A typical amino acid with one amino group and one carboxylic
acid group, such as serine, can exist in water in several ionic forms. (a) Suggest the forms of the amino acid
at low pH and at high pH. (b) Amino acids generally have
two pKa values, one in the range of 2 to 3 and the other in
the range of 9 to 10. Serine, for example, has pKa values of
2.19 and 9.21. Using species such as acetic acid and ammonia
as models, suggest the origin of the two pKa values.
(c) Glutamic acid is an amino acid that has three
pKa's: 2.10, 4.07, and 9.47. Draw the structure of glutamic
acid, and assign each pKa to the appropriate part of the
molecule. (d) An unknown amino acid is titrated with
strong base, producing the following titration curve.
Which amino acids are likely candidates for the unknown?
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