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? asked in Science & MathematicsChemistry · 8 months ago

How to draw the ionization state of Lysine?

I am having trouble solving these problems. 

A. Draw ionization state of Lysine that would predominate in solution at pH 1.B. Draw ionization state of Lysine that would predominate in solution at physiological pH (I believe that is 7.4)C. Draw ionization state of Lysine that would predominate in solution at pH 9.8

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  • 8 months ago
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    Lysine has 4 ionic forms. The fully protonated form on the left will be the form that predominates in solution at pH = 1. This is because at this point, the pH is well be low the pKas for any of the ionizable groups in the molecule and so they are all in their protonated states. 

    When the amino acid is under physiological conditions, the pH is well above the pKa of the alpha carboxyl group, so it will have lost its proton and left a negatively-charged carboxyl group. But, 7.4 is well below the pKa of the next most acidic group, and so the rest of the molecule remains protonated. This is the second figure below. 

    Now, at pH = 9.8, you are slightly above the pKa for the alpha amino group, but about 1 pH unit below the pKa of the side chain amino group. So, at pH = 9.8, the predominant form will be the third figure below, but there will still be a significant amount of the second form. You could calculate what proportion of the molecules will be in that third form compared to the second form by using the Henderson-Hasselbalch equation.

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