A tetrapeptide Cys-Trp-Lys-Pro is in the form that would predominate in aqueous solution at pH=7.0 (pKa values for ionizable groups are

2.2, 8.3, 9.5, and 10.5).

• What is the pI value for this tetrapeptide?
• The tetrapeptide was digested with chymotrypsin and adjusted to pH=0.5 to fully

protonate all digestion products. The digestion products were then applied to a strong

cation exchange column (sulfonic acid). The products were eluted from the column using

a pH gradient from 0.5 to 13. Write the primary amino acid sequence of the products in

the order they would elute from the column. Indicate the pH value at which each product

elutes from the column.