The structure of asparagine is shown at the right along with the pK values of th

Question

The structure of asparagine is shown at the right along with the pK values of the a carboxyl an pK 8.80 CH-C OH pKr 2.02 a-amino groups Asparagine is a diprotic acid which can exist in CH three ionized forms: H2A HAO, and A (HAO consists primarily of the zwitterionic form with a CEO negatively charged carboxyl group and positively charged amino group, but a very small NH amount of the completely neutral species does exist Calculate the fraction (a) of each species present at pH 2.60. Number Number Number 3.8 HA'

Explanation / Answer

pH = -log[H+] = 2.60

[H+] = 2.51 x 10^-3 M

Ka1 = 9.55 x 10^-3

Ka2 = 1.6 x 10^-9

[A] = [H+]^2 + Ka1[H+] + Ka1Ka2

     = (2.51 x 10^-3)^2 + 2.51 x 10^-3 x 9.55 x 10^-3 + 9.55 x 10^-3 x 1.6 x 10^-9

     = 6.3 x 10^-6 + 2.4 x 10^-5 + 1.53 x 10^-11

     = 3.03 x 10^-5

fractions

alpha[H2A+] = [H+]^2/[A] = 0.208

alpha[HAo] = [H+]Ka1/[A] = 0.792

alpha[A^2-] = Ka1Ka2/[A] = 5.05 x 10^-7

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