1. Starter

2. Why are these pigments coloured?

3. Biological pigments
B9 - Biochemistry

4. What you need to know
•Biological pigments are coloured compounds produced by metabolism. • The colour of pigments is due to highly conjugated systems with delocalized electrons, which have intense absorption bands in the visible region. • Porphyrin compounds, such as hemoglobin, myoglobin, chlorophyll and many cytochromes are chelates of metals with large nitrogen-containing macrocyclic ligands. • Hemoglobin and myoglobin contain heme groups with the porphyrin group bound to an iron(II) ion. • Cytochromes contain heme groups in which the iron ion interconverts between iron(II) and iron(III) during redox reactions. • Anthocyanins are aromatic, water-soluble pigments widely distributed in plants. Their specific colour depends on metal ions and pH. • Carotenoids are lipid-soluble pigments, and are involved in harvesting light in photosynthesis. They are susceptible to oxidation, catalysed by light. • Explanation of the sigmoidal shape of hemoglobin’s oxygen dissociation curve in terms of the cooperative binding of hemoglobin to oxygen. • Discussion of the factors that influence oxygen saturation of hemoglobin, including temperature, pH and carbon dioxide. • Description of the greater affinity of oxygen for foetal hemoglobin. • Explanation of the action of carbon monoxide as a competitive inhibitor of oxygen binding. • Outline of the factors that affect the stabilities of anthocyanins, carotenoids and chlorophyll in relation to their structures. • Explanation of the ability of anthocyanins to act as indicators based on their sensitivity to pH. • Description of the function of photosynthetic pigments in trapping light energy during photosynthesis. • Investigation of pigments through paper and thin layer chromatography.

5. L.O -Biological pigments are coloured compounds produced by metabolism.
In order to absorb electromagnetic radiation in the UV-Vis region of the spectrum, molecules must generally contain a double bond in the form of C=C, C=O or a benzene ring. These groups, which give rise to absorptions in the UV-Vis region, are called chromophores.

6. L.O -Biological pigments are coloured compounds produced by metabolism.
EM radiation in UV/Vis region of the spectrum is absorbed to promote electrons from a low energy (molecular orbital) in molecules to a higher energy level(molecular orbital)

7. Additional info

8. L.O - The colour of pigments is due to highly conjugated systems with delocalized electrons, which have intense absorption bands in the visible region.
The bonds highlighted below form a conjugated system

10. Increase conjugation - increase λ

11. How would you explain this trend?

12. Question
Identify all conjugated and isolated double bonds in the structures below. For each conjugated pi system, specify the number of overlapping p orbitals, and how many pi electrons are shared among them.

13. Why chlorophyll appears green

14. What colour will these substances have?

15. Answers
Substance A absorbs most light except red light so appears red Substance B absorbs most light except violet/blue so appears blue

16. L.O - Porphyrin compounds, such as hemoglobin, myoglobin, chlorophyll and many cytochromes are chelates of metals with large nitrogen-containing macrocyclic ligands.
Chelate - compounds consisting of a central metal atom attached to a large molecule, called a ligand, in a cyclic or ring structure.

18. L.O. - Hemoglobin and myoglobin contain heme groups with the porphyrin group bound to an iron(II) ion.

20. L.O - Explanation of the sigmoidal shape of hemoglobin’s oxygen dissociation curve in terms of the cooperative binding of hemoglobin to oxygen.
According to the induced fit model, the binding of a substrate (oxygen molecule) to a free active site (deoxygenated heame) alters the shape of the haemoglobin molecule, including the shapes of the remaining active sites
These changes increase the affinity of the partly oxygenated haemoglobin to molecular oxygen
This is termed cooperative binding
As a result the kinetic curve of haemoglobin-oxygen interaction does not obey the Michaelis-Menton model – Will cover this in SL

21. Cooperative binding in Haemogolobin

22. Cooperative binding in Haemogolobin
Cooperative binding increases the efficiency of oxygen transport
Most of the active sites become occupied by oxygen molecules quickly and carry as much oxygen as possible from the lungs to other tissues
As the partial pressure of O2 decreases some oxygen molecules are released, reducing the affinity of haemoglobin for oxygen and accelerating the loss of remaining O2 molecules
This means that in venous blood, with low partial pressure of oxygen, there is not much affinity for oxygen and so the last oxygen molecules are released and haemoglobin becomes ready for the next cycle of oxygen transport

23. Other factors affecting affinity for oxygen
Temperature:
Too high – active site changes shape
Low – increases affinity of haemoglobin for oxygen
pH:
Low – protons (or CO2) bind to side-chains of amino acids in haemoglobin and act as non-competitive inhibitors

24. Foetal haemoglobin
Structurally different from adult haemoglobin and can bind to oxygen more efficiently
Needed, as by the time the blood gets to the foetus, it is partially deoxygenated
Less sensitive to certain inhibitors
Steeper saturation curve allows HbF to release a greater proportion of oxygen to developing tissues and operate even at a very low partial pressure of oxygen

25. Carbon Monoxide Poisoning
Competitive inhibitor, preventing the delivery of oxygen to body tissues
Complex of CO and haemoglobin, carboxyhaemoglobin is very stable and accumulate in the blood until most of the active sites are occupied by CO molecules
0.2% CO in air is problematic, 0.5% can be fatal
Can be displaced by inhaling pure oxygen
Heavy smokes, truck drivers and traffic police often show symptoms of chronic hypoxia due to constant low level exposure

26. L.O - Cytochromes contain heme groups in which the iron ion interconverts between iron(II) and iron(III) during redox reactions.

27. L.O - Cytochromes contain heme groups in which the iron ion interconverts between iron(II) and iron(III) during redox reactions.

28. Anthocyanin
Here is an example of an anthocyanin Predict the following based on it’s structure -solubility -aromatic -boiling point -colour

29. L.O - Anthocyanins are aromatic, water-soluble pigments widely distributed in plants. Their specific colour depends on metal ions and pH.

30. Anthocyanins

32. Anthocyanins
They are very common pigments in plants. Anthocyanins are the principal pigments responsible for the pink, red, blue and purple colours of many fruits and vegetables including red cabbage and grapes Anthocyanins have molecules that contain aromatic rings but are soluble in water because they also have a large number of OH groups. The presence of metal ions can affect the colour of anthocyanins. Anthocyanins can form vivid, deep-coloured complexes with metal ions such as Al3+ and Fe3+. Complex ions are formed with the anthocyanin molecules acting as ligands.

33. Anthocyanins and metal ligand

34. Changing pH

36. Uses of anthocyanins
Bright colours of flowers and fruits attract insects and animals that provide pollination and seed dispersal
Protect plant tissues from excessive sun exposure to UV
Antioxidants due to their highly conjugated electron systems
Natural acid-base indicators and organic components of dye-sensitised solar cells (in energy topic)
Artificial food colourings in the EU and USA

37. L.O - Carotenoids are lipid-soluble pigments, and are involved in harvesting light in photosynthesis. They are susceptible to oxidation, catalysed by light.
Carotenoids are the most widespread pigment found in nature – this is primarily due to their abundance in algae.
Carotenoids generally absorb in the blue–violet region of the visible spectrum and therefore transmit or reflect longer wavelengths of the visible spectrum and so have colours in the yellow–orange–red region.
Carotenoids are present in carrots, tomatoes, watermelon, sweet peppers and saffron etc.

38. Carotenoids
Most carotenoids are derived from a 40-carbon polyene chain (multiple C=C double bonds). The ends of the chain may terminate in a cyclic (ring) group which may or may not have oxygen-containing functional groups attached.