Question 1

Which of the following best describes how to estimate the net charge of a peptide at a given pH?

Accepted Answer

By summing the charges of all ionizable groups, considering their $$pK_a$$ values relative to the pH.

Question 2

Given the peptide sequence Arg-His-Asp-Glu, what is its net charge at physiological pH ($$pH = 7$$)? (Assume $$pK_a$$ values: Arg = 12.5, His = 6.0, Asp = 3.9, Glu = 4.1, N-term = 9.0, C-term = 2.0)

Accepted Answer

$$+1$$

Question 3

If the peptide Arg-His-Asp-Glu is dropped into a solution of bleach ($$pH \approx 13$$), what is its net charge?

Accepted Answer

$$-2$$

Question 4

Which of the following peptides would have the largest number of ionizable R groups?

Accepted Answer

Arg-His-Glu

Question 5

A peptide has the sequence Ala-Lys-Asp. Estimate its net charge at $$pH = 7. ($$Assume $$pK_a$$ values: Lys = 10.5, Asp = 3.9, N-term = 9.0, C-term = 2.0)

Accepted Answer

$$0$$

Question 6

Which of the following statements about the effect of $$pH on $$peptide net charge is correct?

Accepted Answer

Raising the $$pH$$ above the $$pK_a of an $$acidic group causes it to lose a proton and become negatively charged.

Question 7

Estimate the net charge of the peptide Ala-Asp-Lys at $$pH = 2. ($$Assume $$pK_a$$ values: Lys = 10.5, Asp = 3.9, N-term = 9.0, C-term = 2.0)

Accepted Answer

$$+2$$

Question 8

Which $$pH$$ range would result in a net charge of approximately $$+1$$ for the peptide Ann-Arg-Cys? (Given $$pK_a$$ values: Ann = 8.8, Arg = 12.48, Cys = 1.96, Cys side chain = 8.18)

Accepted Answer

$$pH = 5.0 to$$ $$8.18$$

Question 9

A peptide has the sequence Glu-Lys-Ser. What is the net charge at $$pH = 10$$? (Assume $$pK_a$$ values: Glu = 4.1, Lys = 10.5, N-term = 9.0, C-term = 2.0)

Accepted Answer

$$-1$$

Question 10

Which of the following best explains why only the first and last amino acid residues in a chain contribute their terminal group charges to the net charge calculation?

Accepted Answer

Because only the N-terminus and C-terminus are free to ionize; internal residues are involved in peptide bonds.

Question 11

If the $$pH of a $$solution is much lower than the $$pK_a of an $$amino acid side chain, what is the likely charge on that group?

Accepted Answer

It will be protonated.

Question 12

A peptide with the sequence Asp-Gly-Lys is placed in a solution at $$pH = 12. $$What is its net charge? (Assume $$pK_a$$ values: Asp = 3.9, Lys = 10.5, N-term = 9.0, C-term = 2.0)

Accepted Answer

$$-2$$

Question 13

Which of the following is a correct step in determining the net charge of a peptide at a given $$pH$$?

Accepted Answer

Compare the $$pH to $$the $$pK_a of $$each ionizable group to determine its protonation state.

Question 14

A peptide has the sequence Ala-Glu-Arg. At $$pH = 7$$, which group is most likely to be positively charged?

Accepted Answer

Arginine side chain

Question 15

If a peptide has a net charge of $$-1 at$$ $$pH = 7$$, what is the most likely composition of its ionizable groups?

Accepted Answer

It contains more acidic than basic residues.

Question 16

Which of the following changes would most likely increase the net positive charge of a peptide at $$pH = 7$$?

Accepted Answer

Replacing a glutamic acid residue with a lysine residue.

Net Charge of Peptides: Concepts and Calculations

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