buochem lab midterm exam

• Colloid


  Suspension of particles larger than true solutions but still smaller to settle out by gravity, cannot be filtered by ordinary filters
• Colloid system


  • Dispersed phase (macromolecular solids like proteins and nucleic acids, liquids like oily fats)
  • Dispersion phase (medium in which insoluble materials are dispersed, may be solid, liquid or gas)
• Suspensoid


  Lyophobic colloid where water is the dispersing medium but the suspended particles have little or no affinity toward water
• Emulsoid


  Lyophilic colloid which hydrates readily and has an affinity between the dispersed phase and the dispersion medium
• Differences between Emulsoid and Suspensoid

  • Foam Formation (Stable vs Unstable)
  • Precipitation with Electrolytes (Very stable vs Unstable)
  • Reversibility (Reversible vs Irreversible)
• Dialysis


  Process where solvent molecules, small molecules and hydrated ions pass from a solution through a semipermeable membrane that holds back colloid particles and large molecules
• Types of dialysis membranes

  • Cellulose-based (PES, PAN, PVDF)
  • Regenerated cellulose-based
  • Composite
• Adsorption


  Process where a solid (activated charcoal) is used to remove a soluble substance (like methylene blue dye) from water
• Methylene blue is a monovalent cationic dye with synthetic origin and complex organic aromatic structure, making it inert and difficult to biodegrade
• Adsorbent


  Substance onto which liquid molecules get adsorbed
• Adsorbate


  Liquid molecules that get adsorbed onto the adsorbent
• Physisorption and Chemisorption


  Two categories of adsorption based on type of forces involved between adsorbent and adsorbate
• Methylene blue solution combined with animal charcoal gives a colorless filtrate
• Osmosis


  Spontaneous flow of solvent through a semipermeable membrane from a solution of low concentration to one of higher concentration
• Diffusion alone, without convection currents and gravity, is very difficult to demonstrate in liquids
• Diffusion is faster for small molecules like Potassium Permanganate than larger ones like Methylene Blue
• Protective colloid


  Colloid that helps to stabilize other colloids by adsorbing onto hydrophobic particles and forming a protective layer
• Surface tension


  Tension at the surface of a liquid due to unequal attraction of surface molecules compared to interior molecules
• Factors affecting surface tension

  • Temperature (decreases with increase)
  • Dissolved substances (inorganic salts slightly raise, organic substances lower, alkalis increase, acids decrease)
• Surface tension is involved in the process of digestion, as bile salts reduce the surface tension of lipids and assist emulsification
• Proteins and amino acids used in experiments

  • Egg albumin
  • Casein
  • Gelatin
  • Metaproteins, proteoses, peptones
• Types of amino acids

  • Neutral (Glycine, Alanine, Valine, etc.)
  • Acidic (Aspartic acid, Glutamic acid)
  • Basic (Lysine, Arginine, Histidine)

  
• Other amino acid classifications

  • Aliphatic
  • Hydroxy
  • Sulfur-containing
  • Aromatic
  • Dicarboxylic acid and their amides
  • Diamino acids
  • Imino acids (Proline)

  
• Solubility test


  Amino acids and proteins have varying solubility in water, dilute alkali and dilute acid depending on their structure and pH
• Biuret test


  Reaction that gives a color change (violet, purple, dark pink) depending on the number of peptide linkages in the protein
• Ninhydrin test


  Reaction that gives a purple color with α-amino acids, and a yellow color with imino acids like proline and hydroxyproline
• Biuret Test


  The reaction is so named since biuret (NH2-CO-NH-CO-NH2) formed by the condensation of two molecules of urea when heated
• Biuret Test


  • The colour varies depending on the number of peptide linkages: albumin/globulin give violet, proteoses purple and peptones dark pink colour indicating that albumin/globulins have largest number of peptide linkages and peptones the least

  
• Peptide linkage


  CO-NH is the peptide linkage in biuret. At least two peptide bonds in the molecule are required for a positive test
• Individual amino acids and dipeptides will not answer this test
• Ninhydrin Reaction


  1. amino acid + ninhydrin → aldehyde + hydrindantin + NH3 + CO2; hydrindantin + NH3 + ninhydrin → Ruhemann's purple + 3H2O
• Proteins do not give a true colour reaction; but N-terminal amino group of a protein can react with ninhydrin to produce a faint blue colour
• Ninhydrin Reaction Colours


  • All a-amino acids give purple colour, imino acids proline and hydroxyproline give yellow colour, glutamine and asparagine produce brown colour
• Ruhemann's Purple


  The coloured complex formed in the ninhydrin reaction
• Xanthoproteic Reaction


  Yellow colour is due to the formation of nitro derivatives of benzene ring containing amino acids (tyrosine and tryptophan), the colour turns orange due to ionization when alkali is added
• All proteins usually respond to the xanthoproteic test, nitration of phenylalanine under these conditions normally does not take place
• Hopkins-Cole Test


  Mercuric sulphate cause mild oxidation of indole group of tryptophan, which condenses with an aldehyde to give the colored complex
• Gelatin, poor in tryptophan, does not give the Hopkins-Cole test
• Sakaguchi Test


  Guanidino groups in arginyl residues of proteins react with the a-naphthol
• This test is given by albumin, globulin and gelatin as they contain arginine