Question

The tetrapeptide Ac-Asn-Met-Cys-Lys has its N-terminus blocked by acetylation. Its C-terminal pKa is 4.30. The side chain pKa’s are 7.8 and 10.4.

a. Write this peptide sequence using single letter abbreviations.

b. Draw its predominant chemical structure at pH 8.0, including charges.

c. What fraction of the peptide will exist in the predominant form at pH 8.0? (Discount any species that represent less than 1% of all species.)

d. Calculate the pI of the peptide.

e. If this tetrapeptide is oxidized to link pairs the molecules via disulfide bridges, what will be the pI of the product?

f. Can a mixture of the oxidized and unoxidized peptides be separated on a DEAE-matrix at pH 6.5? If yes, which peptide will elute first? Explain why.

Answer 1