Rule: pH vs pKa and Protonation

If pH pKa, the group is mostly deprotonated. At pH ≈ pKa, both forms are present in similar amounts.

Acidic Group Charge Pattern

Acidic groups (COOH, SH, phenolic OH): protonated form is neutral (0), deprotonated form carries −1 charge.

Basic Group Charge Pattern

Basic groups (NH3+, imidazole, guanidinium): protonated form carries +1 charge, deprotonated form is neutral (0).

Typical pKa: α-Carboxyl

α-carboxyl group pKa is around 2. At physiological pH (~7.4) it is mostly deprotonated, carrying −1 charge.

Typical pKa: α-Amino

α-amino group pKa is around 9. At physiological pH (~7.4) it is mostly protonated, carrying +1 charge.

The pH at which an ionizable group is 50% protonated and 50% deprotonated; a measure of acid strength.

The sum of all individual charges on a molecule at a given pH.

State in which an ionizable group has an extra proton (H+). Basic groups are usually positively charged when protonated; acidic groups are usually neutral when protonated.

A molecule that has both positive and negative charges but an overall net charge of zero.

pKa, Charge, and pH: Predicting Ionization Like a Pro — BCH 204 Essentials: Structure, Properties, and Functions of the 20 Amino Acids

Q: You have a generic carboxyl group with pKa = 4.0. At pH 6.0, which statement is MOST accurate?

It is mostly deprotonated (–COO−) and negatively charged.. pH 6 is 2 units above the pKa of 4. For an acidic group, pH > pKa means it is mostly deprotonated. Two units above pKa corresponds to about 99% deprotonated, so the carboxyl is mainly –COO− (−1 charge).

Step 1 – Why pKa and Charge Matter

Charge Rules Biology

Charge affects solubility, binding, enzyme activity, and folding. To predict charge, you must connect ionizable groups, their pKa values, and the pH of the environment.

Goal of This Module

Rather than memorizing every pKa, you will learn typical ranges and a simple decision rule to decide if a group is protonated or deprotonated, and thus charged or neutral.

Assumptions

We use standard aqueous conditions and approximate pKa ranges consistent with modern biochemistry references as of 2026. Focus on patterns, not perfect numbers.

Step 2 – Ionizable Groups on Amino Acids

Backbone Groups

All amino acids have an α-carboxyl (pKa ~2) and an α-amino (pKa ~9). Carboxyl: neutral when –COOH, −1 when –COO−. Amino: +1 when –NH3+, neutral when –NH2.

Acidic Side Chains

Asp (D) and Glu (E) have carboxyl side chains, pKa ~4. They behave like small acids: neutral –COOH at low pH, −1 –COO− at higher pH (including pH 7).

Basic Side Chains

His (pKa ~6–6.5), Lys (~10.5), Arg (~12.5). They are neutral when deprotonated and +1 when protonated. At pH 7, Lys and Arg are mostly +1; His is partly +1.

Special Ionizable Side Chains

Cys (thiol, pKa ~8.3) and Tyr (phenolic OH, pKa ~10.1) can be deprotonated to carry −1 charge, but at pH 7 they are usually uncharged.

Step 3 – The pH vs pKa Rule (Henderson–Hasselbalch Thinking)

Key Equation

For HA ⇌ H+ + A−: pH = pKa + log([A−]/[HA]). This relates pH, pKa, and the ratio of deprotonated to protonated forms.

pH vs pKa Rules

If pH < pKa, the group is mostly protonated. If pH > pKa, the group is mostly deprotonated. If pH ≈ pKa, you have a mix of both (about 50:50 at pH = pKa).

Protonation and Charge

Acidic groups: protonated 0, deprotonated −1. Basic groups: protonated +1, deprotonated 0. Always decide protonation first, then assign charge.

Step 4 – Visualizing Charge at Low, Neutral, and High pH

Glycine at Low pH

At pH 1, both α-COOH (pKa ~2) and α-NH3+ (pKa ~9–10) are protonated. Glycine has –COOH (0) and –NH3+ (+1), so net charge ≈ +1.

Glycine at Neutral pH

At pH 7, α-COOH is deprotonated (–COO−, −1), α-NH3+ is still protonated (+1). Net charge ≈ 0. This internal +1 and −1 is the zwitterion form.

Glycine at High pH

At pH 12, α-COOH is −1 (–COO−), α-amino is neutral (–NH2). Net charge ≈ −1. Low pH favors positive charge; high pH favors negative charge.

Step 5 – A Systematic 4-Step Method for Net Charge

Step 1: List Groups

Always list all ionizable groups: N-terminus, C-terminus, and any side chains from D, E, H, C, Y, K, R (plus special cases if given).

Step 2: Label and Recall pKa

Mark each as acidic (COOH, SH, phenolic OH) or basic (NH3+, imidazole, guanidinium) and recall typical pKa ranges for each category.

Step 3 and 4: Compare and Sum

Compare pH vs pKa to decide protonated vs deprotonated, then assign charge per group and sum them to get the net charge.

Step 6 – Worked Examples: Single Amino Acids

Glu at pH 7

Glu: α-COOH, α-NH3+, side chain COOH. At pH 7, both carboxyls are deprotonated (−1 each), amino is +1. Net charge: −1.

Lys at pH 7

Lys: α-COOH, α-NH3+, side chain NH3+. At pH 7, carboxyl is −1, both amino groups are +1. Net charge: +1.

Pattern at Physiological pH

At pH ~7, acidic residues (D, E) tend to be −1, basic residues (K, R) +1. Histidine is partially protonated and often near-neutral overall.

