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Chapter 6 of 12

Isoelectric Points and Charge Calculations: From Concept to Exam Problems

Turn abstract pI definitions into a practical toolkit for calculating isoelectric points and net charges on amino acids and short peptides under exam pressure.

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Step 1 – What is the Isoelectric Point (pI) and Why It Matters

What is pI?

Isoelectric point (pI) is the pH at which a molecule has zero net charge. Below pI it is overall positive; above pI it is overall negative. At pI, positives and negatives balance, though individual groups may still be charged.

Why pI Matters

pI is crucial for protein purification (isoelectric focusing, ion-exchange), solubility (proteins are often least soluble near pI), and formulation of biologic drugs, where buffers are chosen away from pI for stability.

Exam Use of pI

On exams, pI is a calculation and reasoning tool: predict net charge at a given pH, approximate pI from pKa values, and decide which ionic form (cationic, zwitterionic, anionic) dominates at a given pH.

Key Idea

Always remember: pI lies between two pKa values that surround the net-neutral form. You will use pKa values for ionizable groups plus Henderson–Hasselbalch reasoning from earlier modules.

Step 2 – Ionizable Groups and Typical pKa Ranges

Backbone pKa Values

Every free amino acid has ionizable backbone groups: α-COOH pKa ≈ 2–3 and α-NH3+ pKa ≈ 8–10. In peptides, the N-terminus and C-terminus have similar pKa ranges.

Side-Chain pKa Values

Key ionizable side chains: Asp ~4, Glu ~4.2, His ~6–6.5, Cys ~8–8.5, Tyr ~10–10.5, Lys ~10.3–10.7, Arg ~12–12.5. Exams typically provide exact values, but these ranges guide intuition.

Acidic vs Basic Groups

Acidic groups: pH << pKa → protonated, neutral; pH >> pKa → deprotonated, −1. Basic groups: pH << pKa → protonated, +1; pH >> pKa → deprotonated, neutral.

Why This Matters

Knowing which groups ionize and their pKa values lets you compute net charge and pI systematically, instead of guessing. Next, we turn this into a step-by-step procedure.

Step 3 – pI for Simple Amino Acids (Two Ionizable Groups)

Two-Group Amino Acids

Amino acids without ionizable side chains (e.g., Gly, Ala, Val, Ser) have only two ionizable groups: the α-COOH and α-NH3+. These are the simplest pI cases.

Titration Pattern

At low pH: net +1. After COOH deprotonates: zwitterion, net 0. After NH3+ deprotonates: net −1. The zwitterion dominates between the two pKa values.

pI Formula for Two Groups

Label pKa1 < pKa2. For these amino acids: pI = (pKa1 + pKa2) / 2. This works because the neutral form lies between the two ionizations.

Example: Glycine

Gly: pKa1 ≈ 2.3, pKa2 ≈ 9.6. pI = (2.3 + 9.6)/2 = 5.95 ≈ 6.0. This is the standard textbook value you often see in biochemistry courses.

Step 4 – Worked Example: pI and Charge of Glycine

Glycine pKa Values

Glycine: pKa1 (COOH) = 2.3, pKa2 (NH3+) = 9.6. No ionizable side chain, so we have only two ionizable groups to consider.

Glycine pI Calculation

Use pI = (pKa1 + pKa2)/2. For glycine: pI = (2.3 + 9.6)/2 = 5.95 ≈ 6.0. This is the standard pI value for glycine.

Net Charge at pH 1 and 7

pH 1: COOH 0, NH3+ +1 → net +1. pH 7: COO− −1, NH3+ +1 → net 0. pH 7 is near the pI, so the net charge is approximately zero.

Net Charge at pH 12 and Shortcut

pH 12: COO− −1, NH2 0 → net −1. Shortcut: pH < pI → net positive; pH ≈ pI → net ~0; pH > pI → net negative, for simple amino acids like glycine.

Step 5 – Strategy for Amino Acids with Ionizable Side Chains

Three-Group Amino Acids

Amino acids like Asp, Glu, His, Lys, Arg, Cys, Tyr have three ionizable groups: α-COOH, α-NH3+, and a side chain. pI is no longer just the average of two fixed pKa values.

