SCH4UC

B.9 Biological Pigments

Coloured compounds made by chemical reactions in cells (examples are anthocyanins, carotenoids, chlorophyll, heme)

Molecules containing a double bond (C=C, C=O, benzene) can absorb light in the UV and visible region of the spectrum (these groups are called chromophores)

In a conjugated system of alternating single and double bonds, pi electrons are delocalized because of overlapping of p orbitals

If there are 2 or more single bonds between the double bonds, the system is not conjugated

How many conjugated double bonds are in

this molecule? 6 Visible and ultraviolet electromagnetic

radiation is absorbed by molecules with conjugated systems

For a compound to be coloured, its molecules must absorb visible light (400-750 nm)

The longer the conjugated chain, the longer the wavelength of light absorbed

This is because the absorption of EM radiation promotes an electron from a lower to higher energy molecular orbital

As the conjugated system gets longer the energy gap between the lower and higher molecular orbitals gets smaller, thus longer wavelength (lower energy) radiation is absorbed

Conjugated systems with around 8 or more double bonds will absorb in the visible region, and thus be coloured

Lycopene, shown above, absorbs light in blue-green part of the visible spectrum, and therefore appears red

Retinol, shown above, with only 5 bonds in the conjugated system, is not coloured

Chlorophyll Both chlorophyll a and b absorb light in the

400-500 nm and the 600-700 nm region, therefore appear green (the colour of the wavelengths not absorbed)

Anthocyanins Common pigment in plants Responsible for pink, red, blue, purple

colours of many fruits and vegetables (e.g. red cabbage, strawberries, blueberries, grapes)

Have characteristic structure base on the same core unit:

Usually soluble in water because of large number of –OH groups

The presence of metal ions can affect the colour of anthocyanins (e.g. vivid deep-coloured complexes with metal ions such as Al3+ and Fe3+)

Complex ions are formed with the anthocyanin molecule acting as ligand

Carotenoids Most abundant pigment found in nature

due to presence in algae Generally absorb in the blue-violet region of

the visible spectrum, thus have colours in the yellow-orange-red region

Also present in carrots, tomatoes, watermelon, sweet peppers

Carotenoids that contain only carbon and hydrogen – carotenes

Carotenoids that contain oxygen atoms - xanthophylls

most are derived from a 40-carbon chain with multiple C=C double bonds, thus are soluble in non-polar solvents or fat-soluble

are involved in light harvesting in plants during photosynthesis

Stability of pigment colours Affected by temperature, pH, oxidation,

presence of metal ions, and oxidation number of metal ions

Changes in the above may affect the structure of the pigment and/or the wavelength of light absorbed

Effect of pH on anthocyanins

Are very sensitive to pH and thus used as indicators (e.g. red cabbage indicator)

Low pH Red flavylium form predominates

pH 4-5

carbinol base form with shorter conjugated system which absorbs only in the UV region and thus is colourless

increase pH yellow chalcone form

further increase pH purple quinoidal base form

Effect of Temperature on Anthocyaninsflavylium cation carbinol chalconethe flavylium cation – the form that is most important for

the colour of species containing anthocyanins – is less stable at higher temperatures

at low temperatures (and low pH), the red form is abundant

as temperature increases, equilibrium shifts to the right, pigments lose red colour

at higher temperatures, anthocyanins also dissociate into smaller molecules that do not absorb visible light

Effect of different conditions on carotenoidsdue to conjugated C=C system, highly susceptible to

oxidation: photo-oxidation catalyzed by light oxidation catalyzed by metal ions and organic

hydroperoxides the conjugation is destroyed and the oxidized species

is discoloured and often has an unpleasant odourstable up to 50C and in acidic conditionswhen heated above 50C, all trans-form is converted into

cis-products (can also be influenced by presence of light or iodine)

Effect of different conditions on chlorophyllshigh temperatures can destabilized chlorophylls

depending on pHchlorophyll is stable in alkaline solution but highly unstable

in acidic conditionsWhy do vegetables lose their green colour when cooked?

