複選題26. Mr. Bean synthesizes a pentapeptide "DPRTH". Choose the correct answer from the following descriptions (pK₂ of the N-terminal amino group of the peptide is 9.1； pK₁ of the C-terminal carboxyl group of the peptide is 2.1； pKa of D is 3.65, pKa of R is 12.5, pKa of H is 6): (A) The pI value of the peptide is around 4. (B) The pI value of the peptide is around 7. (C) The pI value of the peptide is around 11. (D) When 0.1 mg of the peptide is dissolved in 100 mM Tris (pH 9), it could bind to a cation exchange column. (E) All the residues in this peptide contain one chiral center only.