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amino acid methionine, has pKa1 = 2.2 and pKa2 = 9.1. If this amino acid is represented by H2L+, what is the major species at pH 6?

  • chemistry -

    Here is how you do this type.
    alphao = (H2L) = (H^+)^2/D
    alpha1 = ((HL^-) = k1*(H^+)/D
    alpha2 = (L^-2) = k1k2/D
    where D (denominator) is
    (H^+)^2 + k1*(H^+)^ + k1k2

    Plug in 1 x 10^-6 for (H^+) and evalueate alphao, alpha1 and alpha2.
    You can go through the math (I did) and approximations are
    alphao = (H2L) = 0.0002
    alpha1 = (HL^-) = 0.95
    alpha2 = (L^-2) = 0.0009
    Those numbers aren't exact and you should do it to determine the exact values (these are fractions in the form shown); however, you can see that the majority of the H2L is in the form of HL^-

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