The following polypeptide was treated with 2-mercaptoethanol and then with iodoacetic acid. After reacting with maleic anhydride, the peptide was hydrolyzed by trypsin. (Treatment with maleic anhydride causes trypsin to cleave a peptide only at arginine residues.)
Gly-Ser-Asp-Ala-Leu-Pro-Gly-Ile-Thr-Ser-Arg-Asp-Val-Ser-Lys-Val-Glu-Tyr-Phe-Glu-Ala-Gly-Arg-Ser-Glu-Phe-Lys-Glu-Pro-Arg-Leu-Tyr-Met-Lys-Val-Glu-Gly-Arg-Pro-Val-Ser-Ala-Gly-Leu-Trp
a. Why, after a peptide is treated with maleic anhydride, does trypsin no longer cleave it at lysine residues?
b. How many fragments are obtained from the peptide?
c. In what order would the fragments be eluted from an anion-exchange column using a buffer of pH = 5?