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Option B.9 Biological Pigments
Biological pigments are colored compounds produced by metabolism. Their color is due to highly conjugated systems with delocalized electrons, which absorb visible light. The longer the conjugated system, the longer the wavelengths absorbed and the color observed. Chlorophylls are examples of pigments with long conjugated systems that absorb blue-green and orange-red light, appearing green. The color of a pigment depends on the length of its conjugated system and the wavelengths of light it absorbs.
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Option B.9 Biological Pigments
- 1.
- 2. Understanding 1: Biological pigmentsare coloured compounds produced by metabolism
- 3. Chromophores • In orderto 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.
- 4. • Electromagnetic radiationin ultraviolet- visible region of the spectrum is absorbed to promote electrons from a low energy (molecular orbital) in molecules to a higher energy level(molecular orbital)
- 5. Understanding 2: The colourof pigments is due to highly conjugated systems with delocalized electrons, which have intense absorption bands in the visible region
- 6. Conjugated systems • Aconjugated system is a sequence of alternating single and double bonds in a molecule
- 7. • The bondshighlighted in figure below form a conjugated system
- 8. • Not partof this system • They are separated from the other double bonds by more than one single bonds BUT!!
- 9. • The doublebonds must alternate with single bonds
- 10. If ≥ 2single bonds between the double bonds then the system is not conjugated
- 11. • Another exampleof conjugated system…
- 12. • We cansee from this figure that electrons are delocalise in a conjugated system because p orbitals can overlap along the whole chain
- 13. Absorption of electromagnetic radiationand colour • For a compound to be coloured, its molecules must absorb visible light (electromagnetic radiation, wavelength about 400-750 nm) • Therefore, if a molecule absorb absorbs radiation between these wavelengths, it will be coloured.
- 14. • The longerthe conjugated system, the longer the wavelengths of the radiation absorbed. • If a conjugated system involves ≥ 8 doubles bond, the molecules should absorb in the visible region of the spectrum • Hence, be coloured.
- 15. The longer aconjugated chain (delocalised system), the longer the wavelength of radiation absorbed by a molecule. • A system of 11 conjugated double bonds • Absorbs light in the blue-green part of the visible spectrum • Appears red
- 16. • Only havea system with 5 conjugated double bonds • Does not absorb visible light • It only absorbs ultraviolet radiation • Therefore, it is colourless Reti nol
- 17. • Chlorophyll aand b have long conjugated systems • They absorb light in the 400-500 nm and 600-700 nm region
- 18. • The greenlight in the middle of the spectrum is not absorbed, and so these molecules look green in natural light.
- 19. Application & skills: Explanationof the sigmoidal shape of haemoglobin's oxygen dissociation curve in terms of the cooperative binding of haemoglobin to oxygen
- 20. The binding ofoxygen to haemoglobin • Hemoglobin consist of 4 polypeptides sub- units • Each of which contains a heme prosthetic group • With the iron at the centre of the heme • Having oxidation number of +2
- 21. The binding ofoxygen to haemoglobin • Each heme can carry one molecules of oxygen. • So, each hemoglobin unit can transport four molecules of oxygen.
- 22. The binding ofoxygen to haemoglobin • The iron in the heme can bond to 6 ligands • In the unbound state, the Fe2+ is bonded to 5 ligands: 4 : nitrogen atoms (of porphyrin) 1 : amino acid (that attached it to protein) • When molecular oxygen binds, this becomes the 6th ligands. • Called Oxygenated hemoglobin
- 23. The binding ofoxygen to haemoglobin • Binding of the oxygen molecules result in Fe2+ being oxidised to Fe3+ • In hemoglobin, the oxygen binds reversibly, allowing its release to tissue cells
- 24. Affinity of hemoglobinfor oxygen changes as the partial pressure of oxygen changes
- 25. The scale onthe y axis represents the fraction of iron ions bound to oxygen molecules. This is called oxygen binding curve/oxygen dissociation curve
- 26. Partial pressure ofoxygen low, hemoglobin has a low affinity for oxygen
- 27. Oxygen affinity increasesas the partial pressure of oxygen increases
- 28. • This suggestthat its becomes easier for oxygen to bind to hemoglobin when some oxygen molecules have already bound to the iron – cooperative binding
- 29. • Hemoglobin havetetrameric structure with 4 iron-heme complex. • The binding of oxygen to one of the iron ions in the tetramer changes the shape (conformation) of the protein • Its becomes easier for oxygen molecules to bind to the other sites
- 30. • This isan allosteric effect – the binding of a molecule at one site has an effect on another site.