Amino acids and zwitterions
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Explainer Video
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 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.
Firstly, find the longest carbon chain which includes the carbon chain and write down the parent name.
Next, write down the position of the amino group. Amino groups are always on .
Then, write down all the side chains in the amino acid. Valine has one methyl group on .
Therefore the systematic name of valine is .
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.
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 . They are therefore able to rotate plane polarised light and exist as optical isomers.
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
1. | Draw a horizontal line in pencil (as it is insoluble), 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 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 values for each spot on the chromatogram. Note: This value should always be between and . 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 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 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 or alumina (.
FAQs - Frequently Asked Questions
What is the isoelectric point?
The isoelectric point is the pH when the net charge of the amino acid is zero.
What is a zwitterion?
A zwitterion is a species which has both a positive and negative charge within the species, but the overall charge is neutral.
What does amphoteric mean?
Amphoteric means the molecules have both acidic and basic properties.