Download Name: pKa of amino acids and polypeptides Learning Outcome

Name:________________________
pKa of amino acids and polypeptides
Learning Outcome: Students will be able to explain and quantify the relationship between pH
and protein charge.
1. Buffer X has a pKa of 7.5.
a. Draw a pH titration curve for Buffer X. (Hint; the X-axis should be OH equivalents
and the Y-axis should be pH)
b. Label the pKa, buffering region and add a scale for the Y axis
c. Using your own words, define what a pKa is.
2. Find the structure for the amino acid glycine in your table.
a. How many pKas does Glycine have?
b. Draw a pH titration curve for glycine. Be sure to label all pKas.
c. Draw the chemical structure for the majority of glycine molecules in solution at
pH1, pH6, and pH12.
3. There is a polypeptide C-H-E-E-R
a. How many pKas will this polypeptide have?
b. What will be the charge of the majority of C-H-E-E-R in solution at pH 7.5? Draw
the chemical structure.
c. What will be the charge of the majority of C-H-E-E-R in solution at pH 5.0? Show
your work.
d. What will be the charge of the majority of C-H-E-E-R in solution at pH 10.0?
Show your work.
Group Members: _________________________________________________________
Challenge Question
You are purifying the polypeptide C-H-A on a cation exchange chromatography column. All
purification solutions are pH 5.8. You find that a significant portion of your protein did not stick
to the cation exchange resin. Why is this, and what could you do it fix the problem? Show your
work. (Hint; if you are having trouble, draw out the titration curve for this polypeptide.)