Amino acids and zwitterions

Select Lesson

Exam Board

OCR A

OCR AAQAPearson Edexcel

Explainer Video

Tutor: Alisha

Summary

Amino acids and zwitterions

In a nutshell

Amino acids are amphoteric. They can exist as zwitterions at the isoelectric point. Amino acids can be identified using paper chromatography and TLC. This is because each amino acid has a different solubility, therefore have a different RfR_f​ value, in a given solvent. 



Nomenclature

Amino acids are amphoteric. This means they have both acidic and basic properties. This is because amino acids have an amino group (basic) and a carboxyl group (acidic).


Many amino acids have a common name as well as a systematic name.  


Example

The following amino acid is called valine. Give the systematic name for valine.


Chemistry; Organic chemistry III; KS5 Year 12; Amino acids and zwitterions


Firstly, find the longest carbon chain which includes the carbon chain and write down the parent name.


butanoic acidbutanoic \ acid​​


Next, write down the position of the amino group. Amino groups are always on  C2C_2.


2aminobutanoic acid2-aminobutanoic \ acid​​


Then, write down all the side chains in the amino acid. Valine has one methyl group on C3C_3.


Therefore the systematic name of valine is 2amino3methylbutanoic acid\underline{2-amino-3-methylbutanoic \ acid}.



Zwitterions

Amino acids exist as zwitterions. A zwitterion has both a positive and negative charge within the species, but the overall charge is neutral. Amino acids exist as zwitterions at their isoelectric point. The isoelectric point is the pH when the net charge of the amino acid is zero. The isoelectric point is dependant on the R group of the amino acids.


Chemistry; Organic chemistry III; KS5 Year 12; Amino acids and zwitterions
1. Ion at a lower pH than isoelectric point; 2. Zwitterion at isoelectric point; 3. Ion at a higher pH than isoelectric point.

Acidic conditions will lead to the amino group to become protonated. At the isoelectric point, both the amino and the carboxyl group are ionised. Under basic conditions the carboxyl loses a proton and becomes a carboxylate.



Chirality

Most amino acids are chiral because they have four different atoms or groups of atoms attached to C2C_2. They are therefore able to rotate plane polarised light and exist as optical isomers.​

Chemistry; Organic chemistry III; KS5 Year 12; Amino acids and zwitterions


Paper chromatography

Amino acids can be identified using paper chromatography. This is because each amino acid has a different R group which gives rise to different solubilities in a given solvent.


PROCEDURE


Chemistry; Organic chemistry III; KS5 Year 12; Amino acids and zwitterions
1.  Watch glass; Solvent front; 2. A component from the mixture; 4. Baseline; 5. Solvent.


1.

Draw a horizontal line in pencil (as it is insoluble), 1 cm1 \, cm from the bottom of the chromatography paper. This is your base line.

2.

Use a glass capillary tube to apply a spot of a sample of the mixture being investigated on the pencil line. 

3.

Add a solvent to a depth less than 1 cm1 \, cm into a beaker. 

4.

Put the chromatography paper in the beaker, ensuring the baseline is above the solvent level.

5.

Assemble the watch glass on top.

6.

Allow the solvent to travel close to the top of the paper.

7.

Each amino acid will have a different solubility in the solvent. The components in the mixture will separate out as they travel up the chromatography paper at different rates. 

8.

Remove the chromatography paper and using a ruler, draw a pencil line across the chromatography paper at the point which the solvent stopped travelling up to. This is called the solvent front.


You now have a chromatogram.

9.

Some amino acids are not coloured so you will need to spray ninhydrin solution. Circle the spots you identify using the spray.

10.

Measure the distance between the baseline and the solvent front - this is the distance travelled by the solvent. 

11

Measure the distance from the baseline to each centre of the spots the mixture has separated into.

12.

Calculate the RfR_f values for each spot on the chromatogram.


Rf=distance travelled by spotdistance travelled by solventR_f = \frac{distance\,travelled\,by\,spot}{distance\,travelled\,by\,solvent} ​​


Note: This value should always be between 00 and 11. Your value for the distance moved by the solvent is the same in each calculation.

13.

If the experiment was carried out under standard conditions compare your RfR_f values to known reference tables. Otherwise, repeat the experiment with a spot of substance you think is in the mixture to check if they have the same RfR_f value.​


Amino acids can also be separated and identified using thin-layer chromatography (TLC). The difference is that the stationary phase is a thin layer of silica (SiO2)(SiO_2) or alumina (Al2O3)Al_2O_3)​.



Read more

Learn with Basics

Length:
Polymers

Unit 1

Polymers

The structure and function of proteins

Unit 2

The structure and function of proteins

Jump Ahead

Amino acids and zwitterions

Unit 3

Amino acids and zwitterions

Final Test

Create an account to complete the exercises

FAQs - Frequently Asked Questions

What is the isoelectric point?

What is a zwitterion?

What does amphoteric mean?

Beta

Hey there, I'm Vulpy, your AI study buddy! I'm here to make your learning journey not only easier but also a whole lot more enjoyable. Whether you have questions, need some motivation, or just want to chat about your school challenges, I'm here to support you every step of the way.
Choose, in which mode would you like to engage with me today.
You must be logged in to use the AI chat