Systematic Strategy

1) List all pKa values in ascending order. 2) Start at very low pH, assign charges. 3) Raise pH through each pKa, updating charges. 4) Find where net charge is 0. 5) Average the two pKa values around that neutral region.

Acidic Side Chains

For Asp and Glu: net charge path is typically +1 → 0 → −1 → −2. The neutral region lies between the two lowest pKa values (backbone COOH and side-chain COOH). pI ≈ average of those two.

Basic Side Chains

For Lys, Arg (and often His): net charge path is +2 → +1 → 0 → −1. The neutral region lies between the two highest pKa values (side chain and backbone NH3+). pI ≈ average of those two.

Step 6 – Worked Examples: Aspartate and Lysine pI

Aspartate pKa Values

Asp: pKaα-COOH = 2.1, pKa side-chain COOH = 3.9, pKaα-NH3+ = 9.8. Ordered: 2.1 < 3.9 < 9.8. Start at low pH with all groups protonated.

Aspartate Charge Steps

Low pH: net +1. After 2.1: net 0. After 3.9: net −1. After 9.8: net −2. The neutral region is between 2.1 and 3.9, so pI ≈ (2.1 + 3.9)/2 = 3.0.

Lysine pKa Values

Lys: pKaα-COOH = 2.2, pKaα-NH3+ = 9.0, pKa side-chain NH3+ = 10.5. Ordered: 2.2 < 9.0 < 10.5. Start at low pH with all groups protonated.

Lysine Charge Steps and pI

Low pH: net +2. After 2.2: net +1. After 9.0: net 0. After 10.5: net −1. Neutral region is 9.0–10.5, so pI ≈ (9.0 + 10.5)/2 = 9.75.

Step 7 – Thought Exercise: His vs Glu pI Reasoning

Step 7 – Thought Exercise: His vs Glu pI Reasoning

Use reasoning (no calculator) to compare histidine (His) and glutamate (Glu).

Approximate pKa sets (typical teaching values):

  • His: pKaα-COOH ≈ 2.0, pKa side chain ≈ 6.0, pKaα-NH3+ ≈ 9.0
  • Glu: pKaα-COOH ≈ 2.1, pKa side chain ≈ 4.3, pKaα-NH3+ ≈ 9.7

Your task:

  1. For each amino acid, list the pKa values in order.
  2. Decide whether the side chain is acidic or basic.
  3. Predict which two pKa values you will average to get pI.
  4. Without doing exact arithmetic, answer:
  • Which amino acid has the lower pI, His or Glu?
  • Is His’s pI closer to 6 or closer to 9?

Pause and think it through before reading the “solution idea” below.

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Solution idea (do not peek until you have tried):

  • His is basic-ish (side chain pKa ≈ 6), so pI is between the two highest pKa values, around (6 + 9)/2 ≈ 7.5.
  • Glu is acidic (side chain pKa ≈ 4.3), so pI is between the two lowest pKa values, around (2.1 + 4.3)/2 ≈ 3.2.
  • Therefore: Glu has the lower pI, and His’s pI is much closer to 6 than to 9, but actually a bit above 7.

Step 8 – Systematic Net Charge Calculation for Short Peptides

Find Ionizable Groups

For any peptide, list ionizable groups: N-terminus, C-terminus, and side chains of Asp, Glu, His, Cys, Tyr, Lys, Arg. These are the only groups you normally track for charge.

Assign Protonation States

Use pH vs pKa: acidic groups pH << pKa → 0, pH >> pKa → −1. Basic groups pH << pKa → +1, pH >> pKa → 0. Decide protonation for each group at the specified pH.

Sum Charges for Net Charge

After assigning charges to each ionizable group, sum them to get the peptide's net charge. Most exam pH values are ≥1 unit from pKa's, so you can treat groups as fully protonated or deprotonated.

Near-pKa Nuance

At pH ≈ pKa, a group is about 50% protonated. In more advanced questions, you may estimate fractional charges, but many undergraduate exams avoid this by choosing convenient pH values.

Step 9 – Worked Peptide Example: Net Charge of Ala-Lys-Glu at pH 7

Peptide and pKa Values

Peptide: Ala–Lys–Glu. pKa's: N-term ~9.0, C-term ~2.1, Lys side chain ~10.5, Glu side chain ~4.3. We want the net charge at pH 7.0.

