chromatogram at the end

AMINO ACIDS - natural polymers - POLYPEPTIDES, PROTEINS & ENZYMES

Doc Brown's GCSE/IGCSE/O level KS4 science-CHEMISTRY Revision Notes

 

13. Amino acids, proteins, enzymes & chromatography

What is an amino acid? What are proteins? What do proteins do? How are proteins formed from amino acids? How can we use chromatography to investigate protein structure? A spot of protein cooking chemistry! What happens when meat or eggs are cooked? These revision notes on amino acids, protein structure and enzyme function should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (91), (9-5) & (5-1) science courses.


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13. Naturally Occurring Molecules from plants and animals

13a. Proteins and Amino Acidsand DNA

  • Amino acids are carboxylic acids (like ethanoic acid with the -COOH group) but one of the hydrogen atoms of the 2nd carbon atom is substituted with an amino group (a nitrogen + two hydrogens gives -NH2).
    • Another hydrogen on the same 2nd carbon can be substituted with other groups of atoms (R) to give a variety of amino acids.
    • or The simplest is aminoethanoic acid or 'Glycine'
    • and another amino acid called 2-aminopropanoic acid or 'Alanine'
  • All amino acids have the general structure H2N-CH(R)-COOH (see diagram by 5b heading).
    • R can vary, think of it as the 'Rest of the molecule!
    • R = H for Glycine, R = CH3 for Alanine.
    • do better diagrams of amino acids and polypeptides
  • Amino acids can polymerise together, by condensation polymerisation, forming polypeptides.
    • The peptide linkage is formed by elimination of water between two amino acids.
    • The simplest amino acid is glycine H2NCH2COOH and the polymerisation can be written as ...
      • n H2NCH2COOH  ===>  (-NHCH2COO-)n + nH2O,
      • where n can be quite a large number in the polymer.
    • In general the polymerisation to form a protein or polypeptide is ...
    • HNH-CH(R)-COOH + HNH-CH(R)-COOH ==> H2N-CH(R)-CO-HN-CH(R)-COOH + H2O ...
    • ... to form one peptide linkage, so ...
    • n H2N-CH(R)-COOH ==> -NH-CO-CH(R)-NH-CO-CH(R)-NH-CO-CH(R)-NH-CO-CH(R)- etc. n units long
    • ... where R is variable chemical group as there over 20 known amino acids,
    • and so proteins are long chain polypeptides and are natural condensation polymers of amino acids.
    • Each polypeptide, protein, enzyme etc. has its own unique sequence of amino acids (all encoded for in your DNA!)
  • Proteins have the same (amide) linkages as nylon but with different units.
  • Proteins are an important component of tissue structure and enzymes (powerful biological chemical catalysts) are also protein molecules. Proteins tend to adopt a particular three dimensional shape (3D) which aids its function.
    • Apart from the structural proteins in you body e.g. muscle tissue, enzymes are protein molecules wrapped into a specific 3D shape to carry out their catalytic function.
    • For more detailed notes see Enzymes and Biotechnology
  • When proteins are heated with aqueous hydrochloric acid or sodium hydroxide solution they are hydrolysed to amino acids.

    • see chromatography below, about how amino acids are identified in proteins.

  • A spot of cooking chemistry!

    • Food is cooked for several reasons:

      • The high cooking temperature kills harmful microbes-bacteria, as long as cooked for the required time at a high enough temperature.

      • It may improves the texture.

      • It may improve the flavour and taste (but remember some foods might taste better raw e.g. lettuce!)

      • It makes it easier for the body to digest the food.

    • Most of meat from animals consists of protein together with smaller amounts of water and fat. Eggs and fish are also good sources of protein.

    • Protein molecules have a definite shape (diagram 1. above).

    • During the cooking of meat irreversible chemical changes take place.

    • The complex and specific structure of protein molecules is partly broken down in the cooking process.

    • The high cooking temperature promotes particular chemical reactions to happen.

    • The structure changes and some of the chemical bonds are broken and new molecules can be formed that have a different taste-flavour and texture giving the food its own characteristic 'cooked' character.

    • The breaking down of protein complex protein molecules is called denaturing.

    • A similar process happens in the cooking of carbohydrate foods like potatoes which are broken down into far more readily digestible molecules by breaking down the cell walls.

13b. Chromatography - a method of analysis

  • Hydrolysis means breaking down a molecule with water to form two or more products.

    • Hydrolysis is accelerated if the substance is heated with acid or alkali solutions.

  • When proteins are heated with aqueous acid they are hydrolysed to amino acids.

  • Acid hydrolysis of complex carbohydrates (e.g.. starch) gives simple sugars.

  • (1)chromatography at start  (2)chromatogram at the end  (3)chromatography (4)diagram of paper/thin layer chromatography at the end

  • Paper or Thin layer chromatography is used to separate coloured compounds (illustrated above).

    • (1) samples spotted onto start line, paper placed in solvent, but below start line of pencil.

    • (2) Solvent rises up paper.

