Advanced Level Organic Chemistry: UV and visible light absorption spectroscopy - MELANIN pigments

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Doc Brown's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy - MELANIN

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

15.5.2 uv-visible spectroscopy theory, spectrometer, examples of absorption & reflectance spectra explained

15.5.3 uv-visible absorption spectra - index of examples: uses, applications, more on the chemistry of colour

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The uv and visible absorption spectrum of melanin pigments e.g. in the skin

(a) The biological function of melanin

Melanin pigment molecules are very efficient in absorbing potentially harmful ultraviolet (uv) radiation photons and absorb both uv and visible light radiation over a wide range of wavelengths.

The wavelengths of uv radiation are <380 nm (see diagram below) where the strongest absorbances observed.

There are two forms of melanin pigment molecules called eumelanin and pheomelanin that absorb uv.

The more melanin in your skin, the greater protection you have from strong sunlight and the darker your skin tone.

Black eumelanin also causes hair to be black whereas brown eumelanin causes hair to be brown or blonde depending on concentration - albino-white coloured (from lack of melanin pigment in skin, hair and eyes), these animals do not produce sufficient melanin, as is the case of older people whose goes grey - mea culpa!

Red pheomelanin is found in the lips and nipples, but in the hair when mixed with eumelanin, it produces a red or orange colour.

Whatever your origin, it is important for your body to have the right balance of melanin in the skin.

Too little melanin risks harmful effects from uv radiation - DNA damage leading to skin cancer.

Too much and you may inhibit the formation of essential vitamin D which requires uv radiation for its synthesis - too little vitamin D causes rickets - a debilitating condition affecting bone development in children - it causes bone pain, poor growth and soft, weak bones that can lead to bone deformities.

 

(b) The overall absorption spectrum of melanin

uv visible absorption spectrum of melanin skin pigment

Image adapted from https://www.researchgate.net/figure/Ultraviolet-and-visible-spectrum-of-melanin_fig1_236343874

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

The ultraviolet and visible absorption spectrum of melanin (several pigment molecular structures) shows a continuous, if decreasing absorbance, declining from the ultraviolet region through to the red region of the visible spectrum.

This results in a black - dark brown - red skin colour, because a variety of molecules contribute to the final colour of the melanin pigment (some are illustrated below).

 

(c) The molecular structure and adsorption spectra of the individual melanin pigments

structure of eumelanin monomer (DHI) black melanin eumelanin monomer (DHICA) brown melanin pheomelanin pigments uv-visible light absorption spectra of eumelanin (black - brown) and pheomelanin (red) molecules melanin pigment molecules

The conjugated system of the chromophore for melanin pigments is based on a double ring system comprising a benzene ring and a heterocyclic ring involving nitrogen (and sulfur as well in one case).

The molecular structures of eumelanin (black-brown) and pheomelanin (red) above are somewhat simplified, more detailed structures below.  They are described as heterogeneous biopolymers.

The comparative uv-visible absorption spectra are shown on the left-above.

Both melanin pigment spectra are quite similar, both showing some small absorbance into the red region of the visible spectrum and very strong absorbance in the near uv-violet region of the electromagnetic spectrum.

Since they absorb a wide range of visible light photons, it is not surprising they are dark in colour.

The chromophore molecules in melanin are part of a polymeric protein system (indicated by the red squiggle).

Images above adapted from https://omlc.org/spectra/melanin/extcoeff.html

molecular structure of the eumelanin molecule melanin pigment skeletal formula   molecular structure of the pheomelanin molecule melanin pigment skeletal formula

Images above adapted from https://en.wikipedia.org/wiki/Melanin for two of the three melanin pigments, namely eumelanin (left) and pheomelanin (right). The arrows indicate the continuation of the polymeric structure

These larger molecular structures show a larger conjugated systems and more readily explain why the chromophore so readily absorbs visible light photons leading to electronic excitation of the melanin pigment molecules.

 

The third melanin molecule is called neuromelanin, and is different because it is not used as a pigment.

It is instead thought to be a protective molecule in the brain that binds to potentially toxic transition-metal ions, such as iron, preventing them from causing damage to the nerve cells.

 

http://www.chm.bris.ac.uk/motm/melanin/melaninh.htm is a good article on melanin



Key words & phrases: interpreting the uv-visible absorption spectrum of melanin, identifying the maximum absorption peaks in the uv-visible absorption spectrum of melanin, explaining the uv-visible absorption spectrum of melanin, how to use the visible absorption spectra of melanin to explain the colour of melanin, applications of the uv-visible absorption spectrum of melanin explaining the biological function of melanin pigments eumelanin pheomelanin explaining how melanin pigments determine skin and hair colour.


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