Assign Individual Charges

At pH 7: N-term (pKa 9) → +1; C-term (pKa 2.1) → −1; Lys side chain (pKa 10.5) → +1; Glu side chain (pKa 4.3) → −1. Each group is clearly above or below its pKa.

Sum Charges

Add them: +1 (N) −1 (C) +1 (Lys) −1 (Glu) = 0. Thus Ala–Lys–Glu has a net charge of 0 at pH 7.0.

Shortcut Intuition

Shortcut: Lys (+1) cancels Glu (−1); N-terminal (+1) cancels C-terminal (−1). So the overall peptide is neutral at near-physiological pH.

Step 10 – Quick Check: Picking the Right pKa Pair for pI

Step 10 – Quick Check: Picking the Right pKa Pair for pI

You are given an amino acid with three ionizable groups and these pKa values:

  • pKa1 = 2.0
  • pKa2 = 4.0
  • pKa3 = 9.5

At very low pH, its net charge is +1. As pH increases, the net charge sequence is: +1 → 0 → −1 → −2.

Which two pKa values should you average to estimate the pI of this amino acid?

Given pKa1 = 2.0, pKa2 = 4.0, pKa3 = 9.5, and a net-charge path of +1 → 0 → −1 → −2 as pH increases, which pKa pair should you average to estimate pI?

  1. pKa1 and pKa2 (2.0 and 4.0)
  2. pKa2 and pKa3 (4.0 and 9.5)
  3. pKa1 and pKa3 (2.0 and 9.5)
Show Answer

Answer: A) pKa1 and pKa2 (2.0 and 4.0)

The pI lies where the net charge is 0. Here, net 0 occurs between the first and second deprotonations (+1 → 0 → −1), so pI is between pKa1 and pKa2. Average 2.0 and 4.0.

Step 11 – Flashcard Review: Core pI and Charge Concepts

Step 11 – Flashcard Review: Core pI and Charge Concepts

Use these flashcards to lock in the key ideas before you move on.

Isoelectric point (pI)
The pH at which a molecule has zero net charge; positive and negative charges balance, though individual groups may still be charged.
pI formula for amino acids with only two ionizable groups
For amino acids without ionizable side chains: pI = (pKa of α-COOH + pKa of α-NH3+) / 2.
Acidic side-chain amino acids: which pKa’s to average for pI?
For Asp and Glu, the neutral form lies between the two lowest pKa values, so pI ≈ average of pKaα-COOH and pKa of the side-chain COOH.
Basic side-chain amino acids: which pKa’s to average for pI?
For Lys and Arg (and often His), the neutral form lies between the two highest pKa values, so pI ≈ average of pKaα-NH3+ and pKa of the basic side chain.
Charge rule for acidic groups (COOH, phenol, thiol)
pH << pKa → protonated, charge 0; pH >> pKa → deprotonated, charge −1.
Charge rule for basic groups (NH3+, imidazolium, guanidinium)
pH << pKa → protonated, charge +1; pH >> pKa → deprotonated, charge 0.
Steps to find net charge of a peptide at a given pH
1) List ionizable groups. 2) Assign pKa values. 3) Decide protonation state at the given pH. 4) Assign each group’s charge. 5) Sum charges.
Relationship between pH and net charge around pI
For most amino acids/peptides: pH < pI → net positive; pH ≈ pI → net ~0; pH > pI → net negative.

Key Terms

pKa
The negative logarithm of the acid dissociation constant; the pH at which an ionizable group is 50% protonated and 50% deprotonated.
net charge
The sum of all positive and negative charges on a molecule at a given pH.
zwitterion
A molecular form that has both a positive and a negative charge but an overall net charge of zero.
titration curve
A plot of net charge or fraction protonated versus pH, showing stepwise deprotonation of ionizable groups.
basic side chain
An amino acid side chain that tends to accept a proton and carry a positive charge when protonated (e.g., Lys, Arg, His).
acidic side chain
An amino acid side chain that tends to donate a proton and carry a negative charge when deprotonated (e.g., Asp, Glu).
isoelectric point (pI)
The pH at which a molecule carries no net electrical charge; the total positive and negative charges balance.
Henderson–Hasselbalch reasoning
Using the relationship between pH, pKa, and the ratio of protonated to deprotonated species to predict ionization states qualitatively.

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