Predict amino-acid movement in electrophoresis
Use pH relative to the isoelectric point to predict whether an amino acid moves toward the anode, cathode or shows no net movement.
Section 34 · Nitrogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 34. Use the stated reagents, conditions and product scope. Written practice is self-checked This online lesson cannot demonstrate supervised laboratory skills.
Before you begin
- Recall nucleophilic substitution, reduction, acyl-chloride addition–elimination, acid–base equilibria and covalent bonding.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At pH above an amino acid's pI, what is its usual net charge?
What you will learn
- Compare solution pH with an amino acid's pI
- Predict its net charge
- Relate charge sign to electrode direction
Compare pH with pI
At pH below its isoelectric point, an amino acid is more protonated and has a net positive charge. At pH above pI, it is more deprotonated and has a net negative charge.
At pH equal to pI, its average net charge is zero, so it shows no net movement in an electric field.
Match charge to electrode
A positive ion moves toward the negative cathode. A negative ion moves toward the positive anode.
Thus below pI an amino acid moves toward the cathode; above pI it moves toward the anode; at pI it has no net movement. The syllabus asks learners to interpret outcomes, not assemble electrophoresis apparatus.
Pause and explain
Where does a net-positive amino acid move in electrophoresis?
Worked example
An amino acid has pI 6.0. Predict its charge and movement at pH 3.0, 6.0 and 9.0.
Show the worked solution
- Below pI, it is net positive.
- At pI, it has zero net charge.
- Above pI, it is net negative.
Answer pH 3: positive, toward cathode; pH 6: no net movement; pH 9: negative, toward anode.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An amino acid has pI 5.5. Predict its net charge and direction at pH 2.0.
Give me a hint
Compare pH to pI, then match sign to electrode.
Compare my reasoning
- pH 2.0 is below pI 5.5.
- The amino acid is net positive.
- Positive ions move toward the cathode.
Net positive; moves toward the negative cathode.
Look for these in your work
- pH comparison
- Charge
- Direction
A different amino acid has pI 8.0. Predict its movement at pH 10.0 and at pH 8.0.
Give me a hint
One pH is above pI and one equals pI.
Compare my reasoning
- At pH 10.0 it is net negative.
- Negative ions move toward the anode.
- At pH 8.0 its net charge is zero.
At pH 10.0 it moves toward the anode; at pH 8.0 it has no net movement.
Look for these in your work
- Above-pI state
- Anode
- At-pI state
A student predicts an amino acid below its pI will move to the anode because it has gained H⁺. Correct the charge-direction link.
Give me a hint
Amino-group protonation makes the molecule more positive.
Compare my reasoning
- Below pI, net charge is positive.
- Positive charge is attracted to the negative electrode.
- That electrode is the cathode.
Below pI it is net positive and moves toward the negative cathode, not the anode.
Look for these in your work
- Charge sign
- Electrode polarity
- Direction corrected
Common mistakes
- Sending positive ions to the positive anode
- Treating pI as a pH where no ionic groups exist
- Reversing below/above pI
- Assuming every amino acid has the same pI
What can you explain now?
At pH above an amino acid's pI, what is its usual net charge?
Compare with the explanation
Negative
The molecule is more deprotonated above pI.
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Make it stick
How do pH and pI determine amino-acid charge and electrophoresis direction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.