Concept #1: How to Determine the Net Charge of a Peptide
Practice: What is the net charge if you drop the peptide above into bleach (pH 12)?
Practice: Answer the following questions (A, B & C) relating to the 4 tripeptides.
i) Tyr-Lys-Met ii) Asp-Trp-Tyr iii) Asp-His-Glu iv) Leu-Val-Phe
A. Which tripeptide is most negatively charged at pH = 7? _________
B. Which tripeptide contains the largest number of nonpolar R groups? __________
C. Which tripeptide contains sulfur? ____________
Practice: Estimate the net charge for a His-His-His-His peptide at pH 6 (His pK R = 6).
Practice: Estimate the net charge for the following peptide at pH 7: ATLDAK.
Practice: A. Draw the predominant structure of the following peptide at pH 9: Asn-Arg-Cys. What is its net charge?
(Asn pKa1 = 8.8, Arg pKR = 12.48, Cys pKa2 = 1.96, Cys pKR = 8.18).
B. What is the net charge of the same peptide if the pH is lowered to pH = 2? Draw the newly ionized peptide.
C. Within what pH range would the net charge on the peptide above be approximately +1?