Determining Net Charge of a Peptide Video Summary

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Determining Net Charge of a Peptide means identifying every ionizable group in the chain and comparing each group’s pKa to the solution pH. The ionizable groups are the N-terminus, the C-terminus, and any side chains with ionizable R groups. Internal residues do not contribute alpha amino or alpha carboxyl groups, so only the terminal residues and specific ionizable side chains are counted.

The key rule is: if \(\mathrm{p}K_a > \mathrm{pH}\) , the conjugate acid predominates; if \(\mathrm{p}K_a < \mathrm{pH}\) , the conjugate base predominates. For amino groups and basic side chains, the conjugate acid is usually positively charged; for carboxyl groups and acidic side chains, the conjugate base is usually negatively charged. When \(\mathrm{pH} = \mathrm{p}K_a\) , the group is present as a 50:50 mixture and contributes half a charge on average.

After assigning each charge, add them to estimate the net charge. This value is an estimate because the local microenvironment in a peptide or protein can shift pKa values, so using the correct residue pKa set is important.

Concept

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Determining the net charge of a peptide or protein is essential for understanding its behavior in biological systems. The net charge is influenced by the ionizable groups present in the peptide, which include the alpha amino and alpha carboxyl groups of the amino acid residues. To assess the ionization state of these groups, one must compare their pKa values to the pH of the solution.

Ionizable groups can be found in the terminal amino acids and specific side chains of the amino acids. For instance, the terminal amino group typically has a pKa around 8, while the carboxyl group usually has a pKa of about 2. However, when considering amino acid residues within a peptide, these pKa values can shift due to the unique microenvironment surrounding each residue. This shift can significantly affect the net charge, making it crucial to use the correct pKa values for amino acid residues rather than those for free amino acids.

To estimate the net charge of a peptide at physiological pH (approximately 7.4), one must evaluate each ionizable group. For example, if the pKa of an ionizable group is greater than the pH, the group will predominantly exist in its protonated (conjugate acid) form, contributing a positive charge. Conversely, if the pKa is lower than the pH, the group will exist in its deprotonated (conjugate base) form, contributing a negative charge.

For instance, consider a peptide with the following ionizable groups: an amino group at the N-terminus (pKa = 8), an arginine side chain (pKa = 12.5), a histidine side chain (pKa = 6), an aspartic acid side chain (pKa = 3.9), and a carboxyl group at the C-terminus (pKa = 3.5). Evaluating these groups at pH 7.4 reveals:

The N-terminus amino group (pKa = 8) is protonated, contributing +1 charge.
The arginine side chain (pKa = 12.5) is also protonated, contributing +1 charge.
The histidine side chain (pKa = 6) is deprotonated, contributing 0 charge.
The aspartic acid side chain (pKa = 3.9) is deprotonated, contributing -1 charge.
The C-terminus carboxyl group (pKa = 3.5) is deprotonated, contributing -1 charge.

Summing these contributions results in a total charge of:

+1 (N-terminus) + 1 (arginine) + 0 (histidine) - 1 (aspartic acid) - 1 (C-terminus) = 0.

Thus, the estimated net charge of the peptide at physiological pH is 0. This process highlights the importance of understanding the ionization states of amino acids and their contributions to the overall charge of peptides and proteins.

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Problem

What is the net charge if you drop the peptide above into bleach (pH 12)?

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Problem

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? ______

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Problem

Estimate the net charge for a His-His-His-His peptide at pH 6 (His pK R = 6).

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Problem

Estimate the net charge for the following peptide at pH 7: ATLDAK.

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Problem

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?
pH: ______________

Video duration:

12m

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