Step 7 – Try It Yourself: Quick Charge Estimation

Use the 4-step method mentally. Do not worry about exact pKa values; use the ranges given earlier.

Task 1: Histidine at pH 7.4

List ionizable groups:

α-COOH (acidic, pKa ~2)
α-NH3+ (basic, pKa ~9)
Side chain imidazole (basic, pKa ~6–6.5)

Decide protonation at pH 7.4:

α-COOH: pH 7.4 > 2 → deprotonated → −1
α-NH3+: pH 7.4 < 9 → protonated → +1
Imidazole: pH 7.4 > 6–6.5 → mostly deprotonated (but not 100%) → approx 0

Approximate net charge:

−1 (COO−) + +1 (NH3+) + ~0 (imidazole) ≈ 0

Write down your reasoning in this format:

Groups:
Protonation states at pH 7.4:
Charges:
Net charge:

Task 2: Cysteine at pH 8.5

Use the same structure:

Hints:

Cys has α-COOH (pKa ~2), α-NH3+ (pKa ~9), and –SH side chain (pKa ~8.3).
At pH 8.5, the thiol is near its pKa, so a noticeable fraction is deprotonated.

Predict:

Is the side chain mostly –SH (0) or –S− (−1)?
What is the approximate net charge?

Pause and answer before moving on.

Step 8 – Concept Check: pH vs pKa Logic

Answer this quick question to check your understanding of pH vs pKa.

You have a generic carboxyl group with pKa = 4.0. At pH 6.0, which statement is MOST accurate?

It is mostly protonated (–COOH) and neutral.
It is about 50% protonated and 50% deprotonated.
It is mostly deprotonated (–COO−) and negatively charged.

Show Answer

Answer: C) It is mostly deprotonated (–COO−) and negatively charged.

pH 6 is 2 units above the pKa of 4. For an acidic group, pH > pKa means it is mostly deprotonated. Two units above pKa corresponds to about 99% deprotonated, so the carboxyl is mainly –COO− (−1 charge).

Step 9 – Peptide Example: Ala–Lys–Glu at pH 7

Identify Groups in Ala–Lys–Glu

Ionizable groups: N-terminus (Ala), C-terminus (Glu), Lys side chain NH3+, Glu side chain COOH. Internal backbone groups are not ionizable.

Decide Protonation at pH 7

N-term (pKa ~9): +1. C-term (pKa ~2): −1. Lys side chain (pKa ~10.5): +1. Glu side chain (pKa ~4.2): −1.

Net Charge of the Peptide

Sum: +1 (N-term) −1 (C-term) +1 (Lys) −1 (Glu) = 0. Ala–Lys–Glu is overall neutral at pH 7 despite having both acidic and basic residues.

Step 10 – Quick pKa and Charge Flashcards

Use these flashcards to reinforce typical pKa ranges and charge patterns.

Rule: pH vs pKa and Protonation: If pH < pKa, the group is mostly protonated. If pH > pKa, the group is mostly deprotonated. At pH ≈ pKa, both forms are present in similar amounts.
Acidic Group Charge Pattern: Acidic groups (COOH, SH, phenolic OH): protonated form is neutral (0), deprotonated form carries −1 charge.
Basic Group Charge Pattern: Basic groups (NH3+, imidazole, guanidinium): protonated form carries +1 charge, deprotonated form is neutral (0).
Typical pKa: α-Carboxyl: α-carboxyl group pKa is around 2. At physiological pH (~7.4) it is mostly deprotonated, carrying −1 charge.
Typical pKa: α-Amino: α-amino group pKa is around 9. At physiological pH (~7.4) it is mostly protonated, carrying +1 charge.
Side Chain pKa: Asp and Glu: Asp and Glu side chain carboxyl pKa values are about 3.9–4.2. At pH 7 they are deprotonated and carry −1 each.
Side Chain pKa: Lys and Arg: Lys side chain pKa ~10.5, Arg ~12.5. At pH 7 they are strongly protonated and carry +1 each.
Side Chain pKa: His: His side chain pKa is ~6–6.5. Around physiological pH it is partially protonated, often treated as roughly neutral overall unless specified.
Side Chain pKa: Cys and Tyr: Cys pKa ~8.3, Tyr pKa ~10.1. At pH 7 they are mostly uncharged; at higher pH they can deprotonate and carry −1.
Definition: Zwitterion: A molecule with both a positive and a negative charge but overall net charge 0, such as most free amino acids at neutral pH.

Key Terms

pKa: The pH at which an ionizable group is 50% protonated and 50% deprotonated; a measure of acid strength.
net charge: The sum of all individual charges on a molecule at a given pH.
protonated: State in which an ionizable group has an extra proton (H+). Basic groups are usually positively charged when protonated; acidic groups are usually neutral when protonated.
zwitterion: A molecule that has both positive and negative charges but an overall net charge of zero.
deprotonated: State in which an ionizable group has lost a proton (H+). Acidic groups are usually negatively charged when deprotonated; basic groups are usually neutral when deprotonated.
ionizable group: A functional group that can gain or lose a proton depending on pH, changing its charge state.
basic side chain: An amino acid side chain that can accept a proton and is usually positively charged when protonated (e.g., Lys, Arg, His).
physiological pH: The approximate pH of most human body fluids, around 7.4.
acidic side chain: An amino acid side chain that can donate a proton and is usually negatively charged when deprotonated (e.g., Asp, Glu).
Henderson-Hasselbalch equation: An equation relating pH, pKa, and the ratio of deprotonated to protonated species: pH = pKa + log([A−]/[HA]).