    • (3) When solvent near top of paper, remove paper to dry. For colourless amino acids, you spray the paper with ninhydrin which gives a purple spot for each amino acid.

    • (4) You can then measure the Rf values to identify amino acids in the mixture.

    • To illustrate the method I've described the separation of coloured dye molecules. 1 to 5 represent five pure compounds, 6 is a mixture. Red, brown and blue make up the mixture because its spots horizontally line up with the three known colours.

      • The substances (solutes) to be analysed must dissolve in the solvent, which is called the mobile phase because it moves. The solvent may be water or an organic liquid like an alcohol (e.g. ethanol) or a hydrocarbon, so-called non-aqueous solvents.

      • The paper or thin layer of material on which the separation takes place is called the stationary or immobile phase because it doesn't move.

      • The distance a substance moves, compared to the distance the solvent front moves (top of grey area on diagram 2) is called the reference or Rf value and has a value of 0.0 (not moved - no good), to 1.0 (too soluble - no good either), but Rf ratio values between 0.1 and 0.9 can be useful for analysis and identification.

      • Rf = distance moved by dissolved substance (solute) / distance moved by solvent

  • However, amino acids and sugars are colourless, but can still be separated in this way, so read on!

  • Thin layer or paper chromatography can still used to separate and identify the products of hydrolysis of carbohydrates and proteins because you make them coloured by using another chemical reagent.

    • The hydrolysis can be done by boiling the carbohydrate or protein with hydrochloric acid.

    • The hydrolysed mixture is then 'spotted' onto the pencil base line of the chromatography paper.

      • Known sugars or amino acids are also spotted onto the base line too.

      • The prepared paper is then placed vertically in a suitable solvent, which rises up the paper.

    • Since the products are colourless, the dried chromatogram is treated with another chemical to produce a coloured compound.

      • Ninhydrin produces purple spots with amino acids

      • and resorcinol makes coloured spots with sugars.

    • You can then tell which amino acids made up the protein or the sugars from which the carbohydrate was formed.

      • The number of different spots tells you how many different amino acids or sugars made up the natural macromolecule.

      • Spots which horizontally match the standard known molecule spots confirm identity.

      • Starch gives one spot because only glucose is formed on hydrolysis.

        • (C5H10O5)n + nH2O ==> n C6H12O6 (where n is a very large number)

    • More on thin layer/paper chromatography.

    • Note that if organic compounds are gases or volatile (easily vapourised) liquids, they can be analysed using gas-liquid chromatography (in section 6. of the GCSE Extra Industrial Chemistry page).

 

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Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences Chemistry revision notes for GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising AQA GCSE Science AQA GCSE Chemistry, Edexcel GCSE Science, Edexcel GCSE Chemistry, OCR 21st Century Science Chemistry, OCR Gateway Science chemistry, WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10) GCSE chemistry IGCSE chemistry revision notes on structure amino acids polypeptides natural polymer KS4 GCSE Science revision notes on structure amino acids polypeptides natural polymer GCSE chemistry guide notes on structure amino acids polypeptides natural polymer for schools colleges academies science course tutors images pictures diagrams for structure amino acids polypeptides natural polymer science chemistry revision notes on structure amino acids polypeptides natural polymer for revising chemistry module topics notes to help on understanding of structure amino acids polypeptides natural polymer university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 8 grade 9 grade10 AQA chemistry science GCSE notes on structure amino acids polypeptides natural polymer Edexcel chemistry science notes on structure amino acids polypeptides natural polymer for OCR 21st century chemistry science notes on structure amino acids polypeptides natural polymer OCR GCSE Gateway science chemistry notes on structure amino acids polypeptides natural polymer WJEC gcse science chemistry notes on structure amino acids polypeptides natural polymer CCEA/CEA gcse chemistry notes science O level chemistry notes for structure amino acids polypeptides natural polymer IGCSE chemistry revision notes on structure amino acids polypeptides natural polymer O level chemistry notes gcse chemistry revision free detailed notes on chromatography of amino acids from proteins enzymes to help revise igcse chemistry igcse chemistry revision notes on chromatography of amino acids from proteins enzymes O level chemistry revision free detailed notes on chromatography of amino acids from proteins enzymes to help revise gcse chemistry free detailed notes on chromatography of amino acids from proteins enzymes to help revise O level chemistry free online website to help revise chromatography of amino acids from proteins enzymes for gcse chemistry  free online website to help revise chromatography of amino acids from proteins enzymes for igcse chemistry free online website to help revise O level chromatography of amino acids from proteins enzymes chemistry how to succeed in questions on chromatography of amino acids from proteins enzymes for gcse chemistry how to succeed at igcse chemistry how to succeed at O level chemistry a good website for free questions on chromatography of amino acids from proteins enzymes to help to pass gcse chemistry questions on chromatography of amino acids from proteins enzymes a good website for free help to pass igcse chemistry with revision notes on chromatography of amino acids from proteins enzymes a good website for free help to pass O level chemistry

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