Home

Chemistry

Amino acids, proteins and DNA

Amino acids and zwitterions

Select Lesson

Exam Board

Select an option

Practical skills

The scientific method

How to plan experiments

Improving repeatability and reproducibility

Presenting results using tables and graphs

Analysing results: anomalies and correlation

Evaluating experiments

Modern analytical techniques II

NMR spectroscopy

Proton NMR spectroscopy

Chromatography

Combining analytical techniques

Organic chemistry III

Benzene: structure and combustion

Electrophilic substitution

Friedel-Crafts reactions

Phenols

Aliphatic and aromatic amines

Reactions of amines

Forming amides

Condensation polymers

Amino acids and zwitterions

Grignard reagents

Synthetic routes

Practical techniques

Percentage composition and combustion analysis

Organic chemistry II

Optical isomers and chirality

Aldehydes and ketones

Carboxylic acids: properties and reactions

Esters: formation, properties and uses

Acyl chlorides: formation and reactions

Kinetics II

Measuring rates of reaction

Determining reaction orders

The initial rates method

Rate equations and the rate constant

The rate determining step

Halogenoalkane reaction mechanisms

Activation energy and the Arrhenius equation

Transition metals

Transition metals: electron configuration and oxidation number

Complex ions: formation, shape and isomerism

Colours of inorganic ions and complexes

Chromium and its reactions

Ligand substitution reactions

Transition metals as catalysts

Redox II

Electrochemical cells and redox

Electrode potentials and calculations

The electrochemical series

Energy storage cells and hydrogen fuel cells

Redox titrations

Energetics II

Lattice enthalpies and Born-Haber cycles

Polarisation and the Pauling scale

Enthalpy change in solution

Entropy and calculating entropy change

Gibbs free energy change

Acid-base equilibria

Acids and bases

pH calculations

The ionic product of water

pH curves and indicators

Buffer solutions and calculations

Equilibrium II

The equilibrium constant

Partial pressure and gas equilibria

Le Chatelier's principle and equilibrium constants

Equilibrium I

Reversible reactions

Le Chatelier's principle

Kinetics I

Collision theory and rates of reaction

Calculating the rate of reaction

Catalysts and the Maxwell-Boltzmann distribution

Energetics I

Enthalpy changes and reaction profiles

Standard enthalpy changes

Hess's Law and enthalpy cycles

Calculating bond enthalpies

Modern analytical techniques I

Mass spectrometry

IR spectroscopy

Organic chemistry I

Basic concepts of organic chemistry

Organic reactions and mechanisms

Structural isomerism

Alkanes and free radical substitution

Fractional distillation of crude oil

Fuels: combustion and emissions

Alkenes: bonding and reactivity

Stereoisomerism

Reactions of alkenes

Addition polymerisation

Halogenoalkanes and their reactions

Alcohol: classification and reactions

Oxidation of alcohols

Organic techniques

Formulae, equations and amounts of substances

The mole and Avogadro's constant

Calculations involving gases

Empirical and molecular formulae

Balancing equations

Acid-base titrations

Calculating uncertainty and errors

Atom economy and percentage yield

Inorganic chemistry and the periodic table

Group 2: ionisation energy and properties

Thermal stability of nitrates and carbonates

Chemical properties of Group 7

Reactions with halogens

Halide ion reactions

Testing for ions

Redox I

Calculating oxidation numbers

Oxidation, reduction and redox reactions

Bonding and structure

Ionic bonding

Covalent bonding

Shapes of molecules

Giant covalent and metallic structures

Electronegativity and polarisation

Intermolecular forces: types and examples

Hydrogen bonding

Solubility

Predicting structures and properties

Atomic structure and the periodic table

The atom: history, structure and isotopes

Relative masses and mass spectrometry

Electronic structure: shells and orbitals

Atomic emission spectra

Ionisation energies

Ionisation energy trends

Periodicity

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 $R_f$ 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.

$butanoic \ acid$ 

Next, write down the position of the amino group. Amino groups are always on $C_2$ .

$2-aminobutanoic \ acid$

Then, write down all the side chains in the amino acid. Valine has one methyl group on $C_3$ .

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

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.

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 $C_2$ . 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), $1 \, cm$ 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 $1 \, cm$ 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 $R_f$ values for each spot on the chromatogram. $R_f = \frac{distance\,travelled\,by\,spot}{distance\,travelled\,by\,solvent}$ Note: This value should always be between $0$ and $1$ . 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 $R_f$ 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 $R_f$ 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 $(SiO_2)$ or alumina ( $Al_2O_3)$ .

Learn with Basics

Length:

Unit 1

Polymers

Unit 2

The structure and function of proteins

Jump Ahead

Optional

Unit 3