7.68

Cys : 1/2(1.92 + 8.37) = 5.14

Tyr : 1/2(2.20 + 9.21) = 5.70

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