AQA A Level Chemistry复习笔记7.7.2 Proteins

Proteins

 

  • Each amino acid contains an amine (-NH2) and carboxylic acid (-COOH) group
  • The -NH2 group of one amino acid can react with the -COOH group of another amino acid in a condensation reaction to form a dipeptide
    • The new amide bond between two amino acids is also called a peptide link or peptide bond

     

  • Since this is a condensation reaction, a small molecule (in this case H2O) is eliminated
  • The dipeptide still contains an -NH2 and -COOH group at each end of the molecule which can again participate in a condensation reaction to form a tripeptide

 

7.6-Nitrogen-Compounds-Formation-of-Peptide-Bonds

A peptide bond is an amide bond between two amino acids

 

  • A polypeptide is formed when many amino acids join together to form a long chain of molecules

7.6-Nitrogen-Compounds-Polypeptides

A polypeptide is a long chain of amino acid molecules joined together

 

The structure of proteins

  • There are four levels of structure in proteins, three are related to a single polypeptide chain and the fourth level relates to a protein that has two or more polypeptide chains
  • Polypeptide or protein molecules can have anywhere from 3 amino acids (Glutathione) to more than 34,000 amino acids (Titan) bonded together in chains

Primary

  • The sequence of amino acids bonded by covalent peptide bonds is the primary structure of a protein
  • The primary structure is specific for each protein (one alteration in the sequence of amino acids can affect the function of the protein)

Primary-structure

The primary structure of a protein. The three-letter abbreviations indicate the specific amino acid (there are 20 commonly found in cells of living organisms)

 

Secondary

  • The secondary structure of a protein occurs when the weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms to form hydrogen bonds
  • There are two shapes that can form within proteins due to the hydrogen bonds:
    • α-helix
    • β-pleated sheet

     

  • The α-helix shape occurs when the hydrogen bonds form between every fourth peptide bond (between the oxygen of the carboxyl group and the hydrogen of the amine group)
  • The β-pleated sheet shape forms when the protein folds so that two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds
  • Most fibrous proteins have secondary structures (e.g. collagen and keratin)
  • The secondary structure only relates to hydrogen bonds forming between the amino group and the carboxyl group (the ‘protein backbone’)
  • The hydrogen bonds can be broken by high temperatures and pH changes

 

Secondary-structure

The secondary structure of a protein with the α-helix and β-pleated sheet shapes highlighted. The magnified regions illustrate how the hydrogen bonds form between the peptide bonds

 

Tertiary

  • Further conformational change of the secondary structure leads to additional bonds forming between the R groups (side chains)
  • The additional bonds are:
    • Hydrogen (these are between R groups)
    • Disulphide (only occurs between cysteine amino acids)
    • Ionic (occurs between charged R groups)
    • Weak hydrophobic interactions (between non-polar R groups)

     

  • This structure is common in globular proteins

Tertiary-structure

The tertiary structure of a protein with hydrogen bonds, ionic bonds, disulphide bonds and hydrophobic interactions formed between the R groups of the amino acids

 

Summary of the types of bonding in peptides Table

Table_-16-Protein_Structures

 

Exam Tip

You should be able to draw the peptide formed by the joining of up to three amino acids.

Hydrolysis of Proteins

  • Hydrolysis of proteins is the reverse reaction of condensation in which the peptide link is broken and water added, hence the term hydrolysis
  • During hydrolysis reactions polypeptides are broken down to amino acids when the addition of water breaks the peptide bonds
  • The condensation and hydrolysis of a dipeptide is shown here for reference, but the reaction is identical with a protein chain

Dipeptide-formation

Amino acids are bonded together by covalent peptide bonds to form a dipeptide in a condensation reaction

  • The hydrolysis reaction can be carried out by chemical means or using enzymes
  • Concentrated hydrochloric acid is the reagent used and the mixture is boiled for many hours as the reaction is slow
  • With enzyme the reaction occurs at room temperature

Identifying Amino Acids

  • After hydrolysis, the amino acid components from polypeptides can be identified by using the technique of thin layer chromatography (TLC)
  • This technique is described in more detail in a later section
  • Although the amino acids have the same basic structure the R group changes the overall polarity of the molecule so the amino acids will rise up the TLC at different rates
  • Since amino acids are colourless the TLC plate has to either be sprayed with a locating agent such as ninhydrin which stains the amino acids, or the plate must be illuminated under a UV light
  • A TLC plate can be used to calculate Rf values for compounds
  • 1.-Thin-Layer-Chromatography-Basics-equationThese values can be used alongside other analytical data to deduce composition of mixtures
  • The Rf value (retention factor) can be determined and use to identify a specific amino acid

8.1-Analytical-Techniques-Calculating-Rf-Values

Rf values can be calculated by taking 2 measurements from the TLC plate

 

  • Sometimes amino acids have very similar values in the same solvent, so a further technique of two dimensional TLC can be used
  • In this technique the same plate is run through two different solvents
  • A square TLC plate is used and run through the first solvent, then the plate is turned through 90o and run through the second solvent
  • Two Rf  values are determined allowing greater confidence in identifying the amino acids

 

7.7.2-2D-TLC-for-amino-acids

Two dimensional TLC allows greater confidence in identifying amino acids

 

 

转载自savemyexams

 

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