upon heating/cooking plant material, cell membranes break down releasing acid

the Mg2+ ion at the centre of the porphin ring is displaced by the low pH, resulting in the formation of an olive-brown pheophytin complex

loss of green colour can be minimized by shorter cooking time, cooking with lid off to allow escape of volatile acids, and/or adding a small amount of baking soda to raise the pH of the water

Porphyrin Rings – Heme and Chlorophyllporphin ring – a complex, planar macrocyclic unit,

containing a system of conjugated C=C, that the structure of both heme and chlorophyll is based on

porphyrin – a porphin ring with side groups attached at positions 1-8

chlorophylls – a porphyrin unit complexed to a central Mg2+ ion

heme – a porphyrin unit complexed to a central Fe2+ ion

heme acts as a prosthetic group in myoglobin (pigment in muscles) and hemoglobin (pigment in red blood cells)

recall that in a coordination complex, the ligand bonds to the central metal ion with a co-ordinate covalent bond

monodentate or unidentate ligands – bind to central ion through only one atom

polydentate ligands – bind to central ion through more than one atom

the porphyrin ring is a tetradentate ligand – binds to central ion through 4 nitrogen atoms

The Binding of Oxygen to Hemoglobinhemoglobin in red blood cells transports oxygen from the

lungs through the bloodstream to various cellshemoglobin consists of 4 polypeptide subunits, each of

which contains a heme prosthetic group with Fe2+ at centreeach heme can carry one molecule of oxygen thus each hemoglobin unit can transport 4 molecules of O2

Fe2+ in the heme can bond to six ligands 4 of these are the N atoms of the porphyrin 1 is an amino acid that attaches it to the protein the 6th ligand is the O2 molecule

when O2 binds, the Fe2+ oxidizes to Fe3+

oxygenated hemoglobin gives blood its red colourgraph showing how the affinity for hemoglobin to oxygen

changes as the partial pressure of oxygen changes

at low O2 partial pressures, hemoglobin has low affinity for O2 (thus in tissues, hemoglobin releases its O2)

at high O2 partial pressures, hemoglobin easily binds to O2 (i.e. as blood passes through the lungs)

affinity increases sharply as partial pressure of O2 increases, suggesting that the binding of O2 to hemoglobin becomes even easier once some O2 is already bound

when O2 has bound to one of the 4 heme groups of hemoglobin, the shape changes to make it easier for other O2 molecules to bind (allosteric effect)

Effect of various factors on hemoglobin-O2 binding

pH

as pH decreases ([H+] increases), affinity of hemoglobin for O2 decreases (eq’m shifts right)

note that the H+ does not bind to the same site as O2 but to an amino acid side change which changes shape when H+ binds (H+ is a non-competitive inhibitor)

CO2 concentration/partial pressure CO2 produced by cellular respiration in cells diffuses

into red blood cells where it forms carbonic acid, H2CO3

This increases [H+], shifting above eq’m to the right, reducing affinity of hemoglobin for O2

In addition, CO2 binds to the polypeptide chains of the hemoglobin, changing its shape and further reducing the affinity of hemoglobin for O2 (CO2 is a non-competitive inhibitor)

Temperature Affinity of hemoglobin for O2 decreases as

temperature increases

Carbon monoxide CO binds to Fe2+ in hemoglobin more strongly than O2

does CO is a competitive inhibitor

Fetal hemoglobin

Fetuses have a different type of hemoglobin than adults

Fetal hemoglobin has a higher affinity for O2 thus allowing for the transfer of O2 from mother’s hemoglobin to fetus

CytochromesProteins that absorb strongly in the visible region of the

EM spectrum due to presence of hem groups Involved in key redox reactions in cells that result in the

production of energy In these reactions, Fe2+ oxidizes to Fe3+ or Fe3+ is reduced to

Fe2+

Photosynthesis Chlorophyll aborbs light, which promotes electrons to

higher energy levels

As electrons passed via electron-transport chain to a low energy electron acceptor, the energy is converted to chemical energy

Carotenoids also absorb light and transfers energy to chlorophyll (since they absorb different wavelengths than chlorophyll, the amount of energy obtained from light is increased)