Have you ever seen some clearings to the hair when the sun is it very strong, for example at the seaside in summer? This effect of the sun on the hair occurs because UV rays degrade melanin of our hair. In fact, melanin protects them from the action of UVA/UVB rays and free radicals (ROS) that UV rays form by interacting with other hair structures (lipids, proteins, etc.). During this process called photo oxidationmelanin is oxidized and degraded, leading to the loss of hair color. The darker the hair, the greater the amount of melanin it contains and the less lightening it will have. In this article, we will delve into this process from a chemical point of view.
All hair structures absorb UV rays
Melanin, even that of hair, has a function of protection against radiation. It absorbs both visible light and UVA and UVB rays, but it is not the only structure of the hair that interacts with sunlight. Both the cuticle (the outermost layer of the shaft) and the cortex (the inner part) are rich in keratinone protein insoluble which represents between 65 and 96% of the bark itself and is rich in amino acids such as cysteine and tryptophan which absorb part of the UV radiation, especially UVB rays.
In addition to keratin, the cortex is also made up of water, lipids and minerals, which are not able to absorb UV rays, but can instead be damaged by them, resulting in the formation of free radicals (ROS). The ROS that are formed are mainly the superoxide ion O2– and the H2OR2better known as hydrogen peroxidewhich is in fact commonly used as a whitening agent in bleaching treatments.
When UVB interacts with hair proteins, it damages the bonds between sulfur atoms and the hydrogen bonds that hold amino acids together, resulting in ploss of elasticity and the mechanical properties of the hair: this is why we see them more fragile, dull and lifeless.
Since most of the components of the cuticle and cortex absorb UVB, it takes only a few to reach the melanin granules of the cortex, which only make up 3% of the cortex. mostly UVAwhich pass through the other layers of the hair undisturbed.
A little chemistry to understand what happens
When sunlight hits our hair, as well as our skin, melanin absorbs some of the UV radiation. But how does it do it? The explanation is linked to the chemical structure of melanin: as shown in the image, melanin is made up of 5 rings connected to each other, several double bonds married (i.e. alternating and separated by a single bond, an example shown in blue) and carbonyl groups (a carbon atom and an oxygen atom held together by a double bond, an example shown in red).
One of the carbonyl groups present in the molecule is circled in red. 2D Structure Melanin Pubchem CID: 6325210 URL: https://pubchem.ncbi.nlm.nih.gov/
In chemical terms, this system is called a “conjugated system”, which allows a delocalization of electrons, that is, the electrons of the various bonds are shared throughout the molecule, obtaining a more stable system. This characteristic means that when melanin absorbs energy from UV rays and enters a higher energy state (excited), it is able to “spread” this excess energy across the entire molecule and release it in the form of heat.
This process prevents UV radiation from hitting more delicate structures, such as enzymes, lipids, or DNA. Furthermore, melanin, thanks to its ability to accommodate additional energy or electrons, can immobilize many of the free radicals (ROS) formed by the interaction of UV rays with other hair structures.
In the meantime, however, especially when exposure is prolonged, both UV rays and circulating ROS they oxidize melanin. The result of this photo oxidation (i.e. oxidation by light) is literally the breaking of the melanin molecule. Once broken, the melanin loses its pigmentation and consequently the hair bleaches.
But does this happen with all hair?
This discoloration of melanin and hair It happens for all hair typesbut… on some it is more evident than on others. If you have light hair you may notice more discoloration than those with dark hair. Why? According to research, it depends on the melanin content, or more precisely on the differences between eumelanin And pheomelanin.
We know in fact that the term “melanin” actually refers to a family of molecules, among which the most important are eumelaninresponsible for the dark color, and pheomelaninlinked to the red-blond color. Different combinations of these two molecules give life to the infinite shades of our hair.
From a chemical point of view, it seems that eumelanin resists the attack of UV rays better than pheomelanin, and that this greater stability can guarantee greater protection for the entire hair. In fact, even if dark hair on average they have a higher amount of UV-sensitive amino acids, they also contain more eumelanin granules and therefore tend to lighten and deteriorate less.
In any case, the results of the study published in the Journal of Photochemistry and Photobiology demonstrate that radiation UVB is the main responsible for the loss Of proteins in the hair, while the radiation GRAPE is the one responsible for the changes Of color, independently from the type of hair.
