10.1 COMPARE THE SIMILARITIES AND DIFFERENCES IN THE STRUCTURE OF THE NATURAL PIGMENTS:ANTHOCYANINS, CAROTENOIDS, CHLOROP HYLL AND HEME
SIMILARITIES• BETWEEN HEME AND CHLOROPHYLL- Contain a planar heterocyclic unit called a porphin whose structure contain a cyclic system of conjugated double bonds.- Porphins with substituents in positions 1 to 8 are called porphyrins.
Resonance structure of porphine Conjugate double bonds is possible by means of alternating single and double bond.
DIFFERENCESANTHOCYANIN CAROTENOID CHLOROPHYLL HEME1. Has a C6C3C6 1. Derived from a 1. 2 types of 1. Hemoglobinskeleton with 2 40-carbon polyene chlorophyll: contains 4 hemeconjugated benzene chain. -chlorophyll a: blue- subunits whilerings isolated by an green myoglobin containsoxygen-containing -chlorophyll b : one heme ringpyran ring. yellow-green2. Always have a 2. May be 2. Chlorophyll b 2. A haemoglobinsugar residue at terminated by cyclic differs from molecule contains 4position 3 and end-groups and chlorophyll a by protein chainsglucose occur at may be having an aldehyde while myoglobinposition 5,7,3’ and complemented (-CHO) group in molecule contains4’. with oxygen placed of a methyl one protein chain containing group functional groups. (-CH3).
3. It is porphyrin 3. Each heme ringpigments, contains one ironcomposed of 4 ( II) ionpyrrole ring to formtetrapyrrole withmagnesium ioncomplex in thecentre of the ring. 4. The iron in the heme group is coordinated to the four nitrogen atoms in the prophyrin ring and also to a nitrogen atom from the histidine residue of the hemoglobin protein known as the globin
5. The 6 positionaround the iron ofthe heme isoccupied by oxygenwhen thehaemoglobinprotein isoxygenated
10.2 EXPLAIN WHYANTHOCYANINS, CAROTENOIDS, CHLOROP HYLL AND HEME GROUP FORM COLOURED COMPOUND WHILE MANY OTHERS ORGANIC MOLECULES ARE COLOURLESS.
a) Chromophores are unsaturated groups of atoms in organic compounds whose electron absorb radiation in the visible and ultraviolet region of the electromagnectic spectrum.b) The wavelength of light absorb by different chromophores are characteristic of the chromophores and can be used for their identification.c) They can change with temperature, solvent, extend of conjugation and the presence of saturated groups.
d) Anthocyanins• The absorbance spectrum is similar with cyanidin.• Cyanidin is the parent compound of the anthocyanins but lacks of glucose residue.• Under acidic conditions (low pH) cynidin is red.
• Graph• The absorbance at 375 nm lies in the UV region of the electromagnetic spectrum so it has no effect on the observed colour f cyaniding• But, the absorbance at the 530 nm lies in the blue-green region of the visible spectrum hence cyaniding transmits the complementary colour of the eye, namely red.• Although the absorbance spectra of anthocyanins will be similar with cyanidin but it will vary with due to the presence of differential functional group.• The colours of anthocyanins will also vary with temperature and pH due to the presence of forms in equilibrium.
e) In Carotenoids and Chlorophyll• Different molecule absorb light at different wavelength because of differences in their electronic structures.• Figure 2 shows the absorption spectra of the two closely related chlorophylls (a and b) and carotenoid found in plant.
• It is the electronic arrangement of a molecule that is responsible for the absorption of light in the ultraviolet and visible region of the electromagnetic spectrum.• It is the electron in the molecule that are largely responsible for the absorption of light in the visible.• A bond consist of two electron in a spin pair arrangement-one electron spins clockwise and the other spins anticlockwise.• The total potential energy increases as a result of absorption of light.• The absorption of light by electron in a bond can be viewed in terms of an energy level diagram. (Figure 3) * hf (photon) Figure 3
• The diagram shows the light wave (photons) as a squiggly arrow.• One of the electron and has absorbed this energy and moved from the ground state( ) to an excited state ( *).• Now the two electrons will no longer have complementary spins.• This energy jumped occurs only when light of exactly the right wavelength (energy) is absorbed.• The reflected light will have a different energy from the incident light and will have a different colour.
f) Heme• As the heme group is in red colour so the intensity of absorption of electron is high at the blue-green region of visible electromagnetic. spectrum.• Therefore the reflected light is red in colour.
10.3 DEDUCE WHETHER ANTHOCYANINSAND CAROTENOIDS ARE WATER SOLUBLE OR FAT SOLUBLE FROM THEIR STRUCTURE.
Natural pigment Explaination -freely soluble in water but poorly soluble in non-polar organic solvents -presence of 1 or more residues helps the anthocyanins maintains its solubility in water. Anthocyanins -the sugar residue contain 1 or more OH- group which can hydrogen bond with water molecules. -if sugar is hydrolyse, the solubility decreases.
Natural pigment Explaination -poorly soluble in water but freely soluble in non-polar organic solvent because it is essentially hydrocarbon in nature of the solvent. -the polar functional groups outweighed by the much larger polyene backbone, which is Carotenoids hydrophobic. -dissolving carotenoid in water would be an energetically unfavoured process as hydrogen bond between water molecules would be replaced by VDW -carotenoids that are soluble in water contains carboxylic functional group.